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2017 Summer Disease Management Review

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.

Fig. 1 Development of black rot lesions on grapevine leaf (Concord).

 

Fig. 2 Development of black rot lesions on grape berry (Concord).

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.

Fig. 3 Black rot shoot lesions (Concord).

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).

Fig. 4 Limited black rot lesion development from infections occurring toward the end of the susceptibility period (Concord).

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.

Fig. 5 Infection of downy mildew on young cluster and shoot showing curling and thickening of diseased tissue (Chancellor). The white sporulation after a warm humid night can be striking.

 

Fig. 6 Berries of red varieties (Concord (left) and Chancellor (center) at harvest) often turn red or pink after infection and fail to soften and develop properly. Late season leaf infections (far right photo) are yellowish to reddish brown and appear angular or blocky.

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.

Fig. 7 Powdery mildew on young, developing ‘Concord’ berries.

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.

Fig. 8 Numerous lesions concentrated at the base of the oldest internodes result in larger scabby areas that weaken the shoot.

 

Fig. 9 Leaf infections of Phomopsis cane and leaf spot on Concord grape.

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.

Fig. 10 Phomopsis fruit rot on ripe Vignoles and Niagara grapes.

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.

 

References:

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.

Early season grapevine canopy management, Part I: Shoot thinning

By: Maria Smith and Dr. Michela Centinari, Dept. of Plant Science

This is the first of two posts on grapevine canopy management in the early growing season from bud burst to bloom.  The second in the series will be post in two weeks and will focus on pre- or trace-bloom leaf removal for crop level and disease pressure control.

This week, our blog post will focus on shoot thinning, the first canopy management practice of the growing season.  As seen in the pictures below, we spent last week shoot thinning Grüner Veltliner (V. vinifera) vines in a central Pennsylvania vineyard (Figure 1).

Figure 1. (A) Andrew Harner, graduate student at Penn State in the Centinari lab, is shoot thinning Grüner Veltliner (V. vinifera) vines, May 10, 2017, Lewisburg, PA. (B) Grüner Veltliner shoot length at the time of thinning (pencil as a reference for shoot length).

In the following sections, we will highlight the benefits and costs associated with shoot thinning while providing a few general shoot thinning guidelines for both V. vinifera and hybrid cultivars in the Mid-Atlantic region.

Benefits of Shoot Thinning Grapevines

While dormant pruning (https://psuwineandgrapes.wordpress.com/tag/dormant-pruning/) is the primary tool used by grape growers to maintain vine structure, canopy architecture and regulate crop level, shoot thinning provides an additional canopy management tool to bring vines into vegetative and fruiting balance by reducing shoot density and the number of clusters per vine. Cluster thinning later in the season may be needed in order to balance highly-fruitful vines.

In addition to improving balance between vegetative growth and fruit biomass, other benefits of shoot thinning include:

  • Reduction of canopy density and fruit shading: through removal of selected shoots, shoot thinning reduces overcrowding of shoots in the canopy thus reducing the number of leaf layers and improving sunlight exposure to fruit (1).
  • Reduction of disease pressure: reducing canopy density improves air circulation and sunlight penetration that promotes quicker drying of leaves and fruit, as well as increases spray penetration.

Timing of Shoot Thinning

Shoot thinning should be done early in the growing season, when shoots are approximately 5-6 inches long and not more than 10-12 inches long. Shoot thinning should be timed after the date of last ‘expected’ frost, such that secondary or non-damaged primary shoots can be retained in the event of a late spring frost.

When shoot thinning is performed before inflorescences are visible (shoots 0.8 inch to 4 inches), increased vigor of the remaining shoots and lateral shoot growth may occur as a response, negating the benefits of shade reduction (1). When performed too late (shoot longer than 10 inches), shoots become lignified at the base and difficult to remove.  If performing late thinning, pruning shears should be used if there is risk of damaging the arm of the vine. It also takes longer to thin longer shoots, potentially decreasing the cost-effectiveness of this practice.

Shoot Spacing and Density Recommendations

Generally, shoot thinning on cane-pruned vines is easier, faster, and more straight-forward than spur-pruned vines, which require substantially more decisions regarding what shoots to retain or remove, and where shoots should be spaced along the cordon (2; Figure 2).

Figure 2. Before shoot thinning: spur-pruned (left) vs. cane pruned (right) in Grüner Veltliner, May 26, 2016, Lewisburg, PA.

Plant genotype, soil, and climate are all factors influencing vine vigor potential and capacity to fully ripen a crop.  Therefore, these factors indirectly affect the appropriate number of shoots to retain at thinning.  Many Cooperative Extension websites provide recommendations on range of optimal shoot density based on cultivars grown in their region. [Author’s note: for the eastern US see the additional resources section at the bottom of the post.]

Shoot density targets for Pennsylvania regions:

  • For vinifera cultivars it is recommended to leave 3 to 5 shoots per linear foot of canopy (3, 4; Figure 3). The general rule of thumb is to retain fewer shoots in red varieties and more in white varieties. However, other factors (i.e., cultivar disease susceptibility) must be taken into consideration.

Figure 3. Suzanne Fleishman, graduate student at Penn State in the Centinari lab, is shoot thinning spur-pruned Grüner Veltliner vines (May 26, 2016). Note the differences shoot density between the cordons on the right (thinned) and on the left (unthinned) cordons.

  • For most of the hybrid cultivars it is recommended to leave 4 to 6 shoots per linear foot of canopy (5).
  • For Concord and other native cultivars, as many as 15 shoots per linear foot of canopy can be retained (4).
  • In divided canopies trellis systems, the same shoot density along each cordon should be retained (Figure 4).

In addition to the number, the position of the shoots along the cordon is important.  Ideally, the shoots retained should be equally spaced to promote a uniform, balanced canopy.

Figure 4. Proper shoot density at harvest on Gewurtztraminer vines trained on divided Scott-Henry system in Andreas, PA.

What types of shoots should you remove?

  • Weak, non-fruitful shoots especially if they grow in crowded areas of the canopy.
  • Secondary and tertiary shoots, if a primary healthy shoot has emerged.
  • Shoots arising from the trunk that are not retained for renewal wood (e., new trunks and canes or cordons).

