By: Jody Timer, Entomology & Lake Erie Regional Grape Research and Extension Center
Over the last ten years there have been an inpouring of newcomers to the insect community of Pennsylvania’s grape vineyards. These pest, combined with the numerous indigenous pest, have created an ever evolving challenge for the area’s grape growers. In this blog, I will briefly review the grape pest which I feel are becoming ever increasingly problematic for grape growers to control.
The Spotted wing drosophila has become a progressively severe problem in blueberries raspberries, and grapes. Recent research has shown that they are attracted to all cultivars of grapes that we tested. Spotted wing Drosophila, Drosophila suzukii, Matsumura (Diptera: Drosophilae) (SWD) is an invasive vinegar fly of East Asian origin, that was recently introduced into the United States. It was first found in California in 2008 and is now found in all major fruit-growing regions of the country including Pennsylvania. It was first discovered in Pennsylvania’s Lake Erie grape growing region in the late fall of 2011. The potential infestation rate of spotted wing Drosophila differs from other vinegar flies because the female possess a serrated ovipositor that cuts into healthy fruit to lay eggs. Consequently, spotted wing Drosophila (SWD) larvae can be found in fruit that is just ripening: https://youtu.be/dPr61VC2gyo
During egg-laying, it is believed that sour rot and fungal disease can also be introduced, further affecting the fruit quality. During peak temperatures, a female can lay more than 100 eggs a day. Such a high reproduction rate indicates the SWDs’ high potential for fruit infestation and their potential for spreading rapidly through a field or a vineyard. Because of this prolificity it has become increasing important to protect wine grapes starting at veraison. A good YouTube video on how to identify SWD damage is: https://youtu.be/DLNDnMMfWfs
In our research we have seen SWD showing up earlier in the spring each season and their numbers increasing yearly. SWD do attack injured grapes before non-injured, they tend to wait till veraison before attacking grapes, and they will reproduce in fallen berries. For this reason it is important to keep your vineyards as clean as possible and to maintain coverage of these wine grapes through harvest. Trapping and forecasting can lead to improvements in grower’s capability to optimally time pest management decisions which should reduce both the direct cost of pesticide treatments and the indirect cost to wineries. Information can also be found at:
The brown marmorated stink bug (BMSB) is currently a very serious pest in tree fruits and vegetables, and can be a nuisance when they overwinter in houses. Although BMSB prefer other fruits and vegetables to grapes, they do feed on grapes. Their damage can cause ugly scars on table grapes and grapes grown for sale at fruit stands. This type of damage is not important to wine grape and juice grape growers, however, the holes open pathways for fungal and bacteria late season infections. This season, in the Lake Erie region, we have begun to see a small number of BMSB damaged grapes. BMSB may also be easily harvest with the grapes. The insects tend to move to the interior of the cluster when disturbed and are hard to see. When they are killed they give off a foul odor – which is how they got their name. Our research has shown that this odor and resulting taste do survive the pasteurization of juice grapes, but disappears after being stored for longer periods of time. There is conflicting research on whether this taint transfers to wine, more research is ongoing. There are traps commercially available to trap these insect, but their efficacy is very low. BMSB have been found in both grape foliage and grape clusters; they seek the moisture, sugar, and warmth on the inside the clusters (especially overnight) and they often migrate to the cluster’s interior close to harvest. This makes the possibility of BMSB inside the cluster very likely when these grapes are mechanically harvested and transported to the processor.
With the yearly increase of numbers of BMSB in the Pennsylvania vineyards, it is very important for growers to scout for the adults and the presence of the eggs on the underside of grape leaves. There are one to two generations in Pennsylvania. A compilation of research can be accessed at www.STOPBMSB.org
The newest invasive poised to become a major problem to grape growers, the spotted lanternfly (SLF), Lycorma delicatula, (Hemiptera: Fulgoridae) is native to China, India, Japan, and Vietnam and has been detected for the first time in the United States in northeastern Berks County, Pennsylvania. This approximately one inch long insect with piercing-sucking mouthparts has the potential to impact the green industry, grape growers, tree fruit growers, and the forests and wood products industries in Pennsylvania as well as the United States. The host plants of the SLF in its native habitat include grapes, pines, stone fruits, and up to 50 other hosts. Early detection of the SLF is critical for effective control and protection of Pennsylvania’s agriculture and its related businesses. SLF group feeds on grapevines in numbers great enough to cause destruction of the entire grapevine. Grapes are listed as a primary host in its native regions. To date this insect has been confined to areas of Berks and Bucks counties in Pennsylvania. The PDA has issued a general order of quarantine for these areas over the past few years, however this insect is slowly increasing its range.
The following is a link to the PDA’s information on the SLF: www.pda.state.pa.us/spottedlanternfly. You may find a link to a pdf copy of the SLF Order of Quarantine, a PowerPoint on Lycorma Inspection Tips, and the SLF Pest Alert at this website.
What to do if you:
- See eggs: Scrape them off the tree or smooth surface and place the eggs in a tightly sealed container with 70% alcohol or hand sanitizer to kill them.
- Collect a specimen: Send the adult/nymph specimen or egg mass to the PDA Entomology Lab for verification. The mailing address for the lab is: PDA, Entomology Room-111, 2301 N. Cameron St., Harrisburg, PA 17110. First, place the sample collected in 70% rubbing alcohol or hand sanitizer in a leak proof container. Complete the PDA Entomology Program Sample Submission Form. This sample form can be found in the PDA SLF website www.pda.state.pa.us/spottedlanternfly.
- Report a site: Call the Bad Bug hotline at 1-866-253-7189 with details of the sighting and your contact information.
The most destructive insect pest in the Lake Erie region remains the native Grape Berry Moth (GBM), Paralobesia viteana. This insect is becoming increasingly harder to control as result of shorter residual time of insecticides, resistance to insecticides, and abandoned vineyards. GBM larval burrow into the grape berry soon after hatching, making precise timing of spray applications a critical component of control. This insect has four generations per year. Each generation increases in number exponentially if control measures are not applied to the early generations. Spray timings can be calculated by following the NEWA model recommendations (see earlier posts). Growing seasons with large populations of GBM, will require a second spray in July and/or August to control the populations, and to prevent them from moving farther into the vineyards. Scouting for GBM damage often during the season is a critical component of control, as the pheromone traps capture only the males and are not a good indicator of infestation after the first generation. More information can be found on extension pages and on the LERGP Podcasts on Youtube.
Grapevine leafroll associated virus; A brief introduction to an old disease. Should Pennsylvania grape growers be concerned?
By: Bryan Hed, Michela Centinari, and Cristina Rosa
As if wine grape growers don’t have enough challenges in this day and age, the effects of grapevine viruses have been taking on greater importance in eastern vineyards over the past several years. Studies examining grapevine leafroll-associated viruses are developing a growing body of information that will be essential for vineyard managers to continue moving the eastern wine grape industry forward. Grape growers in the eastern United States need not feel they are the only ones with this disease management challenge (as is the case with many fungal diseases of grapes); grapevine leafroll-associated viruses (GLRaVs) are found in vineyards all over the world (Compendium of Grape Diseases). This group of viruses causes a disease known as grapevine leafroll disease, and the association of symptoms with grapevine leafroll viruses was recognized over 80 years ago. As is the case with so many plant pathogens, the worldwide distribution of these viruses occurred as a result of increased movement of plant material/goods across the globe; the ever widening dissemination of infected planting stock (Compendium of grape diseases). The effects of these leafroll viruses is most severe on – you guessed it – cultivars of V. vinifera, where the disease is known to greatly reduce yield, vine vegetative growth or vigor, and cold hardiness; a factor of critical importance for these cultivars grown in the northeastern United States. Grapevine leafroll disease can also delay fruit maturity, reduce color development in red grapes, and fruit quality (decreased soluble solids, increased titratable acidity) of V. vinifera grapes (Fuchs et al. 2009), which can negatively impact perceived wine quality. The severity of the effects of leafroll viruses is dependent on a great number of factors such as grapevine cultivar, virus strain, climate, soil, cultural practices, stress factors, etc. So naturally, the severity of symptoms can vary from one season to the next (Compendium of Grape Diseases). With respect to cultivar, the effects of these viruses on Vitis interspecific hybrids and Vitis labrusca are generally considered to be less serious, but are also less well defined and studied.
Infection by leafroll viruses results in the degeneration of primary phloem tissues in grapevine shoots, leaves and clusters (Compendium of Grape Diseases). As one can imagine, this can have profound effects on all parts of the vine. Symptoms of the disease, which are generally most observable on V. vinifera, consist of cupping and discoloration of older leaves in late summer and fall. On red fruited varieties, leaves of infected vines can display a distinct red coloration of the interveinal tissue, while veins remain green (Figure 1). On white fruited varieties of V. vinifera, symptoms are less striking and leaves tend to look yellowish (chlorotic) and cupped (Figure 2). Leaf discoloration generally affects older leaves first, but these symptoms are not diagnostic of the disease, as they may be due to other causes such as nutrient deficiencies, water stress, and even crown gall. Analysis of grapevine tissues in the laboratory is the only way to confirm the presence (or absence) of these viruses.
Currently, there are about seven GLRaVs found in cultivated grapes, the most common being GLRaV-3. These viruses are easily spread over long distances through the movement of infected nursery stock, but can be spread (vectored) within the vineyard by mealybugs (Compendium of Grape Diseases). Unfortunately, there are no known sources of resistance to GLRaVs among Vitis species and they have been found in many cultivated grape varieties, including V. labrusca, Vitis interspecific hybrids, and V. vinifera. Interest in grapevine leafroll disease and the extent of its effects has been growing in the eastern United States over the past ten years or so. Surveys conducted in New York, Ohio, and Virginia (Fuchs et al. 2009, Jones et al. 2015, Han et al. 2014), have provided confirmation of the presence of GLRaVs in commercial vineyards and have yielded important information necessary to the management of grapevine leafroll disease. For example, infection by GLRaVs is permanent and infected vines must be destroyed to reduce the incidence of grapevine leafroll disease. Therefore, management of the disease would naturally include planting only stock that is free of GLRaVs. Insecticides that target mealybugs and soft scales can prevent vine to vine spread (within the vineyard) of GLRaVs that are known to be vectored by these insects (Compendium of Grape Diseases). Indeed, studies have shown that applications of insecticides like dinotefuran (Scorpion) and spirotetramat (Movento) can significantly reduce mealybug counts and result in a slowing of the progress of the disease in vineyards. One study from New York (Fuchs et al. 2015) showed that insecticide applications should target overwintered and second instar mealybug crawlers from bud swell to bloom and summer generation crawlers later in mid-summer. A study with grape phylloxera as a potential vector of these viruses showed that phylloxera can acquire the virus through phloem feeding on infected vines, but there was no evidence that phylloxera can transmit it (Wistrom et al. 2017).
