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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 which contains much of the information discussed in this blog.



2017 New York and Pennsylvania Pest Management Guidelines for Grapes. Edited by Tim Weigle and Andy Muza. Cornell and Penn State University Cooperative Extension.

Hoffman, L.E., W.F. Wilcox, D.M. Gadoury and R.C. Seem. 2002. Influence of grape berry age and susceptibility to Guignardia bidwellii and its incubation period length. Phytopathology 92:1068-1076.

Hoffman, L.E., W.F. Wilcox, D.M. Gadoury, R.C. Seem, and D.G. Riegel. 2004. Integrated control of grape black rot: Influence of host phenology, inoculum availability, sanitation, and spray timing. Phytopathology 94: 641-650.

Early season grapevine canopy management, Part II: Early leaf removal (ELR)

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

In the previous post, we discussed shoot thinning as a method to achieve vine balance and improve the canopy microclimate (Part I: Shoot Thinning). In this post, we will discuss the use early leaf removal (ELR), a canopy management practice implemented around bloom.  ELR primarily serves to reduce the severity of Botrytis bunch rot infection in susceptible varieties (Wines and Vines:  Benefits and Costs of Early Leaf Removal), but may also be an effective practice for reducing crop yield.

ELR is currently considered an experimental canopy management practice for vineyards.  While it shows great promise within the research and Extension literature (1, 2, Cornell Cooperative Extension 2016), Penn State Extension does not currently recommend implementing ELR as a replacement for traditional methods (i.e., cluster thinning, fungicide sprays) for yield and rot control. However, growers curious about the effects of ELR may find it useful as a supplementary canopy management practice, especially for disease management and crop reduction.

Throughout this post, we will discuss the effects of ELR on:

  • Crop level in highly-fruitful varieties that produce a high number of clusters (3-4 per shoot) or large clusters such as vinifera cvs. Grüner Veltliner, Sangiovese, and Barbera.
  • Botrytis bunch rot infection.
  • Fruit and wine composition.

What is Early Leaf Removal (ELR) and how does it work?

ELR is the removal of basal leaves of the main shoots and, optionally, lateral shoots developed from the basal nodes (; Figure 1).

Screenshot 2017-06-01 12.31.36

ELR is typically performed shortly before (pre-bloom) or at the beginning of bloom (trace-bloom; Figure 2A). In some cases, however, it has been performed later during full-bloom or at the onset of fruit-set (Figure 2B).

Screenshot 2017-06-01 12.31.48

Before and during bloom, the oldest basal leaves have a major role in providing carbohydrates (e.g., sugars) to support the growing shoot and inflorescence (i.e., flower clusters). In contrast, young leaves on the middle and top part of the shoot are still developing and not very photosynthetically ‘active’ at this time (3).  Literature suggests the removal of basal leaves at bloom may starve the inflorescence for a carbohydrates food source (4).  The lack of carbohydrate resources reduces fruit-set (i.e., the percentage of flowers that will develop into berries), which likely reduces the number of berries per cluster at harvest (5). When ELR is performed later, at the onset of fruit-set, removing basal leaves may induce a reduction in berry size and an increase in berry abscission due to carbohydrate limitation at the onset of fruit development (6). Therefore, yield reduction achieved with ELR is the result of reduced cluster weight (reduced number of berries per cluster and/or reduced berry weight). In contrast, yield reduction achieved by cluster thinning is the result of a reduced number of clusters per vine.

Why are ELR practices currently under research investigation?

An increased number of studies is investigating the use of ELR as a potential alternative to cluster thinning techniques used for crop yield control in highly-fruitful wine grape varieties (5, 6, 7). As opposed to traditional cluster thinning, ELR can be more easily mechanized. (Author’s note:  for more information on mechanization, see Additional Resources at the bottom of the post.) ELR may additionally confer benefits such as:

  1. Reduced severity of Botrytis rot infection

Cluster compactness, or the tightness of berries on the cluster, has been positively related to the severity of Botrytis bunch rot infections (8). It is suggested that more compact clusters experience more rot. ELR decreases cluster compactness by reducing the number of berries per cluster and/or the berry size. Decreased cluster compactness through implementing ELR has reduced Botrytis rot infections in several tight-cluster varieties such as Pinot Noir, Riesling, Chardonnay, and Vignoles (1, 9, 10, 14). As an additional benefit, the removal of basal leaves increases sunlight penetration and air movement in the fruiting zone, which is important for improving spray penetration within the canopy (2016 Post Bloom Disease Management Review).

  1. Improved fruit and wine composition

ELR has consistently been reported to alter fruit composition, particularly for red Vitis vinifera varieties in Mediterranean climates (Tempranillo, Sangiovese, Barbera, etc.; 2, 5, 6, 12). In several instances, fruit harvested from ELR vines had higher levels of total soluble solids (TSS, °Brix), phenolic compounds (e.g., flavonols), and total anthocyanins compared to un-defoliated vines (2, 11, 12). ELR can also reduce methoxypyrazines, ‘herbaceous’ aromas found in higher concentrations among immature grapes at harvest, and may contribute to improved wine color intensity (13).  ELR may alter three important parameters associated with berry development and ripening (2):

  • Decreased berry size – Smaller berries tend to have greater skin-to-pulp ratio and higher concentrations of desirable phenolic and aroma compounds which are mainly present in the skin.
  • Increased leaf area-to-yield ratio on a per shoot basis – A greater leaf area-to-yield ratio may translate into higher sugar produced per shoot. More sugar availability could contribute to better fruit ripening.
  • Improved canopy microclimate – ELR, like traditional leaf removal, improves the microclimate of the fruiting zone through decreased leaf density and increased sunlight penetration to the fruit. Higher temperatures coupled with increased sunlight exposure in the fruiting zone can be especially important under cool or cloudy ripening conditions, as they may accelerate berry ripening, resulting in higher TSS, decreased malic acid, increased anthocyanin concentration, and degradation of green volatile aroma compounds such as methoxypyrazines that may mask fruity or floral aromas. Higher ultraviolet (UV) radiation in the fruiting zone in response to increased sunlight penetration may increase production of flavonols, as flavonols biologically act to protect berries from UV exposure (3, 11). Flavonol compounds along with anthocyanin influence red wine color and are used as determinants of quality in fruit (11).