Does shoot thinning improve fruit composition and wine sensory perception?

The associated costs with manual labor and labor shortages are reasonable considerations before implementing vineyard management practices.  This is also true for implementing shoot thinning techniques into a vineyard.  Nonetheless, it is also important to consider the potential benefits from implementing a new practice.

The effects of shoot thinning practices on hybrid varieties are a bit unclear. A previous study on shoot thinning found that shoot thinned Marechal Foch (red interspecific hybrid of Vitis) vines exhibited higher total soluble solids (ᵒBrix) and berry anthocyanin concentrations as compared to un-thinned vines (6). The increase in berry anthocyanin, however, did not translate into higher anthocyanin concentration in the final wine, and furthermore, shoot thinning did not impact the sensory perception of “fruitiness” of the wines (6). In contrast, a study focusing on Corot noir (red interspecific hybrid of Vitis) implementation of shoot thinning provided inconsistent results in grape and wine quality across a two-year (2008-2009) evaluation, which was determined by ᵒBrix, pH, titratable acidity (TA), wine anthocyanin, berry and wine tannin content (7).  Shoot thinning increased berry ᵒBrix, wine alcohol concentration and anthocyanin content only in second year of this study.  While berry TA at harvest was lower (e.g., 2008, un-thinned = 8.6 g/L, shoot thinned = 7.6 g/L), there were no differences in the TA of wine in either year (7).  Shoot thinning also decreased berry seed tannin in 2008 and berry skin and wine tannin in 2009, which could have negative implications for final wine, considering generally low tannin concentrations in hybrid red wines (7).  In an effort to compensate for costs associated with shoot thinning and yield loss, this study on Corot Noir suggested growers increase the price of grapes by 11 to 20% per ton, depending on the average annual market price and yield loss (7).

A study in Fayetteville (Arkansas) on three highly-fruitful French-American hybrid cultivars (Aurore, Chancellor, and Villard noir) found that shoot thinning increased fruit sugar accumulation (ᵒBrix) only in Chancellor and without changes in pH or TA, while a more intense juice color was associated with shoot thinned vines of both red cultivars (Chancellor and Villard noir; 8). In addition, shoot thinning favorably decreased the Ravaz index (yield to pruning weight ratio) for all three cultivars, improving vine balance (8).

The results of these studies suggest that in some situations the costs of shoot thinning may not outweigh the benefits, especially for hybrids that do not command a high market value (Finger Lakes Grape Prices 2016).  However, none of these studies account for potential reduction in disease infections, which may help justify the implementation of shoot thinning in a given vineyard.  For example, it has been found that higher shoot density may contribute to the increased incidence of Botrytis rot infections in susceptible cultivars such as Seyval Blanc (9) and Vignoles (4).

In other cases, shoot thinning improved fruit composition in Pinot Noir and Cabernet Franc for two consecutive vintages (1), and also increased color intensity, phenolic content, and total anthocyanins of Cabernet Franc berries (1). Benefits of shoot thinning on fruit quality and wine sensory perception have been reported for other vinifera cultivars, such us Barbera (10) and Sauvginon blanc (11).

Unless your vineyard is located in a low or moderate vigor site, shoot thinning is strongly recommended for vinifera cultivars growing in the Mid-Atlantic region.

If you want to assess the effects of shoot thinning on fruit composition, plan to leave half of a row of vines un-thinned and thin the remaining half to a consistent number of shoots per foot (e.g., 4 shoots per foot). Alternatively, use two rows (of the same variety and cultivar) to assess the impact of shoot thinning in your vineyard: one row thinned and the adjacent row un-thinned.  These two methods should help evaluate the effect of shoot thinning on berry composition at harvest and if possible, on wine chemistry and sensory perception assuming that the lots of berries can stay separated through wine production.

Effects of shoot thinning on vine physiology

Impacts of shoot thinning on vine physiology and performance are complex.  A study conducted in Italy evaluated the whole-canopy photosynthetic response to shoot thinning using spur-pruned Barbera vines (V. vinifera; 10). Vines were thinned to 5 shoots per foot, reducing the total shoot number by 50% as compared to un-thinned control.  In this study (10) shoot thinning significantly improved grape sugar content, color, and phenolics. Despite the benefits provided by shoot thinning on fruit composition, which has been already reported by other studies, what makes this study unique and interesting it that they investigated the mechanisms behind the improvement in grape quality through the measurement of whole-canopy net carbon assimilation.  Although the shoot-thinned vines had initially lower photosynthesis (carbon assimilation) than un-thinned vines due to the removal of photosynthetic source (leaf), they had regained photosynthetic capacity to levels similar to the un-thinned vines within 17 days of treatment.  This occurred as a result of a substantial increase in both main leaf size and amount of lateral leaves as a result of shoot thinning (10).  Therefore, individual shoots of thinned-vines had a higher supply of assimilates (e.g., sugar) per unit of crop, which can increase sugar accumulation during ripening. This may explain why shoot thinning improved grape composition in Barbera under these growing conditions.

Additional Shoot Thinning Resources

 

References Cited

  1. Reynolds AG., et al. 2005. Timing of shoot thinning in Vitis vinifera:  impacts on yield and fruit composition variables.  56, 343-356.
  2. Intrieri, C and Poni, S. Integrated evolution of trellis training systems and machines to improve grape and vintage quality of mechanized Italian vineyards.  AJEV.  46, 116-127.
  3. Fiola, J. 2017. Canopy Management – Shoot thinning and positioning. “Timely Vit” from UMD Extension.
  4. Walter-Peterson, H. 2013.  Shoot thinning:  Good for the vines, but good for the wines?  Finger Lakes Vineyard Notes.
  5. Martinson, T and Vanden Heuvel, J. Shoot density and canopy management for hybrids. CCE. http://www.fruit.cornell.edu/grape/pdfs/Canopy%20Management%20for%20Hybrids%20-2007.pdf
  6. Sun Q., et al. 2011. Impact of shoot thinning and harvest date on yield components, fruit composition, and wine quality of Marechal Foch.  AJEV. 62:1, 32-41.
  7. Sun Q., et al. 2012. Impact of shoot and cluster thinning on yield, fruit composition, and wine quality of Corot noir.  AJEV. 63:1, 49-56.
  8. Morris, JR. et al. 2004. Flower cluster and shoot thinning for crop control in French-American hybrid grapes.  AJEV. 55:4, 423-426.
  9. Reynolds, AG et al. 1986. Effect of shoot density and crop control on growth, yield, fruit composition, and wine quality of ‘Seyval blanc’.  J. Amer. Soc. Hort. Sci. 111, 55-63.
  10. Bernizzoni, F. et al. 2011. Shoot thinning effects on seasonal whole-canopy photosynthesis and vine performance in Vitis vinifera L. cv. Barbera. Aus. J. Grape Wine Res. 17, 351-357.
  11. Naor et al. 2002. Shoot and cluster thining influence vegetative growth, fruit yield, and wine quality of ‘Sauvignon blanc’ grapevines.  J. Amer. Soc. Hort. Sci. 127(4), 628-634.