As was mentioned earlier, cultivars of Vitis labrusca (Concord, Niagara) can also become infected with GLRaVs, but the infections appear to remain latent or dormant (Bahder et al. 2012) and have not been shown to result in visual symptoms of the disease (Wilcox et al. 1998). On the other hand, cultivars of V. vinifera are severely affected by GLRaVs and make up a very important and growing sector of the PA wine grape industry. Surveys conducted in New York, Ohio, and Virginia (Fuchs et al. 2009, Jones et al. 2015, Han et al. 2014) have revealed the presence of GLRaVs in commercial vineyards to the north, west, and south of Pennsylvania and have led to the development of some important guidelines for management of grapevine leafroll disease.
Given the fact that grapevine leafroll disease is common worldwide and that grapevine leafroll disease can profoundly impact wine quality and grapevine health, researchers at Penn State University are initiating a project to look for GLRaVs in Pennsylvania vineyards. As in other states, the study is targeted to help growers recognize the impact that the disease may be having on the Pennsylvania wine industry and help them to address the effects of these viruses on productivity and fruit quality, reduce their spread and impact, and thereby grow and improve the wine grape industry in Pennsylvania.
The short term, initial objectives of this project will focus on the development of an online survey to collect information from growers with regard to the presence of symptoms of grapevine leafroll disease in Pennsylvania vineyards and their interest in participating in the project. The project will then follow up with tissue sampling from participating, symptomatic and non-symptomatic vineyards throughout the state and serological analysis to determine the presence of Grapevine leafroll virus-1 and Grapevine leafroll virus-3 – the most common of the leafroll viruses – in commercial vineyards in Pennsylvania. The collection of vineyard samples across the state will map the incidence and geographical distribution of these viruses on cultivars of Vitis vinifera and Vitis interspecific hybrid grapevines. The project will also determine and compare the impact of grapevine cultivar and age on infection by Grapevine leafroll virus-1 and -3 in Pennsylvania. Once infected vines have been identified in Pennsylvania vineyards, future objectives will focus on studying the impacts of grapevine leafroll disease on grape quality and productivity in Pennsylvania, and management techniques to mitigate the economic impact of the disease on the Pennsylvania wine industry.
Vineyards will be selected from all parts of Pennsylvania, but the number of locations will favor northwestern and southeastern PA, where the majority of vineyards are located. The study will be expanded as new findings are made and the results will be made available to growers at various meetings throughout the next several years.
Bahder, B., Alabi, O., Poojari, S., Walsh, D., and Naidu, R. 2013. A Survey for Grapevine Viruses in Washington State ‘Concord’ (Vitis x labruscana L.) Vineyards. Plant Health Progress, August 5, 2013. American Phytopathological Society (online).
Compendium of Grape Diseases, Disorders, and Pests. 2nd edition, 2015. Editors Wayne F. Wilcox, Walter D. Gubler, and Jerry K. Uyemoto. The American Phytopathological Society. Pp. 118-119.
Fuchs, M.; Marsella-Herrick, P.; Hesler, S.; Martinson, T.; Loeb, G. M. 2015. Seasonal pattern of virus acquisition by the grape mealybug, Pseudococcus maritimus, in a leafroll-diseased vineyard. Journal of Plant Pathology Vol.97 No.3 pp.503-510
Jones, T. J., Rayapati, N. A., Nita, M. 2015. Occurrence of Grapevine leafroll associated virus-2, -3 and Grapevine fleck virus in Virginia, U.S.A., and factors affecting virus infected vines. European Journal of Plant Pathology 142:209-222.
Wistrom, C. M., G. K. Blaisdell, L. R. Wunderlich, M. Botton, Rodrigo P. P. Almeida & K. M. Daane. 2017. No evidence of transmission of grapevine leafroll-associated viruses by phylloxera (Daktulosphaira vitifoliae). European Journal of Plant Pathology. Volume 147, issue 4. pp 937–941.
By: Bryan Hed
As we move into the post-bloom period, we are reminded that the immediate pre-bloom spray and the first post bloom spray are the most important you’ll make all season. These two sprays protect the nascent crop during its most vulnerable period and are essential to a fruit disease management program for control of the four major grape diseases; powdery and downy mildew, black rot, and Phomopsis. Use ‘best’ materials, shortest intervals, best coverage, etc., for those two sprays, EVERY YEAR! No matter what varieties you grow, those two sprays are the most important for protection of your crop. For growers of Vitis vinifera and many of the French hybrids, the second and perhaps third post-bloom sprays are also of critical importance, especially in a wet year and in vineyards that have already developed some observable level of disease this season. That said, let’s review these major diseases.
First, there’s Black rot caused by the fungus Guignardia bidwellii. This fungus can infect all immature green parts of the vine: fruit, shoots, leaves, and tendrils. On leaves, infections start out as small light green spots visible on the upper surface gradually turning brown to reddish-tan as infected tissue dies (Figure 1). Small, black, pimple-like bodies (pycnidia) develop inside the spot or lesion, usually arranged in a loose ring just inside the dark brown edges of the spot (Figure 1). Spores of the fungus are formed within pycnidia, and are released and splashed around during rainfall periods. Leaves remain susceptible as long as they are expanding and the size of leaf lesions indicate when, during expansion, the leaf was infected. For example, small lesions result when leaves become infected near the end of their expansion. Large lesions indicate the leaf was infected early in expansion. However, numerous small lesions, when clustered, may coalesce to damage large portions of the leaf. The death of large portions of the leaf blade may cause the entire leaf to die and abscise, but this is rare. On petioles, black, elongated lesions may induce wilting or abscission of leaves. Infections on berries initially appear as small, tan spots that develop a dark outer ring and expand rapidly to rot the entire berry. The brown berry shrivels into a hard, black, wrinkled mummy studded with spore producing pycnidia (Figure 2). Once the caps come off during bloom, berries of most varieties are highly susceptible for about 3-4 weeks, gradually developing resistance 5-6 weeks after capfall. Infections that take place during peak susceptibility generally show symptoms within 10-14 days. As berries develop resistance to black rot, the time for infections to become manifest takes longer, and infections that occur toward the end of the susceptibility period (second half of July?) may not develop symptoms until veraison.
On shoots, lesions appear as elongated or elliptical brown cankers. Pycnidia may be clumped in the center of the lesion and/or line the margins of the lesion (Figure 3). These pycnidia produce spores during the current season and can be a source of further infection to fruit. These lesions remain on the shoots after they have “hardened off” and can survive over winter to release spores again the following spring. Large shoot lesions may render the shoots susceptible to breakage by wind, but this is rare.
As berries develop resistance, the appearance of new infections may change: circular lesions are black, expand more slowly, and may remain small, often failing to affect the entire berry (Figure 4). Likewise, leaf infections that take place at the very end of the susceptibility/expansion period may become manifest as small dark pinhead size spots that do not expand (Figure 4).
Cultural and chemical control:
The black rot pathogen survives the winter in infected grape tissue (primarily fruit mummies) which serves as a source of inoculum (spores) the following season. Inoculum that remains in the trellis poses a much greater risk than inoculum dropped to the ground. Therefore, one of the most important methods of cultural control of black rot is removal of infected material, particularly fruit and cluster material, from the trellis. Once on the ground, mummy viability is reduced to further improve control. To take matters a step further, row middles can be plowed and hilling up under the row can bury mummies directly under vines. Maintaining an open canopy where fruit and other susceptible tissue dry out as quickly as possible after rainfall, will also help reduce this disease and improve fungicide penetration and coverage of the fruit.
Chemical control options for black rot mostly include two modern active ingredient classes like the strobilurins (azoxystrobin, kresoxim-methyl, pyraclostrobin, trifloxystrobin) and the sterol inhibitors (tebuconazole, tetraconazole, difenoconazole, myclobutanil) as well as the old standards like captan, mancozeb, and ziram. All are quite effective. The strobilurins and sterol inhibitors are more rainfast than the old standards and the sterol inhibitors have the capacity to stop the progress of an existing infection if applied within about 3 days after the infection period.
Scouting can be an important part of a black rot control program. The presence of pre-bloom leaf infections, especially those in the fruit zone, may indicate the presence of an over-wintering source of inoculum in the trellis and high risk of fruit infection after capfall. Fruit infections can occur during bloom and anytime up to 5-6 (native varieties) to 7-8 (Vitis vinifera) weeks after bloom.
In most parts of Pennsylvania, downy mildew first became active during the second half of May; at about the 5-6 leaf stage of grapevine development. Up here along the southern shore of Lake Erie, our first infection period occurred on May 25 (rainfall with temperatures above about 52 F) and first symptoms were observed at our farm on unprotected suckers of Chardonnay on June 1 (about 6-7 days after infection). On leaves, the first infections of downy mildew appear as yellowish ‘oil spots’ on the top of the leaf that coincide with a white, fluffy or downy patch of sporulation on the lower surface. On young shoots and clusters, early symptoms may first cause cluster rachises and shoots to thicken and curl (Figure 5). As the pathogen, Plasmopara viticola, aggressively colonizes young, expanding grape tissue, infected shoots, clusters, and leaves may turn brown and die. When berries are infected later in the season their development is hindered and they fail to soften at veraison, turning a pale mottled green (white varieties) to red or pink (red varieties, Figure 6). Inflorescences and fruit clusters are most susceptible from about 2 weeks pre bloom to about 2 weeks post bloom. Highly susceptible varieties will require protection through 3-4 weeks post bloom because cluster stem tissue may remain susceptible until later in the season (after fruit have already become resistant) and cluster stem infections can still result in fruit loss. Young leaves and shoots are very susceptible, but become somewhat more resistant as they mature.
Cultural and chemical control:
Because the first inoculum arises from the vineyard soil, cultivation in early spring can help to bury over-wintering inoculum in old leaves and clusters on the ground, reducing primary inoculum in spring (much like with black rot). The first infections in spring often occur on shoots and sucker growth near or on the ground, and prompt elimination of this tissue can delay the occurrence of the first infections in the canopy. Also, the maintenance of an open canopy, where fruit and other susceptible tissue dry out as quickly as possible after rainfall and dew, will help minimize disease development.
There are many chemical options for downy mildew control and the best materials should be applied around and shortly after bloom. Active ingredients found in Ridomil, Zampro, Presidio, and Revus (and Revus Top) have been most effective on downy mildew in our trials. Where strobilurins are still working on this disease (no resistance yet), Abound (except in Erie county), Pristine, and Reason have been very effective too. The phosphorus acid formulations like Phostrol, Prophyt, and Rampart to name a few, have also been very effective against downy mildew, but generally cannot be expected to provide good control beyond 10 days after application, especially under high disease pressure. A tank mix of Ranman (cyazofamid) and phosphorus acid has been shown to be very effective on downy mildew in many university trials. All these aforementioned materials are very rainfast. In addition to these fungicides are the old standards that are strictly surface protectants and are more subject to removal by rainfall. A mancozeb product is probably the best among this group, but fixed copper fungicides can also be quite effective against downy mildew on varieties that are not sensitive to copper. Ziram and captan can also be part of an effective downy mildew program, but are somewhat less effective than mancozeb.