It is important to keep in mind that yield reduction is not desirable in all grape varieties. The use of ELR with varieties that do not typically over-crop may result in under-cropped situations with potential negative effects on fruit quality and vine health, in addition to unnecessary yield reductions and thus revenue loss.

How many leaves should be removed to induce yield reduction?

Unfortunately, there is no “one size fits all” number of leaves to remove when implementing ELR as a vineyard management practice.  The required number of leaves removed to significantly reduce yield through reduced fruit-set depends on several factors, including shoot length and the shoot leaf area at the time of removal. For example, by pulling 5 basal leaves on a shoot with only 8 leaves at trace-bloom, we would remove about 63% of the total number of leaves.  The percentage of leaf area removed would be even higher as the remaining leaves at the top of the shoot are much smaller than those removed from the bottom of the shoot. In contrast, a longer shoot with 15 leaves total will only lose 33% of the leaf area when 5 basal leaves are pulled. Thus, removing 5 leaves from a short shoot would have a more severe effect of depriving the inflorescence of sugar resources than removing the same number of leaves on long shoots (Figure 3).

Screenshot 2017-06-01 12.32.01

Sometimes the degree of ELR is severe in order to induce a yield reduction commensurate with the more traditional cluster thinning technique. For example, Pinot Noir grown in southwestern Michigan showed a reduction in yield from 6.1 tons per acre in non-defoliated vines to 3.6 tons per acre when about half (8 out of 15) of the leaves on the shoots were removed (1). This was a 40% reduction in yield. Comparatively, when 4 or 6 leaves were removed from the Pinot Noir, no significant effect was found in crop yield (1).

With the high potential for crop yield reduction, Dr. Michela Centinari’s lab has been experimenting with ELR for the past two years. We have been examining the effects of ELR at trace-bloom on Grüner Veltliner (V. vinifera) grown in Central Pennsylvania. Grüner Veltliner is highly fruitful, typically producing 2-3 large clusters per shoot. In our experimental practices, we removed 5 basal leaves at trace-bloom. Our objective was to compare the use of ELR to cluster thinning for crop yield reduction. Our first year of data found that the implementation of ELR decreased yields by only about 15% (10.7 tons per acre in the non-defoliated control to 9.3 tons per acre in defoliated vines). In comparison, vines thinned to 1 cluster per shoot had a 45-50% reduction in yield compared to the un-thinned control (10.7 tons per acre to 6.5 tons per acre).

This suggests that a greater leaf removal intensity may be needed for this variety to produce yield reduction comparable to cluster thinning, and we are currently testing different intensity levels of trace-bloom ELR to evaluate if the amount of leaf area removed correlates with reduction of fruit-set and yield at harvest.

Again, ELR is still considered an experimental canopy management technique. For those growers growing high yielding varieties and looking to reduce crop level, cluster thinning is still the recommended practice. For more information on how to implement appropriate CT techniques, please see Cornell Cooperative Extension Fruit thinning in wine grapes and Crop thinning: cluster thinning or cluster removal.

Considerations regarding ELR

Other factors to consider if you are interested in applying ELR:

  • Fruit-set percentage – One of the factors facing the unpredictability of ELR is the weather conditions between bloom and fruit set. Since weather can have a large effect on the percentage of fruit-set (Fruit set in grapes 101), ELR may potentially exacerbate ‘poor’ fruit-set if extended periods of wet, cool (< 59°F), overcast, or very hot (> 90°F) weather conditions occur following leaf removal.  Additionally, berry sunburn may be a potential concern with ELR when performed under chronic high light and temperature intensity.
  • Bud Fruitfulness – While it is generally acknowledged that increased sunlight exposure is positive for bud development, a potential reduction in bud fruitfulness (number of clusters per shoot) may occur in the following season as a result of bud damage from ELR (14). Although still uncertain, bud damage may be the result of physical damage during leaf removal and/or reduction of carbohydrate supply during bud development.
  • Carbohydrate Storage in Cool Climate Grown Vines – Carbohydrates are the main energy source for grapevine growth, stress defense, and fruit ripening. Post-harvest carbohydrate storage in perennial tissues is a determinant of vine overwinter survival and is fundamental for shoot development in the following season. Removing leaves during ELR may alter the amount of carbohydrates produced by the leaves over the season and how carbohydrates are distributed among the vine organs. Currently, limited information is available on how ELR affects carbohydrates storage in perennial tissues and how this relates to dormant tissue (buds and canes) cold hardiness. This is a point of current interest to Centinari’s lab at Penn State, with current research being conducted in vinifera and hybrid wine grape varieties.
  • Crop Estimation – Yield predictions based on ELR use is currently not available. In this regard cluster thinning is a more conservative approach. Unlike ELR, which is performed very early in the season, cluster thinning severity can be decided upon estimation of final yield.



ELR holds potential as a way to reduce yield and Botrytis rot infection for some grape varieties grown in the Mid-Atlantic and other cool-climate regions. However, more research is needed to better understand the consistency of ELR practices on vine physiology, yield reductions, and fruit quality. Current efforts are on-going by the Centinari lab and Bryan Hed at the Lake Erie Grape Regional Extension Center (LEGREC) to evaluate the use of manual and mechanized ELR in hybrid and V. vinifera varieties across Pennsylvania.