 

Maria Smith is a viticulture PhD candidate with Dr. Michela Centinari in the Department of Plant Science.  She specializes in cold stress physiology of wine grapes.  She was the previous recipient of 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.  She is currently funded by the Northeast Sustainable Agriculture Research and Education (NE-SARE) program, a program from the USDA National Institute of Food and Agriculture (NIFA).

2017 Pre-Bloom Disease Management Review and Discussion

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.

 

Integrating Herbicide and Cover Crop Management for Cost Effective Results.

By: Kevin Martin, Penn State Extension Educator (Portland, NY)

We are starting to see increases in herbicide management costs.[1]  Some of you know all to well that 1-2 applications of herbicide do not provide adequate control of weed competition in vineyards.  Complicated tank mixes that cost over $100 per applied acre are not a practice I would consider sustainable.  Some growers, though, would disagree.

The cost of materials are not increasing substantially.  More frequent applications and a need to apply better materials more often is driving costs up.  The majority of herbicides used by growers are off patent these days and available almost exclusively in generic form.  A third or even fourth vineyard pass, could be sustainable.  The cost of materials and materials selected needs to be looked at comprehensively with the number of passes required to obtain adequate control.

There may be a potential for cover crops to improve the effectiveness of weed control.[2]  We can observe this not just in row middle management, but to a lesser extent under trellis management.  Cover crops do not offer the potential to reduce herbicide applications in situations where growers are applying between 1 and 3 per year.  Rather, they offer an option to improve results without adding an additional pass.  This is because cover crops can reduce vine size when row middle competition is undesirable.  In 2016 field trials we observed smaller berry size when cover crops were planted in the late summer of 2015 and were not terminated before June 1, 2016.   Particularly where hard to control species get established, some growers have added a late summer or fall application to bring their total number of herbicide application to 4-5.  In this scenario, the right cover crop mix offers the potential of superior control with one less pass.  Cover crops do require some form of termination (usually chemical).  By selecting the right species, a low rate of round-up may offer excellent row middle control.

Figure 1: Side by side cover crop trial in a commercial vineyard showing cover crop suppression of Horseweed (Marestail) pressure. Image B is the control and shows significant Horseweed (Marestail) pressure. Photos by Luke Haggerty, LERGP

Figure 1: Side by side cover crop trial in a commercial vineyard showing cover crop suppression of Horseweed (Marestail) pressure. Image B is the control and shows significant Horseweed (Marestail) pressure. Photos by Luke Haggerty, LERGP

 

Figure 2: Under vine cover crops in a commercial vineyard. Photo by Suzanne Fleishman, a previous graduate student that worked with Dr. Michela Centinari

Figure 2: Cover crops are under the vines on the left side of the image, while the under vine (or under trellis) area of the vines on the right is managed with herbicide. Photo by Suzanne Fleishman, a graduate student that works with Dr. Michela Centinari.

Cover crop mixes being trialed are similar in cost to an herbicide application.  Low-end rye grass and radish blends are comparable to many post emergent row middle applications. Higher end seed mixes with oats, more radishes or even buckwheat range between $12 and $15 per seeded acre in materials.  Legumes increase costs but potentially reduce fertilizer use.[3]  Easy to kill hybrid crimson clover complicates the economic analysis.  It may reduce urea applications by 50%, but could be more difficult to grow.  Understanding effective seed mixes, their primary benefits and potential secondary benefits will be key to the success of moving cover crops into perennially systems in a cost-effective (saving) way.  Regional differences in seed prices also complicated the matter.  One of our primary suppliers of cover crop seeds in the Lake Erie Region is Ernst Seed Co.  2016 prices were used to calculate the cost of various seed mixes used in trials.

LERGP, led by Luke Haggerty, is taking an integrated look at cover crops in Concord vineyards.  As he observes benefits, I’ll help quantify them.  There is a lot we still do not know.   While preliminary results show promise for increasing economic sustainability where herbicide program prices are spiraling upward, a few years’ worth of data will allow us to clearly observe measurable benefits in herbicide programs.  Right now, it just seems like there are less weeds and more cover crop in row middles that have been seeded.

 

References

[1] Tang, Yijia, Miguel I. Gómez, Gerald B. White. COST OF ESTABLISHMENT AND PRODUCTION OF HYBRID GRAPES IN THE FINGER LAKES REGION OF NEW YORK, 2013.  Cornell, Dec. 2014, http://publications.dyson.cornell.edu/outreach/extensionpdf/2014/Cornell-Dyson-eb1411.pdf Accessed 3 Nov. 2016.

[2] Bowman, Greg, Craig Cramer, and Christopher Shirley. Managing Cover Crops Profitability. 3rd ed.: Sustainable Agriculture Research and Education. SARE, July 2012, http://www.sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition Accessed 3 Nov. 2016. pp. 394.

[3] Id. At 122 – 124.

Resources for Identification and Management of Vineyard Pests

By: Andy Muza, Penn State Extension – Erie County

Another harvest will soon be over for grape growers in Pennsylvania and the winter season is fast approaching. Take the time this winter to explore the resources below to prepare for next season’s pest problems.

Hardcopy References
The following 5 references provide information on identification and management of insect, disease and weed problems in vineyards. I suggest purchasing these items before next season begins. Although the cost will be over $250 it is well worth having these invaluable resources in your viticultural library.