Powdery mildew is caused by the fungus Uncinula necator. Infection on leaves appears mainly on the upper surface as white, powdery patches, though the undersides of leaves can also become infected (Figure 7). As the leaf surface becomes covered with the fungus, leaf function (and photosynthesis) is impaired, with varieties of V. vinifera and highly susceptible French hybrids being most severely affected. Infection by U. necator can stunt growth of new tissues and severe infection of young expanding leaves often results in cupping and distortion of leaves. Cluster infections around bloom may lead to poor fruit set, while later infection can cause berry splitting.
Though primary infections in spring (at least 0.1″ rainfall and greater than 50 F) require rainfall for spore release, secondary disease cycles that result from primary infections, do not require rainfall. Under optimum weather conditions (temperatures in the mid 60s to mid 80s F) secondary disease cycles can be repeated every 5 to 7 days, allowing for explosive increase of disease in the vineyard, especially in highly susceptible wine varieties. Note that optimum temperatures for the fungus are the norm through most of the summer in Pennsylvania and that starting around bloom, nearly every day is an infection period, rain or shine.
In most grape varieties, berries are highly susceptible to infection from the immediate pre-bloom stage until about 2-3 weeks after fruit set, and efforts to protect fruit with fungicides should concentrate on this critical period with timely applications every 7-14 days. Cluster rachises and leaves remain susceptible until harvest and their need for continued protection depends on varietal susceptibility, crop size, and weather. For example, after the fruit susceptibility period, further management of leaf and rachis infections may not be necessary on Concord and other native juice varieties unless vines are heavily cropped or ripening conditions are poor. On the other hand, V. vinifera and susceptible hybrids, may require management of foliar mildew until at least veraison or beyond.
Cultural and chemical control:
There are cultural considerations that can reduce opportunities for powdery mildew disease development. Most involve limiting humidity and promoting sun exposure to all parts of the vine. For example, a training system that improves air movement through the canopy, prevents excess shading and humidity and promotes fungicide penetration to the cluster zone which will help reduce powdery mildew development. Sunlight is lethal to powdery mildew and regular exposure of leaves and fruit can greatly reduce mildew development. Good weed control can also minimize humidity levels that contribute to mildew development.
Unfortunately, cultural measures can only serve as an enhancement to a chemical control program in Pennsylvania and other parts of the northeast. However, we have many effective fungicides for powdery mildew that can provide high levels of control through the critical period around bloom: Vivando, Quintec, Luna Experience, Endura, and now Aprovia. Aprovia is also labeled for black rot control, but our recent tests have indicated that Aprovia’s black rot efficacy is limited especially under high disease pressure on susceptible varieties. The difenoconazole products (Revus Top, Quadris Top, Inspire Super) can also be very effective on powdery mildew, though they may best be used outside the critical two spray period around bloom. Be aware that difenoconazole has been found to cause injury to Concord and a few other varieties (read the label). Sulfur can be an effective powdery mildew material too (on sulfur tolerant varieties) and many wine grape growers rely heavily on it, especially as a tank mix pre-bloom with mancozeb for all diseases. However, it is not recommended as a ‘stand-alone’ material during the critical fruit protection period for powdery mildew control.
There are lots of ‘alternatives’ for powdery mildew control that may be appropriate for late season sprays (to maintain a clean vineyard) that may gradually be used to replace the sulfur and/or synthetics or rotate with synthetics, particularly for reds where late sulfur applications can create wine quality issues. These are materials for which there is little risk of the development of resistance. In fact, these materials can be used to manage the development of resistance to our more risky synthetic fungicides mentioned earlier. Petroleum based oils like JMS Stylet-oil are very effective at 1-2 % solution, but excessive use late in the season (do not apply around or after veraison) may limit sugar accumulation and fruit maturity. And, oils should not be tank mixed with sulfur or applied within 14 days of a sulfur-containing fungicide application. Copper, is moderately effective on powdery mildew and generally applied with lime to reduce the risk of phytotoxicity (read the label). Like sulfur, copper fungicides should not be applied under slow drying conditions as this increases the chance for plant injury. Other materials include potassium bicarbonates such as Kaligreen, Armicarb O, and Milstop. These materials generally produce modest results, and are most effectively applied at short intervals (7 days) to achieve satisfactory control on susceptible varieties. Again, these materials are not appropriate for the critical fruit protection period, but are best integrated during the early season when disease pressure is low OR after the critical fruit protection period to help control leaf infections.
Phomopsis cane and leaf spot is caused by the fungus, Phomopsis viticola. Earlier this spring, growers in many parts of Pennsylvania experienced problems with Phomopsis development on new shoots and leaves. Prolonged wetting/rainfall during the first week of May led to widespread infection by this pathogen on Concord in the Lake Erie region; virtually every shoot of every vine in every Concord vineyard we have examined has some level of Phomopsis development on the first one or two internodes. The infection period(s) occurred when shoots were in the 1-3″ range and inflorescences were just becoming exposed. In some cases, heavy infection of inflorescences is likely to result in problems with fruit rot after veraison (months after the infection period took place!). Fruit are generally at risk of new infections until a couple weeks or so after bloom, but infections of the cluster stem tissue that occur in the early pre-bloom period can move into berries during ripening and cause fruit to rot and shell before harvest. The concentration of heavy infection at the base of the oldest internodes, may result in large scabby areas that weaken the shoot (Figure 8) and green shoots that are severely infected are more apt to break under windy conditions. Leaf infections appear as pinhead sized black spots surrounded by a yellow halo (Figure 9). These infections appear to be of little consequence, other than revealing the presence of the pathogen. Lesions on cluster stems are black and sunken, and can girdle parts of the cluster rachis causing the cluster or parts of the cluster to break off or shrivel.
When berries are infected, they can remain symptomless until ripening when they turn brown and become studded with small pimple-like fruiting structures of the fungus (Figure 10) often resembling black rot infected berries.
However, even though direct fruit infection by both pathogens can occur during the same peak susceptibility period (bloom through 3-4 weeks after bloom), black rot fruit rot symptoms become observable while berries are still green, whereas Phomopsis fruit infections lay dormant until after ripening. Also, leaf symptoms of these two diseases are very different from each other and can be used to determine which pathogen(s) are present and most likely to have caused disease on nearby fruit.
Cultural and chemical control:
Hand pruning to remove dead wood and pruning stubs from the trellis removes much of the over-wintering inoculum of Phomopsis. For this reason, cane pruning can reduce the disease compared to a cordon system that retains a maximum amount of older wood. Trellis systems that train shoots upward also reduce infections on the oldest shoot internodes and clusters. And of course, the maintenance of an open canopy where fruit and other susceptible tissue dry out as quickly as possible after rainfall, will help minimize disease development. For wine grapes, fruit zone leaf removal and shoot thinning reduce canopy density, hasten drying after rainfall, and improve fungicide penetration and coverage of the fruit.
Phomopsis management with fungicides should continue through the first or second post bloom spray, after which inoculum of the fungus is generally spent. Strobilurins, mancozeb products, Captan, and Ziram are generally the only effective materials for Phomopsis control. Some formulations of sterol inhibitor fungicides claim Phomopsis control, but their level of efficacy is still under question and would not be recommended for management of this disease.
Much of the information in this blog can be found in the 2017 New York and Pennsylvania Pest Management Guidelines for Grapes. Be sure to get your copy through Cornell University press. You can also read the publication; Disease Management Guidelines for Organic Grape Production in the Lake Erie Region found online at http://agsci.psu.edu/research/ag-experiment-station/erie/research/plant-pathology/organic-grape-disease-management-trials/DiseaseMgmtGuidelines07.pdf which contains much of the information discussed in this blog.
2017 New York and Pennsylvania Pest Management Guidelines for Grapes. Edited by Tim Weigle and Andy Muza. Cornell and Penn State University Cooperative Extension.
Hoffman, L.E., W.F. Wilcox, D.M. Gadoury and R.C. Seem. 2002. Influence of grape berry age and susceptibility to Guignardia bidwellii and its incubation period length. Phytopathology 92:1068-1076.
Hoffman, L.E., W.F. Wilcox, D.M. Gadoury, R.C. Seem, and D.G. Riegel. 2004. Integrated control of grape black rot: Influence of host phenology, inoculum availability, sanitation, and spray timing. Phytopathology 94: 641-650.
By: Andy Muza, Penn State Extension – Erie County
There are several species of leafhoppers in the genus Erythroneura that feed on grape foliage. Research conducted in New York showed that the eastern grape leafhopper Erythroneura comes (Say) is the most common on American varieties (e.g., Concord, Niagara) while E. bistrata/vitifex complex were more common on Vitis vinifera and interspecific hybrids. Other species found in commercial grapes included E. tricinta, E. vulnerata and E. vitis. (1). Regardless of which of these species is prevalent, their life cycles are similar and the injury caused by these leafhoppers and their management is the same.
Life Cycle and Description
The various Erythroneura leafhoppers overwinter as adults in leaf litter in the vineyard or in plant debris around the vineyard. As temperatures increase in the spring, adults begin feeding on a variety of weeds, bushes and trees. Adults then migrate into vineyards to feed when leaves emerge (2). Eastern grape leafhopper adults are small (only about 1/8”), white-pale yellow, with darker lemon colored markings on the wings, and 3 black spots towards the posterior portion of the wings (Figures 1 & 2). Other Erythroneura species have varying coloration and markings (3).
Initial feeding occurs on sucker growth and basal leaves on shoots in the trellis. Females lay eggs on the undersides of leaves just below the leaf surface. Nymphs of the first generation hatch in mid-late June. Immatures are wingless, pale yellow in coloration with tiny wing pads (Figure 3). Nymphs develop through 5 instars with wings fully developed after the fifth molt (2). Nymphal development to adulthood takes about 30 days or less depending on environmental conditions. In northwestern Pennsylvania nymphs of the second generation can be found in vineyards in mid-late August. There are 1.5 – 2 generations/season in the Lake Erie Region, depending on seasonal temperatures, and in the southwestern portion of the state likely 2.5 – 3 generations.
Grape leafhopper (GLH) adults and nymphs have piercing – sucking type mouthparts and feed on the underside of leaves extracting the contents of leaf cells resulting in white – yellow spotting of the foliage (stippling). Moderate – Heavy feeding causes yellowing and browning of tissue while severe injury can result in premature defoliation (Figure 4).
The greatest risk for economic losses due to grape leafhopper (GLH) feeding occurs during hot, dry years in vineyards with heavy crop loads and high leafhopper populations (4). In most years, the majority of vineyards in Pennsylvania should not require an insecticide treatment specifically for management of grape leafhopper. Therefore, routine, prophylactic insecticide treatments for leafhoppers are unnecessary and not recommended. Insecticide applications should be based on scouting information and threshold levels.