Additional Resources

PSU Wines and Grapes blogs:  An Overview of Cluster-Zone Leaf Removal Strategies for Cool Climate Vineyards and 2016 Post Bloom Disease Management Review

Intrieri C, Filippetti I, Allegro G, et al. 2008.  Early defoliation (hand vs mechanical) for improved crop control and grape composition in Sangiovese (Vitis vinifera L.).  Aus. J. Grape Wine Res. doi: 10.1111/j.755-0238.2008.00004.x

References Cited

  1. Acimovic D, Tozzini L, Green A, et al. 2017. Identification of a defoliation severity threshold for changing fruitset, bunch morphology and fruit composition in Pinot Noir.  J. Grape Wine Res. doi:  10.1111/ajgw.12235
  2. Bubola M, Sivilotti P, Janjanin D, and Poni S.   Early leaf removal has larger effect than cluster thinning on cv. Teran grape phenolic composition.  AJEV.  doi: 10.5344/ajev.2016.16071
  3. Illand P, Dry P, Proffit P, and Tyerman S. Photosynthesis. In The Grapevine, from the science to the practice of growing vines for wine. pp. 91-107.
  4. Coombe BG.   The effect of removing leaves, flowers and shoot tips on fruit-set in Vitis vinifera L. J. Hortic. Sci. 37:1-15.
  5. Poni S, Casalini L, Bernizzoni F, et al. 2006. Effects of early defoliation on shoot photosynthesis, yield components, and grape composition. AJEV. 57: 397-407.
  6. Tardaguila J, Martinez de Toda F, Poni S, and Diago MP. 2010. Impact of early leaf removal on yield and fruit and wine composition of Vitis vinifera Graciano and Carignan. AJEV. 61(3):372-381.
  7. Silvestroni O, Lanari V, Lattanzi T, et al. Impact of crop control strategies on performance of high-yielding Sangiovese grapevines. AJEV. doi: 10.5344/ajev.2016.15093
  8. Vail ME and JJ Marois. 1991. Grape cluster architecture and the susceptibility of berries to Botrytis cinerea. Phytopathology 81:188-191.
  9. Sternad Lemut M, Sivilotti P, Butinar L, et al. Pre-flowering leaf removal alters grape microbial population and offers good potential for a more sustainable and cost-effective management of a Pinot Noir vineyard. J. Grape Wine Res. doi: 10.1111/ajgw.12148
  10. Hed B, Ngugi HK, and Travis JW.   Short- and long-term effects of leaf removal and gibberellin on Chardonnay grapes in the Lake Erie region of Pennsylvania. AJEV.  66(1): 22-29.
  11. Moreno D, Vilanova M, Gamero E, et al. Effects of preflowering leaf removal on phenolic composition of Tempranillo cv. in semi-arid terroir of western Spain.  AJEV. doi: 10.5344/ajev.2014.14087
  12. Risco D, Pérez D, Yeves A, et al. Early defoliation in a temperate warm and semi-arid Tempranillo vineyard: vine performance and grape composition. Aus J Grape and Wine Res. doi: 10.1111/ajgw.12049
  13. Sivilotti P, Herrera JC, Lisjak K, et al. 2016. Impact of leaf removal, applied before and after flowering, on anthocyanin, tannin, and methoxypyrazine concentrations in ‘Merlot’ (Vitis vinifera) grapes and wines. J. Agric. Food Chem.  64:4487-4496.
  14. Sabbatini P, and Howell GS. 2010. Effects of early defoliation on yield, fruit composition, and harvest season cluster rot complex of grapevines.  HortScience 45(12):1804-1808.

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).


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 ( 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.
  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).

How does delaying spur-pruning to the onset or after bud burst impact vine performance? Insights from recent studies

By Michela Centinari

Now that harvest is finally over and wines are tucked away in the cellar, it is time to prepare for the next year. One of the first concerns that many growers feel in a new growing season is that worry of spring frost and the associated potential risk of vine injury. In the spring of 2016, for example, an unusually warm March was followed by a very cold start to the month of April, which resulted in damaging frost incidences in some vineyards of the Mid-Atlantic region.

Susceptibility to frost injury increases with advanced phenological growth stage [1], therefore, growers and scientists have explored different techniques for delaying bud burst of grapevines to increase the chance of avoiding spring frost damaging events. Vegetable-based oils (e.g., Amigo oil) can be sprayed on the canes/buds during the winter to slow down bud de-acclimation and delay the resumption of vegetative growth in the spring [2; 3; study at Penn State]. Delaying pruning until late winter can also be used to delaying bud burst of vines growing in frost prone areas.

Canes of cordon-trained vines can be pruned to 2-3 node spurs late in the winter or even when apical buds begin to open to delay bud burst of basal buds. Due to the strong apical dominance of Vitis vinifera cultivars, apical buds of an unpruned cane tend to burst first, which inhibits development and growth of median and basal buds [4] (Figure 1).

Figure1. Spur pruning vines while the apical buds are bursting. Photo source: McGourty, The case for double—pruning. Practical Winery &Vineyard.

Figure1. Spur pruning vines while the apical buds are bursting. Photo source: McGourty, The case for double—pruning. Practical Winery &Vineyard.

What may happen if we wait until the onset of bud burst or even later to prune the vines?

Spur-pruning the vines when the apical buds of un-pruned canes are already open may not only delay bud burst of the basal nodes, but may also postpone other phenological growth stages such as bloom, fruit-set, or even veraison with potential consequences for vine yield and fruit chemical composition at harvest [4].