  1. New York and Pennsylvania Pest Management Guidelines for Grapes: Every commercial grape grower in Pennsylvania should have a copy of the current guidelines. This guideline provides a wealth of information on insect, disease and weed management with pesticide options, rates, and schedules, as well as, a chapter on sprayer technology.
  2. A Pocket Guide for Grape IPM Scouting of Grapes in North Central & Eastern U.S.:This pocket reference book is for use while scouting in the vineyard. The guide provides concise information and color photographs on insect/mite pests, natural enemies, diseases and disorders.
  3. Compendium of Grape Diseases, Disorders, and Pests, Second Edition: This new edition is an expanded version of the original Compendium with 375 photos and drawings and containing updated information about pathogens including additional diseases. The second edition is divided into 4 parts covering: diseases caused by biotic factors (e.g., fungi, bacteria, viruses etc.); disease – like symptoms caused by insects and mites; disorders caused by abiotic factors (e.g., environmental stresses, nutritional disorders, etc.); and fungicides/spray technology.
  4. Weeds of the Northeast: Described as the first comprehensive weed identification manual available for the Northeast enabling identification of almost 300 common and economically important weeds in the region. The manual contains color photos of vegetative and flowering stages of weeds, as well as, seed photos.
  5. Wine Grape Production Guide for Eastern North America: A comprehensive reference on all aspects of wine grape production (e.g., varieties, canopy management, nutrient management, etc.) including chapters on disease management, insect and mite pests and vineyard weed management.
Important viticulture resources for vineyard managers in the Mid-Atlantic region. Photo provided by: Andy Muza

Important viticulture resources for vineyard managers in the Mid-Atlantic region.

Insect and Disease Resources – 2016 articles

Articles from the 2016 season that should be reviewed include:

GRAPE DISEASE CONTROL, 2016 by Wayne F. Wilcox, Cornell University (74 pages). Dr. Wilcox provides comprehensive coverage of relative research and disease management options.

Grape Insect and Mite Pests – 2016 Field Season by Greg Loeb, Cornell University (21 pages). Dr. Loeb provides a thorough review of insect pests that you might see throughout the season in the vineyard. Included are 18 photos of pests/injury along with management guidelines.

Insect and Disease Resources – Web sites

IPM –Grapes (Cornell): Information is available on diseases, insect and mites, weeds, wildlife, organic IPM, spray technology and pesticides.

NYS IPM : Fruit IPM Fact Sheets (Cornell): Fact sheets on diseases and insects on grapes, tree fruit and small fruit. A total of 22 fact sheets pertain to insects and diseases on grapes.

Identifying Grape Insects (Michigan State University):  The information on this site is from the previously mentioned resource, A Pocket Guide for Grape IPM Scouting of Grapes in North Central & Eastern U.S. and is categorized by: Pests attacking; buds, leaves, fruit, root, during harvest. Also includes beneficial insects and mites.

Mid Atlantic Vineyards Grape IPM (Virginia Tech): Insect fact sheets categorized by: direct pests – fruit; indirect pests – leaves; trunk and cane feeders; and root feeders.

Ontario Grape IPM: This site provides information on a variety of topics including: insects and mites; diseases and disorders; weeds; herbicide injury; identification keys; etc.

Growing Grapes – Vineyard IPM (eXtension): Articles both in English and Spanish on: insects, diseases, weeds, animal pests and problems not caused by insects or diseases.

Weed Resources – Web sites
New Jersey Weed Gallery (Rutgers): Photos and descriptions of weeds found in New Jersey. Weeds can be viewed by common name, Latin name or thumbnail images.

Weed Identification Guide (Virginia Tech): These pages are intended to aide in the identification of common weeds and weed seedlings found throughout Virginia and the Southeastern U.S. The weed pictures are arranged alphabetically by common name.

UMass Extension Weed Herbarium (University of Massachusetts): Identification notes and color photos of over 500 weeds.

UC-IPM Weed Photo Gallery (University of California): Common names link to pages with weed descriptions and photos often showing several stages of development.

 

Late summer/early fall grape disease control; 2016

By: Bryan Hed

We’re in the final leg of the season and it’s time to size up our remaining challenges through the ripening period.  Fruit are no longer susceptible to many of the major diseases like powdery and downy mildew and black rot that can cause crop loss during earlier stages of berry development. But for some grape varieties, particularly wine grapes that produce compact clusters, there is another major hurdle to work through to harvest; late season bunch/sour rot. I am referring to the rotting of fruit in clusters that occurs during the later stages of the ripening period, just a few heartbreaking days or weeks before harvest. Bunch rot can involve the colonization of fruit by a number of different microorganisms, both fungi and bacteria. But the main culprit in most regions of the Northeastern U.S. is the fungus, Botrytis cinerea (Figure 1). Fortunately, we have a number of chemical control options that are quite effective against this fungus that I have listed below. I have organized them according to the FRAC (Fungicide Resistance Action Committee) group that each product belongs to. Basically FRAC groups are fungicide chemistries with the same or similar mode of action, so that pathogen resistance to one fungicide is going to confer cross resistance to another, within that same FRAC group. For example, notice that Vangard and Scala are in the same FRAC group; 9. This means that if a population of Botrytis in a vineyard has developed resistance to the active ingredient in Vangard, then it will also be resistant to the active ingredient in Scala, even though the active ingredients may be different (cyprodinil in Vangard and pyrimethanil in Scala).  The mode of action (the way in which the fungicide disrupts a specific metabolic pathway in the fungus, killing it) of these two chemistries is the same, or similar enough that pathogen resistance to one chemistry will confer resistance to the other.