Scouting – Tim Martinson at Cornell designed a scouting procedure for leafhoppers which corresponds to the timings when sampling for grape berry moth injury are conducted (5).
10 Days Postbloom – Usually population levels and feeding is minimal at this time of the season. If however, early in the season, high numbers of adult leafhoppers migrate into the vineyard this can result in enough leaf feeding to reduce bud fruitfulness in the following year (4). Scouting should be conducted to look for leaf feeding on interior leaves in the canopy. If leaf stippling is noticeable throughout the vineyard then an insecticide application is recommended.
Third week in July – Check 4 different areas in the vineyard (2 exterior and 2 interior). At each area look at lower leaves on shoots and check for leaf feeding. If no – minimal injury is observed, proceed to the next sampling site (Figure 5). If moderate-heavy leaf stippling is observed then begin counting nymphs on the undersides of leaves (Figure 6). Examine 5 leaves (leaves 3-7 from base of shoot)/shoot on 5 different shoots at each location. If a threshold of 5 nymphs/leaf is reached then an insecticide application is recommended.
Late August – The scouting protocol at this time follows the same procedure as the July sampling. However, the threshold for the August sampling period is 10 nymphs/leaf before an insecticide application is recommended.
Based on scouting data, if an insecticide application becomes necessary during the season, there are a number of options available. Consult the “2017 New York and Pennsylvania Pest Management Guidelines for Grapes” (6) for a list of insecticides which are effective for grape leafhopper management.
Shoot and leaf removal practices conducted in many wine grape vineyards may reduce leafhopper population levels, if the removed leaves are harboring nymphs of this pest. In addition, these practices will open up the canopy for better spray penetration.
A number of predators (e.g., spiders, green lacewings, lady beetles, etc.) and egg parasitoids (Anagrus species) which occur in vineyards contribute to reducing leafhopper population levels (7). Therefore conserving these beneficial insects, by avoiding unnecessary applications of broad spectrum contact insecticides, is advised. Good weed control in the vineyard and the prevention of overgrown areas around the vineyard will also reduce leafhopper overwintering sites.
- Martinson, T. E. and T. J. Dennehy. Varietal Preferences of Erythroneura Leafhoppers (Homoptera: Cicadellidae) Feeding on Grapes in New York. Environ. Entomol. 24:550-558 (1995). https://academic.oup.com/ee/article/24/3/550/2394852/Varietal-Preferences-of-Erythroneura-Leafhoppers
- Grape Leafhopper. Grape Insect IPM Insect Identification Sheet No. 4 (1984). NYS. Ag. Exp. Station, Cornell University. https://ecommons.cornell.edu/bitstream/handle/1813/43102/grape-leafhopper-FS-NYSIPM.pdf?sequence=1&isAllowed=y
- Leaf- Stippling Leafhoppers (Ontario GrapeIPM). Ontario Ministry of Agriculture Food & Rural Affairs, Canada http://www.omafra.gov.on.ca/IPM/english/grapes/insects/ls-leafhoppers.html
- Martinson, T. E., et al. Impact of Feeding Injury by Eastern Grape Leafhopper (Homoptera:Cicadellidae) on Yield and Juice Quality of Concord Grape. Am. J. Enol. Vitic., 48:291-302 (1997). http://www.ajevonline.org/content/ajev/48/3/291.full.pdf
- Martinson, T. E., et al. Risk Assessment of Grape Berry Moth and Guidelines for Management of the Eastern Grape Leafhopper. New York’s Food and Life Sci. Bull. 138. 10 pp. (1991). http://nysipm.cornell.edu/publications/grapeman/files/risk.pdf
- Weigle, T. H., and A. J. Muza. 2017. “2017 New York and Pennsylvania Pest Management Guidelines for Grapes”. Cornell and Penn State Extension. 150 pp. https://store.cornell.edu/p-197039-2017-new-york-and-pennsylvania-pest-management-guidelines-for-grapes.aspx
- Williams, L., III, and T. E. Martinson. 2000. Colonization of New York Vineyards by Anagrus spp. (Hymenoptera:Mymaridae): Overwintering Biology, Within-Vineyard Distribution of Wasps, and Parasitism of Grape Leafhopper, Erythroneura spp. (Homoptera: Cicadellidae), Eggs. Biol. Control 18:136-146. https://pubag.nal.usda.gov/pubag/downloadPDF.xhtml?id=43140&content=PDF
By Bryan Hed
Another season of grape growing is upon us and it’s a good time to review important disease management principles and be aware of some of the tools to consider integrating into your vineyard management programs this spring.
First is your annual reminder to check out the NEWA website (Network for Environment and Weather Applications) found at http://newa.cornell.edu. On the home page is a map of the Northeastern U.S. marked with the locations of hundreds of weather stations where historical and ‘up to the hour’ weather data can be viewed. Although is provided free on the internet, it is funded through the New York State IPM program. Click on a weather station near enough to you (denoted by a leaf/rain drop icon) to get weather, insect pest, and disease information you need to make important management decisions that could save you time and money. Clicking on ‘grapes’ under ‘crop pages’ will give you access to forecasting models for all the major diseases, as well as the grape berry moth degree day model that will improve your timing of grape berry moth insecticide sprays later this summer. Each model forecast is accompanied by helpful disease management messages and explanations.
Next, let’s move our minds into the upcoming pre-bloom disease management season. It’s important to recognize that the threat of disease this spring (pre-bloom) is largely determined by the amount of overwintering inoculum in your blocks. The amount of overwintering inoculum is dependent on the amount of disease that developed in your vineyard last year or in previous years. In other words, if you have kept diseases well under control in the past, especially last year, then there will be relatively little for pathogen populations to build on and cause damage, at least initially, this year. Some very practical research by Wayne Wilcox at Cornell nicely illustrates this point with powdery mildew (pm) development in susceptible wine varieties. In blocks where pm was well controlled all season, fewer overwintering structures of the fungal pathogen (chasmothecia) were available the following spring to jump start disease cycles. Early disease pressure was relatively low and early sprays were less critical to good commercial control than in blocks where disease control was poor the previous year. Where there was poor control the previous year, more of the pathogen overwintered to start disease cycles the following spring and early sprays were critical to maintaining successful commercial control. This is not to say that a bad year of pm will automatically be followed by another bad year. But it certainly tilts the odds in favor of the pathogen, especially if for some reason, you can’t manage the timely application of your early disease control program (stuff happens). It also doesn’t mean you can slack off this year if you had good control last year. Remember, there’s the weather. The weather ALWAYS plays an important role too. A good illustration of this is an experience by an organic grape grower who, in an extremely wet season, developed a serious, economically damaging case of black rot. In conventionally managed vineyards there are several very effective chemistries to control black rot, but in organic production there are no real effective fungicides, and control of this disease in organic vineyards must rely heavily on cultural measures that reduce the pathogen’s overwintering population. Of course, the grower did everything he could to sanitize the trellis of overwintering fruit mummies and bury mummies that had fallen to the ground to reduce overwintering inoculum. But fortunately, the following year was bone dry during the fruit susceptibility period and black rot was not even an issue. Had the previous wet season been followed by another wet one, I’m quite certain, the battle for control of black rot in that organic vineyard would have required ‘the kitchen sink’ to avoid losses. Unfortunately, we have no control over the weather and accurate forecasts, especially long term, are not something to rely on. But, we can (and should) strive to control overwintering inoculum levels every year and the best way to do that is good, practical, season-long disease control.
So, begin to wrap your minds around the campaign ahead. If you had poor disease control in some blocks last season, have you reviewed your spray records where control failed AND where it worked well? Where it failed, did you use the wrong material at a critical time? I’ve had growers discuss their control failures with me only to discover that their timing was fine, but their choice of material did not cover the disease(s) they intended to control. The number of spray materials, what disease each one controls, and how well each one controls each disease, can be bewildering at times…and the list keeps growing and changing. Also, materials that used to be good choices may have become ineffective due to the development of resistance by the pathogens. For example, materials like the strobilurins (Abound, Sovran, Flint, Pristine) are no longer effective at controlling powdery and downy mildew in many parts of the east. In vineyards where this has occurred, using them during the critical fruit protection period (which used to be a great idea!) can now prove disastrous. The sterol inhibitor fungicides (Rally, Elite, Orius, Mettle, Tebusol, Tebustar, Procure, Viticure, etc) are also exhibiting the effects of resistance by the powdery mildew fungus. Though in most cases they still work on powdery to some extent, they are not appropriate for the critical fruit protection period anymore, around and shortly after bloom (products that include the more active difenoconazole are an exception on less susceptible varieties). However, they may be acceptable for maintaining a clean vineyard outside the critical period. Do you have an accurate grasp on that?
Do you have a firm grasp on the critical fruit protection period? The critical period for fruit protection from all diseases generally extends from ‘just before bloom’ to about 4 weeks later. This is the period when you need to be especially vigilant about minimizing spray intervals, using your best materials that cover all the major diseases (Phomopsis, black rot, powdery and downy mildew), focus on good coverage, etc. It is never profitable to try to cut corners during the critical period. However, if you had heavy amounts of black rot in your vineyard the year before, you should assume you have an unhealthy dose of overwintering inoculum in your vineyard this spring, and prevention of leaf lesions in the fruit zone (which would need to be addressed during the first 3-12” of shoot growth, well before the fruit protection period) would also prove to be critical. This goes for other diseases as well (refer back to the previous example with Wayne Wilcox’ powdery mildew experiment). The pre-bloom presence of visible disease in the fruit zone is a big red flag; it means you’ve got potential for serious fruit loss ahead, especially if weather conditions favor the pathogen (wet, warm, humid, calm, cloudy) during the fruit protection period that follows.
Did you record the relative levels of disease that developed in years past for each of your blocks? In order to do this, you need to be able to identify the various diseases and then scout regularly for them. This takes up valuable time but you can streamline your scouting efforts in many ways. Do you know when you would expect to first see each disease? Downy mildew doesn’t become active until about the 5-6 leaf stage. So, you know you can’t expect to see it until about that time or shortly after that. In which blocks are diseases most likely to occur first? Your block or rows next to the woods would be a good place to start, or perhaps your most susceptible variety. Blocks with the most disease last year would be a good place to start. On which parts of the vine do you expect to see diseases appear first? Can recent weather data help you to determine where to look for the disease? For example, if a black rot infection period occurred 2 weeks ago (and you can find this out easily by searching the NEWA website), would you examine the newest growth, the oldest growth, or would you look for lesions on leaves that were currently expanding and most susceptible 2 weeks ago? The answers to these questions can help you streamline your scouting efforts, save time, and improve your expertise.