I recently read two articles on this topic published in the American Journal of Enology and Viticulture (Post-bud burst spur-pruning reduces yield and delays fruit sugar accumulation in Sangiovese in central Italy [5] ) and in Frontiers in Plant Sciences (Phenology, canopy aging and seasonal carbon balance as related to delayed winter pruning of Vitis vinifera L. cv. Sangiovese grapevines [6]).

The studies described in these articles aimed to assess if and how delaying winter spur-pruning of Sangiovese vines to the bud swelling stage or later, after bud burst, impacted the annual growth cycle of the vines and its productivity.

The studies were conducted in Italy and the researchers were specifically interested in assessing if vines pruned around or after bud burst exhibited a delay in grape ripening as compared to those pruned during the winter, resulting in lower sugar accumulation and higher acidity in the fruit at harvest. A steady trend of increased warming is, indeed, pushing some Mediterranean grape growing regions toward accelerated ripening [7], which could lead to excessive or overly fast sugar accumulation in the fruit, high alcohol in the wine, unacceptably low acidity, high pH, and also atypical grape flavors and aromas [5].

Although excessive or overly fast sugar accumulation may not be a problem in our region, it’s still important to understand if delaying winter pruning to extremes could be used to delay bud burst and reduce risk of frost damage, as well as the impact this practice may have on vine yield, and fruit and wine chemistry. This is a topic of further interest in light of changing climatic conditions and the potential increase of unpredictable weather patterns like early spring warming and late spring frosts [8].

Below, I will summarize the two previously mentioned studies emphasizing results which can be of interest to wine grape growers in our regions.

Both studies were conducted on mature Sangiovese (Vitis vinifera L.) vines. The first study was established in a commercial vineyard in central Italy, whereas the second study was conducted on vines growing outdoors in 10-gal pots at a research station in northern Italy. Groups of vines were assigned to different pruning treatments. Vines assigned to the standard grower practice treatment were spur-pruned to 2 basal nodes during the winter when buds were dormant. Vines assigned to the other treatments were spur-pruned at more unusual times from the bud swelling to full bloom (Figures 2 and 3A).

Figure 2. Phenological growth stages of the apical shoot at the time vines were pruned to 2-node spurs. *BBCH scale was used to assess phenological stages [9] in studies described below.

Figure 2. Phenological growth stages of the apical shoot at the time vines were pruned to 2-node spurs. *BBCH scale was used to assess phenological stages [9] in studies described below.

Did delaying vine spur-pruning to after bud-burst consistently delay the whole annual growing cycle?

Basal buds of Sangiovese vines spur-pruned when apical shoots were about 1.6″ long (called late pruning treatment; Figure 3A central panel) burst 17 days later than those of vines pruned in the winter, when buds were dormant (called standard pruning treatment; Figure 3A left panel). Pruning the vines even later, when apical shoots were about 4.7-5.5″ long (called very-late pruning treatment;  Figure 3A right panel) extended the delay in bud burst to 31 days as compared to vines pruned in the winter Unfortunately no phenology data were recorded for vines pruned at bud swelling stage (study 1).

The delay in phenological growth stage decreased over the season. For example, late-pruned and very-late pruned vines reached veraison 3 and 13 days, respectfully, after those pruned in the winter (Figure 3C). Shoots of vines pruned after bud burst developed later in the season under higher air temperature than those of vines pruned during the winter. Greater air temperature may have helped shoots of late- and very-late pruned vines to reach bloom and veraison in fewer days as compared to those pruned earlier [6].

By harvest the delay was fully off-set for the late-pruned vines: they reached the sugar level set for ripening (~ 19 ºBrix) three days before those pruned in the winter. Grapes of very-late pruned vines reached 19 ºBrix 6 days after those pruned in the winter.

Figure 3. Vine appearance at the time of pruning (A) and at bloom (B) and veraison (C). Note: standard winter pruning was taken as a reference for bloom and veraison. Photo courtesy Dr. Stefano Poni (professor of Viticulture, Universita’ Cattolica del Sacro Cuore, Italy).

Figure 3. Vine appearance at the time of pruning (A) and at bloom (B) and veraison (C). Note: standard winter pruning was taken as a reference for bloom and veraison. Photo courtesy Dr. Stefano Poni (professor of Viticulture, Universita’ Cattolica del Sacro Cuore, Italy).

Spur-pruning vines after bud burst significantly reduced crop yield compared to standard winter pruning

Vines pruned at bud swelling growth stage had similar crop weight, number of clusters per vine, and cluster weight than those pruned when buds were still dormant. Pruning vines after bud burst, however, reduced yield as compared those pruned during the dormant season. For example, late pruned vines (spur-pruned when apical shoots were about 1.6″ long; Figure 3A central panel) had 26% lower crop yield as compared to those of the standard pruning control group (spur-pruned before bud burst). Reduction of crop yield was related to lower cluster weight and lower number of berries per cluster. While there is not a clear explanation on why late-pruned vines had fewer berries per cluster, several hypotheses were presented including increased production of gibberellins during the initial flush of growth in the late-pruned vines [6].

Waiting even longer to prune the vines had a detrimental effect, reducing not only cluster weight but also the number of clusters per vine. For example, vines pruned to two basal nodes when the apical shoots were already flowering had no crop at harvest. When vines were pruned so late the basal shoots did not develop flowers and remained vegetative after pruning. I am not sure why any growers would want to wait until bloom to prune the vines, but it’s still interesting to see how such a drastic treatment may limit sources of carbohydrates for developing cluster primordia [5].

Although uncertainty still exists, the authors suggested that delaying spur-pruning until after bud burst, but not to extremes, may have the potential for reducing crop yield in high-yielding cultivars such as Sangiovese planted in specific regions of Italy. However, long-term field studies are necessary to assess if it is possible to calibrate winter pruning date for managing yield reductions and/or fruit maturation rate.