  1. FRAC group 2: Rovral, 7 day pre-harvest interval
  2. FRAC group 7: Endura, 14 day pre-harvest interval
  3. FRAC group 7 (and 3, which is not for Botrytis): Luna Experience, 14 day pre-harvest interval
  4. FRAC group 7 and 11: Pristine, 14 day pre-harvest interval
  5. FRAC group 9: Vangard, Scala, 7 day pre-harvest interval
  6. FRAC group 9 (and 3, which is not for Botrytis): Inspire Super, 14 day pre-harvest interval
  7. FRAC group 9 and 12: Switch, 7 day pre-harvest interval
  8. FRAC group 11: Flint, 14 day pre-harvest interval
  9. FRAC group 17: Elevate, 0 day pre-harvest interval

No doubt many wine grape growers have already applied a bloom, pre-bunch closure, and veraison spray to bunch rot susceptible varieties. However, one or more applications may be necessary in some vineyards. Populations of the Botrytis fungus are quite adept at developing resistance to these fungicides; be mindful to rotate FRAC groups and limit the application of any one FRAC group to one or two per season to delay the development of that resistance. If you have to use a FRAC group more than once per season, it would be better to compose one of those two applications with a material that contains a second FRAC group for Botrytis. For example, if you already used Scala, it would probably be better to apply Switch (after you’ve already rotated to FRAC group 2, 7, 11, or 17) than to apply Vangard or another Scala spray.  Most of these materials are considered ‘high risk’ for resistance, so rotation is extremely important to maintaining the effectiveness of these products.  Also, pay attention to pre-harvest intervals which range from 0 to 14 days. That said, you can’t spray your way completely out of the damage that Botrytis and other microorganisms can cause; consistently effective bunch rot control programs must be integrated with a generous dose of cultural practices like fruit zone leaf removal, sanitation, canopy management, and vine balance. And, unfortunately, these chemistries listed above are specific for Botrytis and will not control many of the other microorganisms that may make up the bunch rot complex or that lead to the dreaded sour rot complex.

Figure 1. Botrytis cinerea sporulating on damaged grapes of Vitis interspecific hybrid ‘Vignoles’. Such damage often occurs as a result of berry overcrowding in overly compact clusters. The damage leaves fruit open to colonization by the ever present Botrytis fungus and by many other fruit rot organisms.

Figure 1. Botrytis cinerea sporulating on damaged grapes of Vitis interspecific hybrid ‘Vignoles’. Such damage often occurs as a result of berry overcrowding in overly compact clusters. The damage leaves fruit open to colonization by the ever present Botrytis fungus and by many other fruit rot organisms.

I’ve already alluded to one of the major predisposing factors for bunch rot (including sour rot) in grape clusters, and that is cluster compactness. The compactness of clusters is responsible not only for initiating much of the fruit rot that occurs in clusters, but perhaps more importantly, for the rapid spread of rots throughout the cluster (Figure 2). Rots can be initiated in loose grape clusters as well (by bird or insect damage, for example), but generally do not spread beyond the damaged berry or berries. However, in compact clusters, a single damaged berry can spread rot to large sections of the cluster by virtue of the close contact between those berries. Contact between berries in compact clusters also reduces cuticle thickness, an important barrier to rot pathogens, and reduces pesticide penetration into clusters for protection of berry surfaces against Botrytis and damage by insects. Cluster compactness also increases the effects of retained bloom trash (dead flower parts) inside clusters that can provide a substrate for Botrytis, increasing fruit rot by harvest. Taken together, this generally makes berries in compact clusters much more susceptible to invasion by fruit rot pathogens than berries in loose clusters.

A series of greenhouse experiments we conducted years ago also suggested that latent (dormant) infections of Botrytis can be activated by the kind of berry injury that occurs in compact clusters.  Latent Botrytis infections are infections that occur during bloom and the early fruit development period for which you apply that bloom and pre-closure spray. Years ago, we monitored the incidence of latent infections in our block of Vignoles and found that even though the incidence appeared to increase throughout the berry development period, most of these infections did not lead to fruit rot by harvest. In fact, when we inoculated clusters of potted, greenhouse grown Chardonnay vines with Botrytis shortly after bloom, generating high levels of latent infection in berries, the berries did not rot during ripening if they remained intact in the greenhouse, unexposed to weather, birds, insects, or compactness (the clusters were thinned after inoculation and thinned berries were used to determine latent infection levels). However, when we surface sterilized the berries (to eliminate any Botrytis on the outside of berries) and created small injuries at the berry/pedicel interface of ripe berries (the kind of injury that commonly occurs in overcrowded clusters) the vast majority of the inoculated berries quickly rotted compared to berries that were not inoculated with Botrytis (checks).

By loosening clusters, damage from berry overcrowding can be minimized and bunch/sour rot development can be greatly alleviated. Unfortunately, loosening clusters in a consistently effective AND cost effective way is not always an easy thing to accomplish. Over the years we have examined a number of potential methods for cluster loosening with varying levels of success. Treatments such as pre-bloom fruit zone leaf removal have provided the most consistently significant reductions in cluster compactness and fruit rots in most years. The pre-bloom timing of fruit zone leaf removal simply combines the benefits of an open, sun lit fruit zone (which has been well documented by many investigators over the past several decades) with a reduction in cluster compactness and rot susceptibility. In our experiments, this treatment has typically been applied by hand, but the technology exists to mechanically remove leaf tissue around inflorescences (pre-bloom) without serious damage to them, and trials are being conducted to evaluate the mechanization of the pre-bloom leaf removal on a number of grape varieties. So far, results have been mixed depending on variety and trellis training system. In vineyards where we were able to compare pre-bloom mechanized leaf removal with pre-bloom leaf removal by hand and post-bloom mechanized leaf removal, the effects of pre-bloom mechanized leaf removal (increased light exposure of clusters, looser clusters, less rot, yield reduction) generally fell somewhere between the two latter treatments. The hope of this research is to expose growers to some new possibilities for fruit rot control and increase the potential for its adaptation to commercial vineyards and adoption by growers. We’ve examined other technologies with potential for cluster loosening and improved fruit rot control, but unfortunately their adoption is more problematic.  For example, we have found that inexpensive gibberellin sprays around bloom have also been effective at loosening clusters and enhancing rot control on Vignoles and Chardonnay with little or no serious negative side effects. But they are currently ‘off label’ and are very unlikely to ever become legal applications in the United States. Also, the effects of gibberellin sprays are variety specific and therefore must be examined and defined for each variety: in our experience, low rates (5-20 ppm) can have serious negative side effects on Vitis vinifera Riesling, whereas rates as high as 100 ppm have had little or no effect on Vitis interspecific hybrid ‘Chancellor’.