Do you fully comprehend the susceptibilities of all the varieties you’re growing? You cannot spray premium Vitis vinifera like the hybrids or natives and expect the same results. What are you going to change this year to address disease control breaches in your vinifera? If you had good control last year, are you ready to do it again this year? OR, do you feel lucky and plan to back off until close to bloom to apply your first spray? I always plan for the worst when it comes to the weather and assume it’s going to be wet, cloudy, and warm; ideal for fungal disease epidemics. Consider that here in the east we are growing a highly vulnerable, susceptible host (wine grapes) on the pathogen’s ‘turf’ (the wet, humid eastern U.S.). The good news is that disease control during the pre-bloom period is generally easier (good spray coverage not a problem, low initial disease/inoculum levels, etc.) and cheaper (can use lower fungicide rates, lower spray gallonage, less expensive materials, less time, etc) than in the post bloom period, and a well prepared pre-bloom disease management program will provide extra insurance against problems during bloom and early fruit set, when your fruit ($) is most vulnerable. Now let’s review the common diseases with some of these questions and concepts in mind.
Phomopsis cane and leaf spot is often the first disease problem we face in the pre-bloom period, particularly where trellis systems maintain lots of old and/or dead wood. That’s because old and/or dead wood is where the pathogen overwinters. Therefore, the more old wood you have in your trellis, the more inoculum you can expect to be battling with this spring. Conversely, cane pruned systems have fewer problems with Phomopsis, and cane pruning/minimizing older wood is an important cultural control for this disease. Fortunately, many areas of PA and other parts of the east experienced a relatively dry spring in 2016, helping to minimize new overwintering infections on year-old wood. But, older cordons and especially dead wood and pruning stubs, can carry overwintering inoculum into many subsequent springs. So, if there was little opportunity for new Phomopsis infections to occur last year, you can still be carrying a fair amount of overwintering inoculum in old cordons and pruning stubs.
During early spring rains, Phomopsis spores flush from lesions on wood and are splashed about to invade any new shoot, leaf, and inflorescence they land on…provided the wetting period/temperature combination falls within a minimum range for infection. The basal-most (oldest) internodes of new shoots are the most susceptible to shoot infections simply because they are closest to the inoculum source; wood. In every trial where I have rated shoot infection of Phomopsis, the most severe lesion development was ALWAYS found (on average) on the first (oldest) internode region of the shoot. Lesion development typically got less severe as my rating progressed through internodes 2, 3, 4, and 5. However, once these internodes become fully expanded after the first few weeks in the season, they are no longer susceptible to lesion development. I rarely see Phomopsis lesion development beyond the fifth internode region. That’s why this disease is best dealt with preventatively, very early, during the first few inches of shoot growth. Infections that occur on the first few internodes of new shoots are not only the most likely to occur, but also the most critical; infections of inflorescences (generally on nodes 2-5) can lead to crop loss early (parts of the inflorescence may be ‘bitten off’ by the pathogen) or later during ripening (cluster stem infections in spring move into berries and cause fruit rot and shelling after veraison). And, infections that occur on the basal-most internodes, can’t all be eliminated by judicious hand pruning during the dormant season. So, in blocks where you suspect any risk of early Phomopsis infections, applications of a fungicide (mancozeb or captan are good choices) at no later than 3-6” of shoot growth are a good investment, particularly if you are not cane pruning. Following up with fungicides at 8-12” shoots and immediate pre-bloom are also important pre-bloom applications. Below are some pics from last year’s blog (Figures 1, 2) to help you get a handle on the appearance of lesions on year-old canes. Unfortunately, determining the presence of Phomopsis on older wood generally involves more than just a visual assessment.
Figure 1. Dark brown lesions on the first few internodes on these Chancellor canes are from Phomopsis infections that occurred during early shoot growth in the previous year (when these were green shoots). The buds present are just ready to burst open with new shoot growth that will be very vulnerable to infection during subsequent rain periods.
Figure 2. Although the 1” shoot stage can be vulnerable to damage from this pathogen, the more critical stage is at 3-6” shoots, when more shoot, leaf, and cluster tissue is exposed and is highly susceptible (below). Note the inflorescence in the upper right picture from which Phomopsis has “bitten off” whole branches, dramatically limiting yield potential for that cluster.
Pre-bloom fungicide applications for Powdery mildew are also prudent during early shoot growth for Vitis vinifera cultivars and highly susceptible hybrids, especially in vineyards where control of this disease may have slipped last year (again, because of lots of overwintering inoculum). The primary inoculum for this pathogen generally comes from overwintering structures of the fungus that are lodged within cracks in the bark of cordons and trunks. Spring rain periods of at least 0.1” of precipitation and temperatures of 50 F or more, are the requirements for release of primary inoculum (ascospores) from the overwintering structures. The more mildew that was allowed to develop the year before, the larger the release of spores in early spring, the more primary infections that are likely to occur, and the more critical the need to control the disease early. Sulfur, oils, monopotassium phosphate, and potassium bicarbonate materials can be good choices for mildew management early on. All of these materials can eradicate small existing powdery mildew infections on leaves and cluster stems. Most do not generally offer any protection from future infections and therefore work best if applied often. Sulfur is an exception, and has the added benefit of providing a week or more of protection against future infections. Many of the more experienced growers like to utilize a mancozeb/sulfur combination to control all diseases during the pre-bloom period. This combination is relatively inexpensive, there are no resistance issues, and it works. Remember to read labels and be aware that you can’t mix sulfur and oils, or oils and captan. The tebuconazole products can be used during early pre-bloom to control powdery mildew as well, especially at the 8-10” shoot stage. These materials are very inexpensive and generally provide enough powdery mildew control to keep vines healthy until the immediate pre-bloom spray (they will also nicely control early black rot infections). At immediate pre-bloom and first post bloom, you want to apply your best powdery mildew chemistries like quinoxyfen (Quintec), difenoconazole (Revus Top), metrafenone (Vivando), fluopyram/tebuconazol (Luna Experience), etc. For native juice grapes, powdery mildew is rarely a concern during the early shoot growth stages, especially in the cooler Lake Erie region of Pennsylvania.
A note about fungicide resistance management and powdery mildew: It’s important to plan your powdery mildew management choices ahead of time with resistance management in mind. The easiest way to do this is to become familiar with FRAC (fungicide resistance action committee) codes listed prominently on the first page of fungicide labels. Fungicides with the same FRAC group number can be considered similar enough in their mode of action/chemistry that resistance to one is resistance to all others within that group. Therefore when you rotate fungicides for resistance management, you’re essentially rotating FRAC groups. Some good rules to remember are to avoid using the same FRAC group consecutively, or more than twice in a given season. The development of powdery mildew resistance is always a concern when using materials like the strobilurins (FRAC 11), the sterol inhibitors (FRAC 3), Quintec (FRAC 13), Vivando (FRAC U8), Luna Experience (FRAC 7, 3), Torino (FRAC U6), and Endura (FRAC 7) to name a few. Resistance is generally not a concern for uses of sulfur, oils, bicarbonates, and the potassium salts (mentioned above), or copper.
Next, black rot: One of the best ways to reduce overwintering inoculum of black rot is to scout your vineyard for old fruit mummies and eliminate them from the trellis. Black rot infected fruit mummies that have overwintered in the trellis are the most potent source of inoculum for infections the following spring. No matter how cold it gets over the winter, the pathogen survives just beautifully in colonized fruit remaining in the trellis. But, dropping this inoculum source to the soil, allows microbial degradation/weathering to reduce the potential for mummies to release spores the following spring. It also places the inoculum source much farther from new, susceptible plant tissue up in the trellis. The best time to ‘sanitize’ the trellis is during dormant pruning; weathering has already accomplished some of the removal of last season’s infected fruit from the trellis, and what remains is relatively easy to see and remove by hand. Experiments we conducted several years ago clearly showed that the earlier the mummies are knocked to the ground during the dormant period, the more time for decomposition to break them down before the next season, and the fewer spores released from the ground the following spring to start new disease cycles. Nevertheless, some inoculum on the ground will survive to release spores in spring, and burial of mummies with cultivation will go a step further to eliminate the threat. Removal of ALL old cluster material from the trellis before bud break is important to maintaining good control of this disease.
It may not be necessary to apply a fungicide for black rot at early shoot stages IF good control of this disease was achieved the previous year AND conscientious scouting and trellis sanitation has been implemented. However, the importance of early shoot infections should not be underestimated as I mentioned above, especially if they result in leaf lesions in the fruit zone. For example, inoculations we performed from early May to early June (simulating wet weather and an overwintering inoculum source (mummies) in the trellis) resulted in leaf and shoot lesions in the cluster zone (Figure 3). Those lesions went on to release spores during the critical fruit protection period, resulting in crop loss of 47-77% on those shoots with infected leaves!
An application of mancozeb, ziram, or captan for Phomopsis will also provide control of early black rot infections. The sterol inhibitor fungicides and strobilurins are also good materials for black rot that are more rainfast than mancozeb, ziram, and captan. The sterol inhibitors also provide excellent post infection activity that can be very useful at terminating an infection that has already occurred (but not yet manifested itself).
Figure 3. Early (pre-bloom) black rot leaf infections in the cluster zone provide inoculum that can add to problems with controlling fruit infection after capfall. The two small tan lesions on the leaf at node 2 are just inches from the developing inflorescence found at node 3 (picture on the right). These lesions will release spores during rainfall periods that could easily be splashed to highly susceptible cluster stems pre-bloom, and developing fruit after capfall. Resulting fruit infections will lead to crop loss.
Downy mildew and the 5-6 leaf stage: This stage marks the point at which the downy mildew pathogen first becomes active and is capable of releasing primary spores from inoculum sources that have overwintered on the ground (leaves and other plant material that was infected during the previous season). As with all other diseases, vineyards that developed a fair amount of downy mildew leaf/cluster infection last year will be at higher risk this spring than vineyards that were kept clean. However, overwintering structures of the downy mildew pathogen can survive more than one season in the soil.
Periods of rainfall with temperatures of at least 52 F meet the requirements of spore release and the first infections; plant surfaces must be wet for infection to occur. While scouting for this disease, expect to see it first in wetter areas of your acreage and pay close attention to leaves near the ground (sucker growth, grape seedlings that germinated from shelled berries last fall) which are most likely to become infected first. Therefore, keeping such low growth to a minimum in spring is a prudent control measure that can delay the development of the disease. It also suggests that if you’re planning vine trunk renewal from sucker growth, you will need to apply fungicides to protect that growth from the ground up as the pathogen becomes active.
Spring leaf infections are identified by the yellow ‘oil-spots’ seen on the tops of leaves (Figure 4), coinciding with white, downy sporulation of the pathogen on the undersides of leaves. Inflorescences can be blighted and show sporulation as well. Sporulation occurs during darkness under high relative humidity, and can typically be seen during a morning scout of the vineyard following a wet/humid night. Under optimum temperatures (70-75F), only an hour or two of plant surface wetness may be required for infection to occur, and new infections can produce their own spores with just 5 days.