Did delaying vine spur-pruning to bud swelling stage or after bud burst consistently impact fruit chemistry?

The effect of the timing of spur-pruning on fruit composition at harvest varied between studies and with the extent of the pruning delay. For example, Sangiovese vines late pruned (apical shoots 1.6″ long) had higher total soluble solids (+ 1 °Brix), total anthocyanins and phenolics than winter-pruned vines. However, vines growing in a commercial vineyard (study 1) and spur-pruned to two basal nodes later in the season, when inflorescences of apical shoots were already swelling (Figure 3C), had lower sugar concentration (-1.6 °Brix) and higher TA (+1.8 g/L tartaric acid) than those pruned during the winter, but at the same time they also had higher anthocyanins and phenolic concentrations.  This result suggests that spur-pruning canes after bud burst may decouple the accumulation patterns of total soluble solids and anthocyanins, phenolic metabolites. This could be intriguing for growers trying to delay fruit sugar accumulation and acid degradation, while maintaining wine color, but on the another hand it could also come with a  reduction in crop yield, quantified to over 50% in this study [5].

Did delaying winter spur-pruning have negative carry-over effects on the following season?

Pruning vines at the bud swelling stage did not have negative effects on vine growth in the following year. It did not impact bud fertility (number of clusters per shoot) or winter carbohydrate storage, which is important for winter vine survival and following year resumption of growth. However, pruning the vines to two-nodes after bud burst, specifically when inflorescences of apical shoots were already swelling (Figure 3C), reduced bud fertility by 50% in the following year. Those vines were able to recover once standard winter pruning was applied again at the end of the study.

In conclusion:

These studies conducted on Sangiovese vines grown in Italy found that:

  • Winter spur-pruning can be applied up to bud swelling without adversely affecting vine yield, grape composition at harvest or bud fertility in the following year.
  • Vines pruned after bud burst show pronounced delay in shoot development at the beginning of the season, which increased as the pruning time was further delayed. Under the warm conditions of these studies the delay in phenological growth stage decreased or even disappeared over the season. This could be partially explained by the fact that late-pruned vines needed less time than vines pruned during the winter to reach maximum photosynthesis efficiency.
  • Delaying spur-pruning to after bud-burst may reduce vine yield, decrease sugar accumulation and bud fertility in the following year.

Delaying winter pruning of vines located in frost prone areas to the onset of bud burst or shortly after that may be used as frost avoidance technique. However, we need to further understand how a delay in shoot development and potentially a shorter growing season (number of days from bud burst to harvest) may impact fruit ripening, yield component, vine over-winter carbohydrate storages and susceptibility to winter cold temperatures, as well as the following year growth. The studies summarized here were conducted in a warm region with a growing season longer than many areas of Pennsylvania.  Further research is necessary to corroborate those results under our regional climatic conditions.


Literature cited

  1. Centinari M, Smith MS, Londo JP. 2016. Assessment of Freeze Injury of Grapevine Green Tissues in Response to Cultivars and a Cryoprotectant Product. Hortscience 51: 1–5.
  2. Dami I, and Beam B. 2004. Response of grapevines to soybean oil application. J. Enol. Vitic. 55: 269–275.
  3. Loseke BJ, Read PE, and Blankenship EE. 2015. Preventing spring freeze injury on grapevines using multiple applications of Amigo Oil and naphthaleneacetic acid. Scientia Hort. 193: 294–300.
  4. Friend AP, and Trought MCT. 2007. Delayed winter spur-pruning in New Zealand can alter yield components of Merlot grapevines. J. Grape Wine Res. 13: 157–164.
  5. Frioni T, Tombesi S, Silvestroni O, Lanari V, Bellincontro A, Sabbatini P, Gatti M, Poni S, Palliotti A. 2016. Post-bud burst spur pruning reduces yield and delays fruit sugar accumulation in Sangiovese in central Italy. J. Enol. Vitic. 67:419–425.
  6. Gatti M, Pirez FJ, Chiari G, Tombesi S, Palliotti A, and Poni S. 2016. Phenology, canopy aging and seasonal carbon balance as related to delayed winter pruning of Vitis vinifera cv. Sangiovese grapevine. Frontiers in Plant Sciences 7:1–14. Article 659.
  7. Jones GV, White MA, Cooper OR, and Storchmann K. 2005. Climate change and global wine quality. Climatic Change 73: 319–343.
  8. Mosedale JR, Wilson RJ, and Maclean IMD. 2015. Climate change and crop exposure to adverse weather: Changes to frost risk and grapevine flowering conditions. PLoS One 10:e0141218.
  9. Lorentz DH, Eichorn KW, Bleiholder H, Klose R, Meier U, and Weber E.1995. Phenological growth stages of thegrapevine (Vitis vinifera ssp. vinifera). Codes and descriptions according to the extended BBCH scale. Aust. J. Grape Wine Res. 1, 100–103.

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.



[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, 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, 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.


Assessing and managing potassium concentration in the vineyard

By Michela Centinari

Potassium (K) plays a critical role in many plant physiology and biochemistry processes (e.g., photosynthesis, osmoregulation, enzyme activation, etc.). Inadequate supply of K can result in reduced shoot, root and fruit growth as a result of reduced xylem sap flow, and can also increase the risk of drought stress [1]. Potassium deficiency leads to inhibition of photosynthesis and to sugar (sucrose) being “trapped” in the leaves which adversely affects yield, fruit ripening and berry soluble solid concentration [1].

While grape growers should monitor vine health to avoid K deficiency, at the same time they should also be on the lookout for excessive concentration of K in vine tissues (e.g., leaf, berry) because of its potential negative impact on vine health and wine quality.