Figure 2. Botrytis bunch rot. The compactness of these bunches has contributed to rapid and severe rotting of large portions of these clusters (left). Loose clusters of the same variety are far less affected by the spread of rot within the bunch (right).

Figure 2. Botrytis bunch rot. The compactness of these bunches has contributed to rapid and severe rotting of large portions of these clusters (left). Loose clusters of the same variety are far less affected by the spread of rot within the bunch (right).

More recently, work conducted by Megan Hall, a grad student of Wayne Wilcox at Cornell University, has shown that additional pesticide applications during the latter stages of ripening can significantly reduce the development of sour rot. Her work has shown a close connection between fruit flies and sour rot development; the presence of the flies is important to the accumulation/generation of acetic acid in rotting fruit. Treatments composed of weekly, tank mix applications of an insecticide (to control the flies) and an antimicrobial (to kill bacteria) have been found to reduce sour rots by 50-80% over unsprayed vines. So far, the best results appear to occur when weekly sprays are initiated before sour rot symptoms are observed (preventive sprays before about 15 brix). This exciting work should provide yet another effective option for sour rot control in the wet, humid parts of the eastern U.S. and we are looking forward to hearing more about this rot control option in the near future.

LATE SEASON LEAF DISEASE CONTROL

Beyond the management of bunch rot on susceptible wine varieties, there is also the matter of keeping canopies (leaves) as clean and functional as possible, for as long as possible. Diseases like powdery and downy mildew can continue to be of concern into late summer and early fall, especially for growers of Vitis vinifera. The mildews can greatly reduce leaf function if allowed to spiral out of control.  The ability of the canopy to continue to photosynthesize is crucial to the ripening of the crop and canes and the storage of sugars (starch) in trunks, arms, and roots, which relates to winter hardiness.  The winters of 2014 and 2015 are harsh reminders of just how important this can be. Allowing grapevines to go into winter dormancy with less than optimal preparation can leave them more susceptible to damage by severe cold and another plague of crown gall to have to deal with for years to come.

Good control of powdery mildew up to about Labor Day can also go a long way to reducing overwintering inoculum and disease pressure the following spring. This finding was the result of some excellent research conducted by Wayne Wilcox, Dave Gadoury and graduate students at Cornell University. When powdery mildew infected leaves die by that first hard frost in fall, the mildew on those leaves stops developing and also dies…unless it has had time to form fully mature, winter resistant resting structures called chasmothecia. If the chasmothecia in a powdery mildew colony do not have time to fully mature before the grape tissue dies (as from infections that were roughly initiated after early September), they will not survive the dormant period (winter) and will not contribute to the bank of primary inoculum that infection periods draw upon the following spring.  Knowing this, a grower can get a better handle on the ‘size’ of the powdery mildew problems he/she will potentially face next spring. If, for example, you had heavy mildew development earlier in this season (on clusters and/or leaves), expect to have to deal with powdery mildew early next season and take appropriate action during early shoot growth stages with preventive fungicide sprays. This is particularly important if you are growing Vitis vinifera and much less important for growers of native varieties like Concord and Niagara.

Downy mildew appears to be much less a widespread problem this year. In fact, in our ‘neck of the woods’ along the southern shore of Lake Erie, droughty conditions have prevailed throughout most of the season, and only now are we even beginning to see a few downy mildew infections on leaves close to the ground. At this point in the season regular scouting for this disease is the first line of defense, and in areas that remain relatively dry, perhaps the only control measure needed (?). However, in areas where the disease has remained active throughout the season, be vigilant about keeping it under tight control. Late season epidemics of this disease can quickly strip susceptible wine varieties of their leaves, effectively bringing an early halt to ripening.

For further reading on this and many other disease management topics, refer to the 2016 New York and Pennsylvania Pest Management Guidelines for Grapes. If you don’t have a copy, you can get one through Cornell University press. Every commercial grape production operation should have one!

2016 Post Bloom Disease Management Review

By: Bryan Hed

Once again, we’ve arrived at that part of the season just beyond the immediate pre bloom and first post bloom spray. For many years now, research has shown that those two sprays are absolutely essential to a fruit disease management program, at least for control of the four major grape diseases (powdery and downy mildew, black rot, and Phomopsis). We have always emphasized the use of ‘best’ materials, shortened intervals, best coverage, etc., for those two sprays, EVERY YEAR!…a no brainer. No matter what varieties you grow, those two sprays are most often the most important for protection of your crop.

Fortunately for some of these diseases, fruit susceptibility is short lived and most control of fruit diseases like powdery mildew is achieved by management right around/shortly after bloom. Indeed, work conducted by Wayne Wilcox and his grad students over the years has shown that fruit are generally susceptible to powdery and downy mildew for only about 2-3 weeks after capfall. Varieties of Vitis vinifera tend to be susceptible a little longer than native varieties like ‘Concord’, but for all varieties, the period of time during the first 2-3 weeks of fruit development is the most critical for fruit protection. Whenever I am approached with questions from growers as to why they ended up with a boatload of fruit disease in a given year, the answer almost invariably lies within the spray program during early fruit development.

Beyond that, things tend to get a little more complicated in terms of ‘what do I spray now?’ It depends on a number of things like the weather (past, present, and future), the variety grown/susceptibility of the host, your overwintering inoculum load (how much disease you had last year and the amount of old wood and debris in your trellis system this year) and your current disease levels. If you’re growing Concord grapes in the Lake Erie region in 2016, where rainfall during June has been scarce and sunshine and low humidity have dominated, diseases have been relatively easy to control so far. For example, there have been but four brief downy mildew (Figure 2) infection periods to date. As a result, this disease simply cannot be found in most maintained vineyards in the Lake Erie region, despite an abundance of downy mildew overwintering inoculum from the widespread occurrence of this disease last year. And, there have been just two mild black rot (Figure 1) infection periods since bud break. The immediate pre bloom and first post bloom spray probably provided all that was needed for control of powdery and downy fruit infections. The threat of black rot fruit infection remains (Concord is susceptible to this disease for about 4-6 weeks after capfall; V. vinifera about a week or two longer), though in vineyards that did not have black rot problems last year and where current disease is almost non-existent it is unlikely that black rot will spiral out of control at this point unless the current weather pattern suddenly turns very wet. Regular scouting of your vineyard will reveal whether or not this disease has gotten started in your vineyard (at this point in the season, it takes about 14 days for symptoms of black rot fruit infections to manifest themselves after an infection period).