Many parts of the northeast experienced drought conditions last year, which severely inhibited the development of this disease. Up in Erie County PA, the disease basically took a vacation in 2016, and I could barely find a handful of lesions on unsprayed ‘Chancellor’ leaves and fruit near the ground all summer: it was the perfect year to start renewal trunks! It wasn’t until later in August that rains finally returned and we began to see a few more infections, but for the most part the disease literally could not get off the ground in Erie county PA in 2016. What does this mean for 2017? The great lack of downy mildew in drought hit areas last year means that pre-bloom disease cycles this year will have to rely on overwintering inoculum from previous years (although spores of downy mildew can travel long distances between vineyards, the first infections will arise from inoculum within your vineyard). I have not found any detailed information as to how long the pathogen can survive in the soil, but I guarantee that if you’ve had downy mildew before, then it’s still there. Whether your area was wet or dry last spring, the principle described earlier still applies: vineyards devoid of downy mildew last year (whether from drought or just plain good control) will be easier to keep ‘clean’ in the pre-bloom period this year.
Mancozeb products are good options for the first downy mildew, Phomopsis, and black rot sprays in the pre-bloom period. Ziram and Captan have a similar spectrum of control, but Ziram is a little weaker on downy mildew, and Captan a little weak on black rot. However, these may be a viable option if these diseases are not a huge threat early on (that is if you had good control last year). These materials are all surface protectants subject to wash-off by rainfall, which means that under heavy, frequent rainfall conditions, application intervals will need to be minimized (7-10 days?) especially for highly susceptible varieties. For that more critical ‘immediate pre-bloom’ spray (and the first post bloom spray), there are other materials like Presidio, Revus, Revus Top, and Zampro that are quite rainfast, very effective, and will provide longer range protection under wet conditions (when you need the protection most and are least likely to be able to stick to shorter spray intervals). However, products like Presidio also require a second active ingredient (like mancozeb) in a tank mix for resistance management purposes (which isn’t a bad idea at this critical spray timing in any case). Other materials like the phosphonates, Ranman, and the strobies /Reason, are probably best utilized outside the critical two sprays around bloom (especially for V. vinifera and highly susceptible hybrids), unless they’re used as tank mix partners with other effective materials. They’re very good materials, but they’re just not the ‘best of the best’.
Figure 4. Yellow oil-spot symptoms of downy mildew on young spring leaves.
One more time for emphasis: the immediate pre bloom and first post bloom (7-14 days later) fungicide applications are the most important you’ll make all year, regardless of variety grown and disease pressure. These two sprays protect your fruit from all the major fungal diseases (Phomopsis, black rot, downy and powdery mildew). Make sure sprayers are properly calibrated and adjusted for best coverage on a bloom-period canopy, spray every row at full rates and shortest intervals, and NEVER extend the interval between these sprays beyond 14 days.
‘Newer’ Fungicides: Aprovia (solatenol) may be worth a try for powdery mildew control (received federal registration in 2015). The active ingredient is related (same FRAC group) to Boscalid (found in Endura and Pristine) and Fluopyram (found in Luna Experience). It also has activity against black rot, but should not be expected to control this disease under high pressure on a susceptible variety.
***Lastly, to help you with all your grape management decisions this year, you should have…
New York and Pennsylvania Pest Management Guidelines for Grapes. An inexpensive, excellent source of research based information for commercial growers; some information in this blog was gleaned from it and it is revised every year to include the newest information. Copies can be purchased at the Cornell Store at https://store.cornell.edu/c-875-pmep-guidelines.aspx. It sells for about $31.
By: Andrew Harner and Michela Centinari
As we move forward through the winter season, many growers have begun or are planning on beginning their annual dormant pruning within the coming weeks. Though a routine task within the vineyard, dormant pruning is essential to maintaining a balanced vineyard that produces quality fruit. With that in mind, this post will both review the basics of dormant pruning and present a series of important considerations to keep in mind when pruning and planning to prune.
With that being said, we will begin with the basics: What is dormant pruning?
In short, dormant pruning is the intentional removal of grapevine tissue, in the form of canes, cordons, trunks, etc., during the annual period of plant dormancy.
What are we trying to gain or change through dormant pruning?
In order to understand the rationale and goals behind dormant pruning, it is first important to understand the biology of grapevines and their physical characteristics that have evolved over thousands of years. Grapevines behave as lianas, or woody, lignified vines that lack a specific growth form on their own; instead, they use other means of support for their growth (i.e., trees or trellis wires). Moreover, grapevine shoots exhibit an indeterminate growth pattern and will continue to grow as long as growing conditions allow and are hospitable. This helps explain why the wild grapevine species endemic to North America tend to be sprawling masses of extensive shoots, and overall have a vigor that differentiates grapevines from many other fruit crops. As a whole, they will remain in this vegetative state until there is access to sufficient sunlight to induce floral development.
Both trellising and pruning are means of harnessing this inherent productivity, with the goal of transitioning it into reproductive growth and consistent, quality fruit yields. Dormant pruning is the primary tool used by grape growers to maintain vine shape, as defined by the training system, and to effectively regulate crop load (fruit mass/vine size) so that a vine’s bearing capacity matches its vegetative vigor capacity. This balance is especially important, as over- or underestimating a vine’s capacity to ripen a fruit crop may result in overly vigorous or overly cropped vines, both of which can have long- and short-term negative consequences. Additional crop load adjustments through shoot thinning and cluster thinning may also be necessary during the growing season to fine-tune grapevine crop load.
Principles of vine balance and essential considerations
Various efforts by researchers to quantify the effects of pruning on vine performance have resulted in the establishment of a few metrics that can be used to guide dormant pruning. Perhaps one of the most basic but important ways to measure vine size is through the collection and weighing of the canes removed by pruning (Figure 1). These numbers could be used to compare final vegetative biomass between vines of any given season, and when combined with the crop yield measurement—taken on the same vines at the previous harvest—can be used to calculate the ratio of fruit yield to vegetative mass. This ratio is the basis for the Ravaz index (yield/pruning weight), an early metric of vine balance first pioneered by the French viticulturist Louis Ravaz during the early 20th century.
Otherwise called crop load, optimal Ravaz index values vary by grapevine species and variety: research on Vitis vinifera has suggested that optimal crop load values fall between 5 and 10; a Ravaz index below 5 indicates that vines were potentially under-cropped (a small vine with a large crop), while a Ravaz index close or above 10 indicates that vines may have been over-cropped (a small vine with a large crop). American and Canadian studies have suggested that interspecific hybrid varieties, more of which are grown in Pennsylvania and other regions of the northeast and Midwest US, are capable of achieving higher crop load values without compromising fruit quality.
Caution is still necessary when thinking about crop load, however, as these general ranges fail to detail variations in a vine’s capacity to ripen its crop due to genotype, weather, soil, and management strategies. Exceptions can occur, and often do occur. While any variety may produce high yields with good fruit quality at one site – and subsequently attain high Ravaz index values – the same variety may not be able to ripen the same amount of fruit at less vigorous sites and under different weather conditions (i.e., locations with shorter growing seasons and/or with lower heat accumulation).
The concept of balanced pruning focuses on cropping vines at yields that are tailored to the vine’s vigor potential and size. The goal is to prevent under- or overcropping and ensure proper shoot maturity and winter-hardiness through a conscious approach to pruning: for example, more vigorous vines are allocated a greater number of buds so the vegetative growth potential is spread across a greater number of nodes, resulting in lower individual shoot vigor. In terms of managing weaker vines, fewer buds are to be retained so the remaining buds will produce more vigorous shoots.
Simple model equations have also been developed for balanced pruning that allocate specific numbers of nodes based on total pruning weights. A classic example is the equation for Concord vines: 30+10, where 30 nodes are retained for the initial pound of pruning weight, and 10 nodes retained for every additional pound of pruning weight thereafter. This specific formula is unsuitable for hybrid and V. vinifera vines, however, as Concord vines are typically cropped at higher levels. Other suggested formulas are based on cluster size, with large clustered varieties (e.g., Chancellor) at 20+10, small clustered varieties (e.g., Marechal Foch) at 20+10, and medium clustered varieties at 10+10. Varieties with large clusters and highly fruitful shoots tend to overcrop, so additional cluster and shoot thinning may be necessary for optimal balance; this counteracts any overcropping that could potentially occur if the formula is followed strictly. Again, these formulas are not the rule, and exceptions to them can and will occur. Instead, it is immensely important to use them as a guideline and tailor final node counts to the individual vine or variety, while keeping in mind site and variety vigor, climate, soil type, and training system.
What differences exist between cane and spur pruning?
Depending on the type of training system implemented and the variety being pruned, dormant pruning methodology consists of either cane pruning or spur pruning. The difference lies in the length of bearing unit, or the one-year old wood, retained: spurs are typically 2-3 nodes long, whereas canes are longer – usually between 8 to 15 buds.
With spur pruning, the one-year old fruiting canes are pruned back to spurs of 2-3 nodes, being the fruiting wood that will yield new shoots in the subsequent growing season. This allows for the retention of cordons and mature, wooden arms (Figure 3).
Conversely, cane pruning entails the removal of one-year old growth back to the head or crown of the vine, with the retention of two canes – one for each side of the trunk – for the bilateral training systems that are used in many vineyards in the eastern US (Figure 4). In sites with high vigor potential, growers may choose to leave four canes instead of two (Figure 2A & B, and Figure 5), which will help to accommodate more vigor and leave the vine more balanced. Furthermore, choosing canes of the right size is especially important—the preferred cane diameter is within a range of 3/8 to 1/2 inch, easily represented by the diameter of a pencil—as thick, excessively vigorous bull canes are as unsuitable as thin, spindly canes. Canes that are too thin or too thick will only yield shoots and fruit of inconsistent quality, whereas well-matured canes with diameters within the mentioned range will only help with maintaining full canopies and fruit-bearing capacity.
Choosing the right pruning system is dependent upon many factors: the variety being grown is especially important, as basal buds of some varieties (e.g., Sauvignon blanc, Nebbiolo) have low fruitfulness and are therefore unsuitable for spur pruning. Vine spacing, mechanization, available labor, and time availability may also affect the choice of a pruning technique. Cane pruning requires more labor in the form of tying down canes, but cane-pruned vines generally require less shoot thinning during the growing season. It is therefore important to select canes with equally spaced internodes, as this allows for equally spaced shoots and reduced shoot crowding.
If the vines are spur-pruned, retaining equally spaced spurs is crucial in order to obtain uniform canopy density and improved sunlight penetration, though shoot thinning during the growing season will likely be necessary as well. Regardless of the pruning method chosen, maintaining sunlight into the renewal zone is essential, as poor light penetration will inhibit bud fruitfulness with negative consequences for future yield and fruit quality.
How do I know when to prune? Pruning strategies for cold climate viticulture
Regardless of the seemingly obvious answer that dormant pruning should be implemented during the dormant, winter season, the timing of pruning could have major implications for the following season’s growth.
In grape-growing regions where there is risk of exposure to damaging cold events, such as Pennsylvania, pruning during the late winter is preferred. Low winter temperature events can damage buds and vascular tissues of mainly cold-tender vine varieties – all vines have limits to their cold-hardiness, however, and even very cold-hardy hybrids can sustain injury to buds and other tissues under exceptionally cold events.