When looking back at the past two years of inquires received from Pennsylvania wine grape growers related to vine nutrient or nutrient-related problems, we (Denise and I) found that the number of those concerning excessive concentration of K and related issues (e.g., high/unstable wine pH) were more common than inquiries related to K deficiency, which mostly occurred in young vineyards.

This short article will review problems related to high/luxury absorption of potassium, briefly discuss how soil mineralogy and pH can affect K uptake, why it is important to regularly monitor vine nutrient status, and what environmental and cultural factors may impact K uptake and accumulation in plant tissues. In-depth information on mode of uptake and transport of K in the plant, and functions of K can be found in “A review of potassium nutrition in grapevines with special emphasis on berry accumulation” by Mplelasoka et al. 2003 [2].

Also, you can refer (as I did) to the valuable web-resources recently published by Virginia Tech University (Dr. Tony Wolf and Russel Moss), which includes edited information to supplement Chapter 8 of The Wine Grape Production Guide of Eastern North America:

Viticulture Notes, July 2016

Potassium in Viticulture and Enology, May 2016

Why high/excessive concentration of K in the grape berries may negatively impact wine quality?

Grape berries are a strong sink for K during ripening. Potassium accumulates mainly in the berry skin tissues (Figure 1) and is the most abundant cation (K+, hereafter referred to as K) in grape juice [2]. Mature grapes may have, indeed, almost twice as much K as nitrogen [1]; for example one ton of mature grapes contains about 11 lbs (5 kg) of K [3].

Figure 1. Potassium concentrations in different berry tissues. Adapted from Mpelasoka et al. 2003 [2] . Data from Walker et al. 1998 [4].

Figure 1. Potassium concentrations in different berry tissues. Adapted from Mpelasoka et al. 2003 [2] . Data from Walker et al. 1998 [4].

High concentration of K in grape juice (e.g., > 50 mmol/L) may result in a high juice pH (e.g., > 3.8) and negatively impact wine quality [5]. During winemaking, high concentration of K causes precipitation of free acids, mainly tartaric acid, leading to an increased wine pH [2]. The high pH may reduce color stability of red wines due to a shift of anthocyanins to the non-colored forms [2]. High concentration of K may also reduce respiration and the rate of degradation of malic acid and consequently increase malolactic fermentation [5].

While many studies linked high juice/wine pH to high juice/wine K concentrations (see for example Figure 2) it is also true the other factors, aside from K, can affect wine pH and that wines with low pH (e.g., < 3.25) may also have high K concentration [6]. Moreover, there are varietal differences in the relationship between juice pH and K concentration (see Chardonnay vs. Shiraz, Figure 2), as well as between wine K and juice K [7].

Figure 2. Relation between grape juice pH and K concentration for Chardonnay and Shiraz vines at 4 Australian vineyards. Within each site, a data point represents a different rootstock. Data adapted from Walkers et al. 2012 [7].

Figure 2. Relation between grape juice pH and K concentration for Chardonnay and Shiraz vines at 4 Australian vineyards. Within each site, a data point represents a different rootstock. Data adapted from Walkers et al. 2012 [7].

pH can be adjusted during winemaking through addition of tartaric acid, adding additional costs for the wineries. Ensuring adequate concentration of K in the grapes at harvest will not only help reduce winemaking costs but is also likely to improve wine quality [2].

In next week’s blog post, Denise Gardner will discuss options for dealing with high K concentrations in juice and wine. However, the first step is to determine if there is or there may be a potential problem with K in your vineyard (either deficiency or excess level).

Potassium availability in the soil

It is well known that concentration and availability of K vary with soil type and is greatly affected by the physical and chemical properties of the soil [8]. Potassium in soil is classified into four groups in relation to its availability to the plants: 1) water-soluble (K dissolved in soil solution), 2) exchangeable (on cation exchange sites of surfaces of clay minerals and humic substances), 3) non-exchangeable and 4) structural forms [8].

The water-soluble and exchangeable pools (1 & 2) represent only » 0.1-0.2% and 1-2% of the total soil K, respectively [8]. Both forms are readily available for plant root absorption. The majority of the K is bound in mineral structures, such as mica and feldspar, or it is part of secondary minerals such as vermiculite [6] and thus considered a (very) slowly-available source of K for plants. Clays also have different capabilities of binding K as well as different rates of K release [6]. For example, micas release K at a remarkably faster rate than feldspar [9].

Since clay mineralogy impacts the release of K into the soil solution over time and the K supplying power of the soil, it is important to have detailed information concerning the soil structure and composition of your vineyard (i.e., does your site have high levels of exchangeable K?).

Other important factors affecting K availability are soil pH and relative concentration of K to that of the other cations, such as magnesium (Mg2+) and calcium (Ca2+). Low/acidic soil pH (<6) increase K availability and potentially increase its uptake while reducing uptake of Ca2+ and Mg2+ [10]. Excess K can decrease concentrations of Ca2+ and Mg2+ in plant tissues and induce symptoms of Mg deficiency [1] (Figure 3).

Figure 3. Leaf symptoms of magnesium deficiency. Photo credit: Andrew Harner (graduate research assistant, Penn State University).

Figure 3. Leaf symptoms of magnesium deficiency. Photo credit: Andrew Harner (graduate research assistant, Penn State University).

Assessing K concentration in the vine

Conducting plant tissue (petiole) testing on a regular basis (annual or every two years) to monitor vine nutritional health (K and other essential nutrients) and promptly correcting problems related to nutrient imbalance is strongly recommended. Visual observations of foliar symptoms of nutrient deficiency or toxicity are important clues (Figure 4), but a nutrient management program should not be exclusively based on visual observations because: 1) it is possible to be misled by symptoms that are not nutrient related (e.g. mite injury, virus, etc.) and 2) to develop an appropriate nutrient management program it is crucial to understand the nutritional requirements of the vines [11].