Figure 1. Black rot fruit (left) and leaf (right) infections. Note the one mummified berry at the top of the cluster in the picture on the left. It was likely the source of spores for infections on several other berries of the same cluster just below it.

Figure 1. Black rot fruit (left) and leaf (right) infections. Note the one mummified berry at the top of the cluster in the picture on the left. It was likely the source of spores for infections on several other berries of the same cluster just below it. Photos By: Bryan Hed

 

 

Figure 2. Downy mildew on pea-sized Chancellor fruit (left) and mature Concord leaves (right).

Figure 2. Downy mildew on pea-sized Chancellor fruit (left) and mature Concord leaves (right).  Photos By: Bryan Hed

The threat of Phomopsis infections depends to a large degree on how much overwintering inoculum is available. Since current season Phomopsis infections (Figure 3) generally do not produce spores until the following seasons (unlike the other fungal diseases we deal with each year), the development of this disease is dependent on overwintering inoculum sources that are normally ‘milked out’ by seasonal rainfall from May through mid-July. That means that in an average rainfall year, there are few spores left to cause infections by mid-July, even though fruit of many varieties do not appear to lose their susceptibility to Phomopsis (research by Mike Ellis and his students at Ohio State University). If spore sources are not being depleted in regions that have experienced a dry spring this year (like the Lake Erie region), enough inoculum may still be available in overwintering sources to cause fruit infections (Figure 3) past the mid-July period, should conditions turn wet. As one would expect, this is more of a concern for vineyards with a previous history of this disease. In addition, vineyards trained to trellis systems that retain lots of older and/or dead wood (cordons as opposed to canes, machine pruning as opposed to hand pruning) and/or vineyards that have not been receiving early shoot (3-6” shoots) sprays in previous years, will be more at risk of retaining significant amounts of overwintering inoculum of Phomopsis past the mid-July period during years with dry springs.

Figure 3: Heavy early Phomopsis infections on shoots and leaves (left) of Concord grape. Fruit infection of Niagara grape (right) that manifests itself during the ripening period.

Figure 3: Heavy early Phomopsis infections on shoots and leaves (left) of Concord grape. Fruit infection of Niagara grape (right) that manifests itself during the ripening period. Photos By: Bryan Hed

For Lake Erie region juice grape growers, powdery mildew remains in spite of the dry weather. Recall that powdery mildew primary infection periods require rainfall of at least 0.1” (and temperatures above 50 F). However, once primary infections have occurred, the disease can proceed to build without rainfall from spores produced by those primary infections. I suspect the dry, sunny weather will keep disease development moving at a slower than average pace (direct sunlight kills powdery mildew), but the disease will continue to build as it always does. Juice grape vineyards with low to average size crops may require little beyond the first or second post bloom spray for mildew. But keep in mind that we’ve a long way to go and if cloudy, humid conditions become entrenched, it can speed epidemic development. Also, poor ripening conditions after veraison can greatly reduce a mildewed canopy’s ability to ripen a crop, especially a large crop.

For wine grape growers, wherever you are in PA, it’s a ‘given’ that protection against all the major diseases should continue well past the first post bloom spray, for fruit and for leaves. As mentioned earlier, fruit are still susceptible to black rot and Phomopsis, and if you’re in an area experiencing at least some rainfall this year, downy mildew is definitely a continuing threat. As detailed above, the threat of powdery mildew goes without saying and every day is a powdery mildew infection period. So, for the second post bloom spray on wine grapes, include active ingredients for control of all diseases.

For continuing summer sprays, pay close attention to chemical classes for resistance management and always rotate modes of action. The loss of a mode of action (like the QOIs (strobies)) is a big deal to wine grape growers who have to apply many sprays within a given season for control of diseases like powdery and downy mildew. Rotation and resistance management should be an important component of your summer spray program. You’ve applied your best materials around bloom, now it’s time to rotate to other modes of action. Fortunately, we have lots of effective options for powdery and downy mildew control; use as many of them as you can, never applying consecutive sprays of anything (except the old standards like copper, sulfur, mancozeb products, ziram, captan). In Pennsylvania, we have many effective modes of action for powdery mildew like those found in Vivando, Torino, Quintec, the difenoconazole products (the newest, most powerful sterol inhibitor), and Luna Experience, Endura, and now Aprovia (the succinate dehydrogenase inhibitors). And of course, there’s always sulfur, but beware its use on red hybrids. I should mention that Aprovia is also labeled for black rot control. However, our recent tests have indicated that Aprovia’s black rot efficacy may be limited and that further testing is needed to better define this activity before it can be recommended for control of this disease.

For downy mildew we have products like Revus, Presidio, Ranman, Zampro, the old standards (copper, mancozeb products, ziram, captan), and the phosphorous acid products. Unfortunately in parts of PA, the powdery and downy mildew pathogens have developed resistance to the strobilurins, and they may not be reliable choices any longer. Also, the active ingredient in the product known as Reason, has the same mode of action against downy mildew as the strobilurins, and for resistance management purposes, Reason should be considered the ‘same’ as the strobilurins. One last thing: if you use the phosphorous acid products for downy mildew control, keep in mind that although they are extremely rain-fast, do not expect them to provide more than 10 days of protection against this disease, especially under high disease pressure.

For wine grape growers in more southerly regions of PA that have been receiving regular or heavy rainfall, Protection against all diseases obviously needs to continue. Once past the fruit protection period (which may be up to 6-7 weeks past capfall for black rot on V. vinifera), leaves of V. vinifera and some of the more sensitive hybrids will need continued protection from powdery mildew up to veraison or longer. As long as conditions remain wet, downy mildew will also remain a threat deep into the season. A clean canopy is essential for maximum ripeness and fruit/wine quality, maximum winter hardiness (recalling the cruel winters of 2014 and 2015), and minimal overwintering inoculum. For late season powdery mildew control, alternative materials may gradually be used to replace the synthetics and sulfur (particularly for reds where late sulfur applications can create wine quality issues). Avoid oils around/after veraison for powdery mildew control to avoid reducing photosynthesis. I have heard good things about potassium salt use (potassium bicarbonates and nutrol) from colleagues in Ontario, to maintain clean canopies late into the season. There are other alternatives currently available for late powdery mildew control, but for many their efficacy, especially on V. vinifera, is modest at best.