Pruning in the late winter would allow growers a chance to assess vine injury and accordingly adjust the number of buds/nodes to retain. Yet due to labor and time constraints, it is often not possible to do all pruning in the late winter; instead, it is best to begin with the most cold-hardy varieties and leave more cold-susceptible varieties until later in the season. Moreover, in an instance where a damaging cold event does occur, various levels of additional buds are recommended for retention during pruning, depending on the percentage of bud necrosis (Table 1). A high level of bud injury might require differing pruning strategies, such as retraining new trunks and renewing larger parts of the vine, but these topics will not addressed within this post.
When forming a plan for pruning, it is equally important to consider the topographic and microclimatic variability of a vineyard site, as this has implications for air drainage. Vines, rows, or blocks that are at lower elevations than the rest of the vineyard may be more susceptible to cold temperature injury if dense, cold air drains to these low points and pools there. This creates pockets of air that will have lower temperatures than the ambient temperatures of any surrounding blocks, rows, etc. that are at higher elevations.
This is also a consideration worth keeping in mind as the season progresses to bud-break, as these same microsites are also more susceptible to damaging spring frosts and could have any early season growth quickly curtailed. In these cases, double-pruning can be a potential strategy when spur-pruning: canes are first pruned back to long spurs/canes of 5-8 buds, which will allow for terminal bud growth first and will suppress basal bud growth due to apical dominance. Once the risk of frost has passed, a final pruning cut should be made to cut the spurs back to 2-bud spurs. An alternative for cane-pruned vines would entail leaving long canes and extra canes until the threat of frost has passed, and then subsequently making a final pruning cut that leaves the canes at the desired bud number.
Through this post we have hoped to provide an overview of balanced pruning methodology, as well as emphasize considerations that are essential to successful pruning and maintaining balanced, fruitful vines. We realize that many growers tend to have their own styles and methods of pruning, however, and their own rationale for using specific strategies that may differ from the ones listed here. We would be happy to hear about their systems, and any strategies that have proven successful for their vineyards and vines, or adaptations they have implemented to handle specific circumstances or issues within the vineyard. Please feel free to contact us with your ideas and experiences regarding dormant pruning.
In addition to a number of books and publications that detail balanced pruning methodology, principles, and philosophy, there exists an extensive online reservoir of material that can be easily accessed for guidance related to pruning. More detailed explanations and even experiment findings related to pruning can be found in the Wine Grape Production Guide for Eastern North America, The Grapevine: From the Science to the Practice of Growing Vines for Wine, Sunlight Into Wine, and myriad other texts.
Many cooperative extension services (Penn State University, Washington State University, Cornell University, etc.) can be particularly helpful with providing information about pruning, and instructional presentations and in-field videos can easily be found online through a simple web-based browser search or through the aforementioned universities’ cooperative extension websites.
Iland, P., P. Dry, and T. Proffitt. 2011. The grapevine: From the science to the practice of growing vines for wine. Patrick Iland Wine Promotions Pty Ltd, Adelaide, Australia.
Jordan, T.D., R.M. Pool, T.J. Zabadal, and J.P. Tomkins. 1966. Cultural practices for commercial vineyards. Miscellaneous Bulletin 111. Cornell University, Ithaca, NY.
Reynolds, A.G., and T.K. Wolf. 2008. Pruning and training. In Wine Grape Production Guide for Eastern North America. Tony K. Wolf (ed.), pp. 98-109. NRAES, Ithaca, NY.
Willwerth, D., K. Ker, and D. Inglis. 2014. Best management practices for reducing winter injury in grapevines. Cool Climate Oenology & Viticulture Institute, Brock University, Ontario, Canada.
Andrew Harner is a graduate student at Penn State, where he is pursuing a MS degree as a member of the Plant Science Department and studying how climatic and environmental factors influence rotundone synthesis and concentration in Noiret wine grapes with Dr. Michela Centinari. He is currently funded by the John H. and Timothy R. Crouch Program Support Endowment, an endowment founded and funded by the Crouch brothers, original owners of Allegro Winery in Brogue, PA. Drew graduated from Cornell University in 2016, where he focused his studies on Horticulture and Viticulture/Enology and was first introduced to the world of grapevines and plant-based research. Following the conclusion of his MS program he hopes to continue onwards within academia and pursue a PhD, and spends much of his free time either reading, cooking, or outside exploring the natural areas and parks of the U.S.
By Michela Centinari
Potassium (K) plays a critical role in many plant physiology and biochemistry processes (e.g., photosynthesis, osmoregulation, enzyme activation, etc.). Inadequate supply of K can result in reduced shoot, root and fruit growth as a result of reduced xylem sap flow, and can also increase the risk of drought stress . Potassium deficiency leads to inhibition of photosynthesis and to sugar (sucrose) being “trapped” in the leaves which adversely affects yield, fruit ripening and berry soluble solid concentration .
While grape growers should monitor vine health to avoid K deficiency, at the same time they should also be on the lookout for excessive concentration of K in vine tissues (e.g., leaf, berry) because of its potential negative impact on vine health and wine quality.
When looking back at the past two years of inquires received from Pennsylvania wine grape growers related to vine nutrient or nutrient-related problems, we (Denise and I) found that the number of those concerning excessive concentration of K and related issues (e.g., high/unstable wine pH) were more common than inquiries related to K deficiency, which mostly occurred in young vineyards.
This short article will review problems related to high/luxury absorption of potassium, briefly discuss how soil mineralogy and pH can affect K uptake, why it is important to regularly monitor vine nutrient status, and what environmental and cultural factors may impact K uptake and accumulation in plant tissues. In-depth information on mode of uptake and transport of K in the plant, and functions of K can be found in “A review of potassium nutrition in grapevines with special emphasis on berry accumulation” by Mplelasoka et al. 2003 .
Also, you can refer (as I did) to the valuable web-resources recently published by Virginia Tech University (Dr. Tony Wolf and Russel Moss), which includes edited information to supplement Chapter 8 of The Wine Grape Production Guide of Eastern North America:
Potassium in Viticulture and Enology, May 2016
Why high/excessive concentration of K in the grape berries may negatively impact wine quality?
Grape berries are a strong sink for K during ripening. Potassium accumulates mainly in the berry skin tissues (Figure 1) and is the most abundant cation (K+, hereafter referred to as K) in grape juice . Mature grapes may have, indeed, almost twice as much K as nitrogen ; for example one ton of mature grapes contains about 11 lbs (5 kg) of K .High concentration of K in grape juice (e.g., > 50 mmol/L) may result in a high juice pH (e.g., > 3.8) and negatively impact wine quality . During winemaking, high concentration of K causes precipitation of free acids, mainly tartaric acid, leading to an increased wine pH . The high pH may reduce color stability of red wines due to a shift of anthocyanins to the non-colored forms . High concentration of K may also reduce respiration and the rate of degradation of malic acid and consequently increase malolactic fermentation .
While many studies linked high juice/wine pH to high juice/wine K concentrations (see for example Figure 2) it is also true the other factors, aside from K, can affect wine pH and that wines with low pH (e.g., < 3.25) may also have high K concentration . Moreover, there are varietal differences in the relationship between juice pH and K concentration (see Chardonnay vs. Shiraz, Figure 2), as well as between wine K and juice K .pH can be adjusted during winemaking through addition of tartaric acid, adding additional costs for the wineries. Ensuring adequate concentration of K in the grapes at harvest will not only help reduce winemaking costs but is also likely to improve wine quality .
In next week’s blog post, Denise Gardner will discuss options for dealing with high K concentrations in juice and wine. However, the first step is to determine if there is or there may be a potential problem with K in your vineyard (either deficiency or excess level).
Potassium availability in the soil
It is well known that concentration and availability of K vary with soil type and is greatly affected by the physical and chemical properties of the soil . Potassium in soil is classified into four groups in relation to its availability to the plants: 1) water-soluble (K dissolved in soil solution), 2) exchangeable (on cation exchange sites of surfaces of clay minerals and humic substances), 3) non-exchangeable and 4) structural forms .
The water-soluble and exchangeable pools (1 & 2) represent only » 0.1-0.2% and 1-2% of the total soil K, respectively . Both forms are readily available for plant root absorption. The majority of the K is bound in mineral structures, such as mica and feldspar, or it is part of secondary minerals such as vermiculite  and thus considered a (very) slowly-available source of K for plants. Clays also have different capabilities of binding K as well as different rates of K release . For example, micas release K at a remarkably faster rate than feldspar .
Since clay mineralogy impacts the release of K into the soil solution over time and the K supplying power of the soil, it is important to have detailed information concerning the soil structure and composition of your vineyard (i.e., does your site have high levels of exchangeable K?).
Other important factors affecting K availability are soil pH and relative concentration of K to that of the other cations, such as magnesium (Mg2+) and calcium (Ca2+). Low/acidic soil pH (<6) increase K availability and potentially increase its uptake while reducing uptake of Ca2+ and Mg2+ . Excess K can decrease concentrations of Ca2+ and Mg2+ in plant tissues and induce symptoms of Mg deficiency  (Figure 3).
Assessing K concentration in the vine
Conducting plant tissue (petiole) testing on a regular basis (annual or every two years) to monitor vine nutritional health (K and other essential nutrients) and promptly correcting problems related to nutrient imbalance is strongly recommended. Visual observations of foliar symptoms of nutrient deficiency or toxicity are important clues (Figure 4), but a nutrient management program should not be exclusively based on visual observations because: 1) it is possible to be misled by symptoms that are not nutrient related (e.g. mite injury, virus, etc.) and 2) to develop an appropriate nutrient management program it is crucial to understand the nutritional requirements of the vines .
Soil testing is an extremely useful tool in the pre-planting stage for determining the potential of a vineyard site and the amendments needed, and also for monitoring soil pH over the years (after the vines are planted). However, soil testing only tells one side of the story: what is potentially available to the vine. Again, the recommended and preferred method to assess vine nutritional health and to effectively identify potential nutrient (in this specific case K) deficiency or excess is plant tissues (petiole) testing.
Some limitations of the soil testing include:
- Soil samples are often limited to the first 10-20 inches. Roots of mature vines tend to be sparse and, in deep soils, they can grow much deeper than 10-20 inches. Thus soil testing may not be a good indicator of the soil/plant interaction.
- Soil testing may underestimate the reservoir of K available to the vines . The laboratory nutrient extraction analysis is run over minutes while vines have much longer to absorb/extract nutrients [6,9].
Thus, don’t be too surprised if results of K soil testing are poorly correlated with those of plant tissue testing [6,7,9].
When is the best time to conduct leaf petiole testing?