Soil testing is an extremely useful tool in the pre-planting stage for determining the potential of a vineyard site and the amendments needed, and also for monitoring soil pH over the years (after the vines are planted). However, soil testing only tells one side of the story: what is potentially available to the vine. Again, the recommended and preferred method to assess vine nutritional health and to effectively identify potential nutrient (in this specific case K) deficiency or excess is plant tissues (petiole) testing.

Some limitations of the soil testing include:

  • Soil samples are often limited to the first 10-20 inches. Roots of mature vines tend to be sparse and, in deep soils, they can grow much deeper than 10-20 inches. Thus soil testing may not be a good indicator of the soil/plant interaction.
  • Soil testing may underestimate the reservoir of K available to the vines [9]. The laboratory nutrient extraction analysis is run over minutes while vines have much longer to absorb/extract nutrients [6,9].

Thus, don’t be too surprised if results of K soil testing are poorly correlated with those of plant tissue testing [6,7,9].

When is the best time to conduct leaf petiole testing?  

The preferred time for leaf petiole testing is bloom and late-summer (70 to 100 days after bloom). Assuming bloom was in June, you may still have time left this season to conduct the test. In case of suspected K or other nutrient deficiency the samples can be collected anytime during the season [11]. It is important to have a standardized tissue-sampling procedure. For example, at bloom collect 60 to 100 petioles of healthy leaves opposite to the flower cluster (first or second) for each cultivar. Don’t collect more than one or two petioles per vine. Late summer samples should be collected from “the youngest fully expanded leaves of well-exposed shoots, usually located from 5 to 7 leaves back from the shoot tip”. If the shoot has been hedged, collect “primary leaves near the point of hedging” [11].

More information on how to collect leaf petiole samples and interpret the results can be found at Monitoring Grapevine Nutrition ( and in the Wine Grape Production Guide for Eastern North America, chapter 8 [11].

Reference values of K concentration in grape leaf petioles for bloom- and late summer-collected samples are reported in Table 1. (Note: the source used is the Wine Grape Production Guide for Eastern North America [11], sources from other regions may provide slightly different standards).

Table 1. Reference values for K concentration in grape leaf petioles for bloom- and late summer-collected samples.


In table 2, I included the results of a petiole test conducted at a research vineyard in south central Pennsylvania. At bloom, K concentrations in the leaf petiole of Cabernet Sauvignon and Merlot vines were slightly above the ‘excessive’ range. We repeated the analysis around veraison and found that at that time K concentrations were greatly above the 2% ‘excessive’ threshold. Not too surprisingly, at harvest, both varieties had high grape juice pH and high K concentration according to reference values reported by Mpelasoka et al. 2003 [2] (Table2). Soil testing showed a high level of exchangeable K, and low pH (below 6), the vines were highly vigorous: all factors that can contribute to luxury uptake of K.

Table 2. Potassium concentrations in the petiole of Cabernet Sauvignon and Merlot leaves at bloom and veraison. Potassium concentration and pH of the grape juice was measured at harvest.


What to do next?

In case of K deficiency (petiole and soil testing and visual observations) (Figure 4) you can consult with an extension educator in your county or a viticulture consultant who can assist with the development of an appropriate fertilization program.

Figure 4. Symptoms of potassium deficiency on Cabernet franc leaves.

Figure 4. Symptoms of potassium deficiency on Cabernet franc leaves.

For those of you who may have missed it, Tony Wolf (professor and viticulture extension specialist at Virginia Tech university) recently issued an important update on K fertilization recommendations [6]. The lower limit of optimal soil K reported in the “Wine Grape Production Guide for Eastern North America” [11] has changed from 75 ppm (150 lbs/ac) to 40 ppm (80 lb/acre) (Note: those values are based on Mehilch-3 extraction protocol which is the one used by Penn State Agricultural Analytical Services Lab but not by Virginia Tech).  In the July issue of Viticulture Notes Tony Wolf wrote:

Potassium fertilizer is not recommended pre-plant or to existing Virginia vineyards if the soil test results are at or above 40 ppm (80 lbs/acre) actual K as determined by Mehlich-3 test procedures, or 28 ppm (56 lbs/acre) actual K as determined by Mehlich-1 test procedures. However, young vines should be visually monitored and irrigated under drought conditions to avoid potential K deficiency on soils that are inherently low in exchangeable K.”

How can K concentration and uptake by the vines be reduced? 

In the pre planting stage, if the soil selected for planting the vineyard has high exchangeable K levels, an option is to select rootstocks that accumulate low concentration of K.  Rootstocks, and grapevine varieties in general, differ in their capacity of K uptake and translocation [2]. For example, rootstocks with V. berlandieri genetic background tend to have reduced K uptake as compared to others, as those with V.champini parentage [12]. In northern California, Chardonnay, Cabernet Sauvignon and Zinfandel vines grafted on 101-14 Mgt and 3309C (V. riparia x V. rupestris), two commonly-used rootstocks in the eastern US, consistently had leaf petiole K concentrations within the intermediate range compared to those of the same varieties grafted on V. berlandieri (lowest K concentrations) and V. champinii (highest K concentrations) crosses [12]. A study conducted at Winchester, VA, by Tony Wolf research group found that the use of 420-A (V. berlandieri x V. riparia) rootstock reduced juice pH in Cab Sauvignon vines as compared to those grafted on 101-14 Mgt and Riparia [6].

However, it is important to consider that the performance of rootstocks in terms of K uptake varies depending on rootstock-scion combination (i.e., the same rootstock may have variable effects on different scion varieties), soil type, climate, and management practices.

Another aspect to consider when selecting rootstocks is their vigor or growth-potential. Vigorous rootstocks or rootstocks that convey high vegetative growth and yield potential to the scion may cause increased K uptake as a result of increasing vine demand.