Late Summer sprays are for leaf protection, especially for varieties of V. vinifera. Sprays at this time primarily target powdery mildew, but may also include downy mildew if disease has gotten a foothold in the vineyard and conditions remain wet into fall. Regular scouting and strict attention to weather conditions at this time are very beneficial to making prudent late season spray decisions. For downy mildew, rainfall and leaf wetness is critical for epidemic development and dry late summer periods can sometimes offer relief from this disease. However, beware of heavy over-night dews which can continue to fuel downy mildew infections and sporulation without rainfall and keep the ‘fire’ alive on leaves at a slow burn. Early defoliation by downy mildew will effectively terminate the fruit and cane ripening process and leave vines weakened going into winter.

For bunch rot control, wine grape growers of bunch rot susceptible varieties may have already applied a Botrytis specific fungicide at full bloom.  This is because Botrytis infections of the inflorescences can occur during bloom under wet conditions. These infections usually remain dormant and do not result in active rot…until after veraison, when injury to berries or high humidity, or some other factor (research has not completely determined all the factors involved) may lead to activation of a percentage of these infections and cause clusters to rot.

The next Botrytis fungicide application is commonly applied at just before closure of the clusters (soon). In varieties with very compact clusters, this application may be extremely important as it represents your last opportunity to get fungicides into the interior surfaces of clusters. This spray may also help to reduce latent infections that research has shown can continue to accumulate throughout the berry development period. It may also be an opportunity to ‘blow out’ bloom trash (dead cap and stamen tissue that got stuck in the clusters after bloom) from the insides of clusters. Bloom trash can provide substrate for fungi like Botrytis and serve as a focal point for bunch rots to develop later in the season, from inside clusters. The compactness of clusters plays an important role in not only the retention of bloom trash (the tighter the cluster, the more bloom trash retained), but also the effect of retained bloom trash on cluster rot; as compactness increases, the enhancement of bunch rot by retained bloom trash increases.

Another bunch rot control measure is leaf removal around clusters. Most often applied shortly after fruit set, fruit zone leaf removal exposes fruit to better air, sunlight and pesticide penetration which can improve control of ALL fungal diseases. This practice is most commonly applied to varieties of Vitis vinifera that produce tight clusters, but it is an expensive operation to add to your production costs and is most cost effectively applied by machine (machinery costs aside). It can be mechanized most effectively if vines are trained to a VSP or some other two dimensional trellis system with a relatively focused and narrow cluster zone (Figure 4A and B).

Figure 4A (top) and B (bottom). Canopy of VSP trained Riesling before (top) and after (bottom) mechanized leaf removal utilizing air-pulse technology. Note the dramatic increase in exposure of inflorescences after leaf removal, with little or no damage to inflorescences.

Figure 4A (top) and B (bottom). Canopy of VSP trained Riesling before (top) and after (bottom) mechanized leaf removal utilizing air-pulse technology. Note the dramatic increase in exposure of inflorescences after leaf removal, with little or no damage to inflorescences.  Photos By: Bryan Hed

Research generally indicates that the earlier this practice is applied, the larger the effects for bunch rot control. For example, when applied at trace bloom (first flowers opening), it tends to reduce fruit set in addition to exposing clusters. This can be beneficial for varieties that naturally produce compact clusters (Pinot Noir, Pinot Gris, Chardonnay, Vignoles, Riesling) that are very susceptible to rot during ripening (Figure 5). Clusters that are looser (as a result of reduced fruit set) are easier to penetrate with pesticides, and are less apt to become damaged by overcrowding of berries before harvest. However, the potential for yield reduction may make the trace bloom timing unnecessary or undesirable on varieties that do not suffer from compactness/high susceptibility to late season rots. One note of caution: in more southerly climates, some growers remove leaves only on the east (north-south running rows) or north (on east-west rows) side of the trellis to avoid sun damage to fruit in late summer.

Fruit zone leaf removal can also reduce bloom trash retained in clusters: when comparing clusters of vines treated with and without leaf removal, we noted a significant reduction in bloom trash where leaves were removed, regardless of timing or method (by hand or machine). This can be particularly effective if utilizing air-pulse technology to remove leaves. This type of leaf removal mechanization applies high speed pulses of air to shatter leaves in the cluster zone, while blowing bloom trash from clusters.

The next fungicide application for Botrytis is made just before or at veraison. As fruit begin to soften and skins become thinner and more ‘breachable’ by fungal pathogens like Botrytis, an application at this time, to rot prone varieties, is a good way to stave off bunch rot development as fruit become more susceptible and more likely to become injured by birds, insects, excess moisture/humidity, and overcrowding of berries in tight clusters. Botrytis fungicides can protect intact fruit surfaces and may help to reduce the spread of Botrytis rot on fruit, even after they have become injured. Lastly, an application about 2-3 weeks after veraison, especially under wet weather conditions, can reduce further rot development during the last stretch of ripening. Keep in mind that Botrytis fungicides control Botrytis, and will not provide protection against sour rot organisms that often destroy fruit of overly compact clusters, despite the application of a full Botrytis fungicide program.

Figure 5. Botrytis bunch rot developing very aggressively on compact Vignoles grape clusters.

Figure 5. Botrytis bunch rot developing very aggressively on compact Vignoles grape clusters. Photo By: Bryan Hed

Don’t forget that there is abundant information available in the 2016 New York and Pennsylvania Pest Management Guidelines for Grapes. This is one of the very best guides for grape growers in NY and PA (and the Northeastern U.S. in general). It represents the compilation of many years of excellent grape research and includes the most recent updates on pesticide use and disease and insect pest control. If you don’t have a copy, get one through Cornell University press. Every commercial grape production operation should have one! At about half the cost of a single pesticide spray, it is well worth it.