The preferred time for leaf petiole testing is bloom and late-summer (70 to 100 days after bloom). Assuming bloom was in June, you may still have time left this season to conduct the test. In case of suspected K or other nutrient deficiency the samples can be collected anytime during the season . It is important to have a standardized tissue-sampling procedure. For example, at bloom collect 60 to 100 petioles of healthy leaves opposite to the flower cluster (first or second) for each cultivar. Don’t collect more than one or two petioles per vine. Late summer samples should be collected from “the youngest fully expanded leaves of well-exposed shoots, usually located from 5 to 7 leaves back from the shoot tip”. If the shoot has been hedged, collect “primary leaves near the point of hedging” .
More information on how to collect leaf petiole samples and interpret the results can be found at Monitoring Grapevine Nutrition (eXtension.org) and in the Wine Grape Production Guide for Eastern North America, chapter 8 .
Reference values of K concentration in grape leaf petioles for bloom- and late summer-collected samples are reported in Table 1. (Note: the source used is the Wine Grape Production Guide for Eastern North America , sources from other regions may provide slightly different standards).
Table 1. Reference values for K concentration in grape leaf petioles for bloom- and late summer-collected samples.
In table 2, I included the results of a petiole test conducted at a research vineyard in south central Pennsylvania. At bloom, K concentrations in the leaf petiole of Cabernet Sauvignon and Merlot vines were slightly above the ‘excessive’ range. We repeated the analysis around veraison and found that at that time K concentrations were greatly above the 2% ‘excessive’ threshold. Not too surprisingly, at harvest, both varieties had high grape juice pH and high K concentration according to reference values reported by Mpelasoka et al. 2003  (Table2). Soil testing showed a high level of exchangeable K, and low pH (below 6), the vines were highly vigorous: all factors that can contribute to luxury uptake of K.
Table 2. Potassium concentrations in the petiole of Cabernet Sauvignon and Merlot leaves at bloom and veraison. Potassium concentration and pH of the grape juice was measured at harvest.
What to do next?
In case of K deficiency (petiole and soil testing and visual observations) (Figure 4) you can consult with an extension educator in your county or a viticulture consultant who can assist with the development of an appropriate fertilization program.
For those of you who may have missed it, Tony Wolf (professor and viticulture extension specialist at Virginia Tech university) recently issued an important update on K fertilization recommendations . The lower limit of optimal soil K reported in the “Wine Grape Production Guide for Eastern North America”  has changed from 75 ppm (150 lbs/ac) to 40 ppm (80 lb/acre) (Note: those values are based on Mehilch-3 extraction protocol which is the one used by Penn State Agricultural Analytical Services Lab but not by Virginia Tech). In the July issue of Viticulture Notes Tony Wolf wrote:
“Potassium fertilizer is not recommended pre-plant or to existing Virginia vineyards if the soil test results are at or above 40 ppm (80 lbs/acre) actual K as determined by Mehlich-3 test procedures, or 28 ppm (56 lbs/acre) actual K as determined by Mehlich-1 test procedures. However, young vines should be visually monitored and irrigated under drought conditions to avoid potential K deficiency on soils that are inherently low in exchangeable K.”
How can K concentration and uptake by the vines be reduced?
In the pre planting stage, if the soil selected for planting the vineyard has high exchangeable K levels, an option is to select rootstocks that accumulate low concentration of K. Rootstocks, and grapevine varieties in general, differ in their capacity of K uptake and translocation . For example, rootstocks with V. berlandieri genetic background tend to have reduced K uptake as compared to others, as those with V.champini parentage . In northern California, Chardonnay, Cabernet Sauvignon and Zinfandel vines grafted on 101-14 Mgt and 3309C (V. riparia x V. rupestris), two commonly-used rootstocks in the eastern US, consistently had leaf petiole K concentrations within the intermediate range compared to those of the same varieties grafted on V. berlandieri (lowest K concentrations) and V. champinii (highest K concentrations) crosses . A study conducted at Winchester, VA, by Tony Wolf research group found that the use of 420-A (V. berlandieri x V. riparia) rootstock reduced juice pH in Cab Sauvignon vines as compared to those grafted on 101-14 Mgt and Riparia .
However, it is important to consider that the performance of rootstocks in terms of K uptake varies depending on rootstock-scion combination (i.e., the same rootstock may have variable effects on different scion varieties), soil type, climate, and management practices.
Another aspect to consider when selecting rootstocks is their vigor or growth-potential. Vigorous rootstocks or rootstocks that convey high vegetative growth and yield potential to the scion may cause increased K uptake as a result of increasing vine demand.
Growth drives K uptake: Factors such as high vine vigor, leaf area, and extensive root system can enhance K uptake, translocation, and accumulation in the grape berries. Soil moisture increases the dissolution of K from clay particles, thus facilitates K supply and uptake by the roots. High soil water availability also leads to increase vegetative growth which may indirectly affect K uptake and its accumulation in the berries.
Shaded leaves are a source of K translocation to the grape berries: Canopy microclimate and mainly foliage shading can affect the accumulation of K in the berries. For example, artificial (shading cloths)  or natural (canopy) shading  was found to increase K concentration in berries and juice. We don’t know exactly why this happens yet, but it is possible that in conditions of low sugar accumulation, as under foliage shading, the increasing accumulation of K in the berries helps regulating osmotic potential, maintaining cell turgor and thus minimizing reduction in berry growth which may occur with low sugar content .
Can crop load be regulated to reduce K accumulation in the fruit?
Since berries after veraison are the primary sink for K, we would expect that regulation of crop load (commonly defined as the ratio of fruit weight to pruning weight or to leaf area) can affect K translocation and accumulation in the berries . However, results from previous studies are inconclusive. For example, in hot climate regions (Israel, California) cluster thinning reduced berry K in one study  while having no effect on juice K in another study . Factors such as grape variety, timing of thinning, and the amount of crop retained can greatly affect outcomes and explain why the effect of crop load on accumulation of K in the berries still remains unclear. Making matters more complicated, manipulation of crop load may also affect vegetative growth, and the degree of foliage shading, thus indirectly impacting K translocation into the berries.
Generally speaking, over cropping may result in a lower or insufficient amount of K in the vine tissues (K deficiency tends to be more pronounced on heavily cropped vines after veraison). At the same time if yield is very (too) low the shoots may become competitive sinks for K and as a result its accumulation in the berries may be reduced .
Vineyard management practices that decrease leaf shading may reduce K accumulation in the berries. Reducing canopy density and shading, either through a) the removal of lateral shoots, b) lateral and top-hedging, or c) basal leaf removal reduced K concentration and, in some cases, pH in juice and wine of Tannat vines in Uruguay (Figure 5) . The use of divided-trellis systems could also be a method to manage highly vigorous vines and decrease leaf shading .
In conclusion, if you suspect a problem with high vine K and/or high juice/wine pH, here a few things you can do:
- Plant tissue (petiole) analysis to assess the K level of your vines and to confirm that high/excessive K is the real issue. Again, don’t rely exclusively on soil testing, which is still useful to assess soil pH and other factors that may affect K uptake.
- If your vines are highly vegetative you could test the effect of reducing foliage shading on juice/wine pH and K. Reducing canopy density may also have additional benefits for the health of the grapes and quality. Canopy practices such as basal leaf removal, hedging, shoot positioning and thinning can be used. It is a good practice to leave a block of untreated vines (no additional canopy management) as a control. At harvest measure juice pH and K concentration in the treated (less shaded) and untreated (more shaded) vines to assess if the extra canopy management mitigate the K/pH level in the grape juice.
If you have tried or panning on trying to manage K levels in your vineyards we would be happy to hear about your experience and methods/results.
- Keller M. 2010. The Science of Grapevines: Anatomy and Physiology. Publisher: Academic Press.
- Mpelasoka BS, Schachtman BP, Treeby MT, Thomas MR. 2003. A review of potassium nutrition in grapevines with special emphasis on berry accumulation. Aust. J. Grape Wine Res. 9, 154–168.
- Coombe BG, Dry PR. 1992. Viticulture Volume 2 – Practices. Publisher: Winetitles.
- Walker RR, Clingeleffer PR, Kerridge GH, Rühl EH, Nicholas PR, Blackmore DH. 1998. Effects of the rootstock Ramsey (Vitis champini) on ion and organic acid composition of grapes and wine, and on wine spectral characteristics. J. Grape Wine Res. 4, 100–110.
- Kodur 2011. Effects of juice pH and potassium on juice and wine quality, and regulation of potassium in grapevines through rootstocks (Vitis): a short review. Vitis 50, 1–
- Wolf TK. Viticulture Notes. Vol 31 No. 5. 23 July 2016. Virginia Tech University Cooperative Extension. Available at: http://www.arec.vaes.vt.edu/alson-h-smith/grapes/viticulture/extension/news/vit-notes-2016/vit_notes_july_2016.pdf
- Walker RR, Blackmore DH. 2012. Potassium concentration and pH inter-relationships in grape juice and wine of Chardonnay and Shiraz from a range of rootstocks in different environments. J. Grape Wine Res. 18, 183–193.
- Zörb C, Senbayram M, Peiter E. 2014. Potassium in agriculture – Status and perspectives. J. Plant Physiol. 171, 656–669.
- Beasley, E, Morton L, Ambers C. 2015. The role of soil mineralogy in potassium uptake by wine grapes. Progress report to the Virginia Wine Board http://www.vawine.org/wpcontent/uploads/2015/11/REPORT-2-Beasley-Final-Report-SEPT-2015-2.pdf
- Moss R. 2016. Potassium in viticulture and enology. Virginia Tech University Cooperative Extension. Available at: http://www.arec.vaes.vt.edu/alson-h-smith/grapes/viticulture/extension/news/vit-notes-2016/kinvitandeno.pdf
- Wolf TK. 2008. Wine grape production guide for Eastern North America. Natural Resource, Agriculture, and Engineering Service: Ithaca, NY USA.
- Wolpert JA, Smart DR, Anderson M. 2005. Lower petiole potassium concentration at bloom in rootstocks with Vitis berlandieri genetic backgrounds. Am. J. Enol. Vitic. 56:163-169.
- Rojas-Lara BA, Morrison JC. 1989. Differential effects of shading fruit or foliage on the development and composition of grape berries. Vitis 28, 199–208.
- Dokoozlian N, Kliewer MW. 1996. Influence of light on grape berry growth and composition varies during fruit development. J. Am. Soc. Hortic. Sci. 121, 869–874.
- Hepner Y, Bravdo B. 1985. Effect of crop level and drip irrigation scheduling on the potassium status of Cabernet Sauvignon and Carignane vines and its must and wine composition and quality. J.Enol. Vitic. 36, 140–147.
- Freeman BM, Kliewer WM. 1983. Effect of irrigation, crop level and potassium fertilization on Carignane vines II. Grape and wine quality. J.Enol. Vitic. 34, 197–207.
- Coniberti A, Ferrari V, Fariña L, Disegna E. 2012. Role of canopy management in controlling high pH in Tannat grapes and wines. Am. J.Enol. Vitic. 63:554-558.