Growth drives K uptake:  Factors such as high vine vigor, leaf area, and extensive root system can enhance K uptake, translocation, and accumulation in the grape berries.  Soil moisture increases the dissolution of K from clay particles, thus facilitates K supply and uptake by the roots. High soil water availability also leads to increase vegetative growth which may indirectly affect K uptake and its accumulation in the berries.

Shaded leaves are a source of K translocation to the grape berries: Canopy microclimate and mainly foliage shading can affect the accumulation of K in the berries. For example, artificial (shading cloths) [13] or natural (canopy) shading [14] was found to increase K concentration in berries and juice. We don’t know exactly why this happens yet, but it is possible that in conditions of low sugar accumulation, as under foliage shading, the increasing accumulation of K in the berries helps regulating osmotic potential, maintaining cell turgor and thus minimizing reduction in berry growth which may occur with low sugar content [2].

Can crop load be regulated to reduce K accumulation in the fruit?

Since berries after veraison are the primary sink for K, we would expect that regulation of crop load (commonly defined as the ratio of fruit weight to pruning weight or to leaf area) can affect K translocation and accumulation in the berries [2]. However, results from previous studies are inconclusive. For example, in hot climate regions (Israel, California) cluster thinning reduced berry K in one study [15] while having no effect on juice K in another study [16]. Factors such as grape variety, timing of thinning, and the amount of crop retained can greatly affect outcomes and explain why the effect of crop load on accumulation of K in the berries still remains unclear. Making matters more complicated, manipulation of crop load may also affect vegetative growth, and the degree of foliage shading, thus indirectly impacting K translocation into the berries.

Generally speaking, over cropping may result in a lower or insufficient amount of K in the vine tissues (K deficiency tends to be more pronounced on heavily cropped vines after veraison). At the same time if yield is very (too) low the shoots may become competitive sinks for K and as a result its accumulation in the berries may be reduced [2].

Vineyard management practices that decrease leaf shading may reduce K accumulation in the berries. Reducing canopy density and shading, either through a) the removal of lateral shoots, b) lateral and top-hedging, or c) basal leaf removal reduced K concentration and, in some cases, pH in juice and wine of Tannat vines in Uruguay (Figure 5) [17]. The use of divided-trellis systems could also be a method to manage highly vigorous vines and decrease leaf shading [2].

Figure 5. Effect of canopy management treatments on K concentration and pH of Tannat wines over three years. Asterisks (*) indicate a significant difference with respect to the control (p ≤ 0.05) based on Tukey’s test.

Figure 5. Effect of canopy management treatments on K concentration and pH of Tannat wines over three years. Asterisks (*) indicate a significant difference with respect to the control (p ≤ 0.05) based on Tukey’s test.

In conclusion, if you suspect a problem with high vine K and/or high juice/wine pH, here a few things you can do:

  1. Plant tissue (petiole) analysis to assess the K level of your vines and to confirm that high/excessive K is the real issue. Again, don’t rely exclusively on soil testing, which is still useful to assess soil pH and other factors that may affect K uptake.
  2. If your vines are highly vegetative you could test the effect of reducing foliage shading on juice/wine pH and K. Reducing canopy density may also have additional benefits for the health of the grapes and quality. Canopy practices such as basal leaf removal, hedging, shoot positioning and thinning can be used. It is a good practice to leave a block of untreated vines (no additional canopy management) as a control. At harvest measure juice pH and K concentration in the treated (less shaded) and untreated (more shaded) vines to assess if the extra canopy management mitigate the K/pH level in the grape juice.

If you have tried or panning on trying to manage K levels in your vineyards we would be happy to hear about your experience and methods/results.


Literature Cited:

  1. Keller M. 2010. The Science of Grapevines: Anatomy and Physiology. Publisher: Academic Press.
  2. Mpelasoka BS, Schachtman BP, Treeby MT, Thomas MR. 2003. A review of potassium nutrition in grapevines with special emphasis on berry accumulation. Aust. J. Grape Wine Res. 9, 154–168.
  3. Coombe BG, Dry PR. 1992. Viticulture Volume 2 – Practices. Publisher: Winetitles.
  4. Walker RR, Clingeleffer PR, Kerridge GH, Rühl EH, Nicholas PR, Blackmore DH. 1998. Effects of the rootstock Ramsey (Vitis champini) on ion and organic acid composition of grapes and wine, and on wine spectral characteristics. J. Grape Wine Res. 4, 100–110.
  5. Kodur 2011. Effects of juice pH and potassium on juice and wine quality, and regulation of potassium in grapevines through rootstocks (Vitis): a short review. Vitis 50, 1–
  6. Wolf TK. Viticulture Notes. Vol 31 No. 5. 23 July 2016. Virginia Tech University Cooperative Extension. Available at: 
  7. Walker RR, Blackmore DH. 2012. Potassium concentration and pH inter-relationships in grape juice and wine of Chardonnay and Shiraz from a range of rootstocks in different environments. J. Grape Wine Res. 18, 183–193.
  8. Zörb C, Senbayram M, Peiter E. 2014. Potassium in agriculture – Status and perspectives. J. Plant Physiol. 171, 656–669.
  9. Beasley, E, Morton L, Ambers C. 2015. The role of soil mineralogy in potassium uptake by wine grapes. Progress report to the Virginia Wine Board
  10. Moss R. 2016. Potassium in viticulture and enology. Virginia Tech University Cooperative Extension. Available at:
  11. Wolf TK. 2008. Wine grape production guide for Eastern North America. Natural Resource, Agriculture, and Engineering Service: Ithaca, NY USA.
  12. Wolpert JA, Smart DR, Anderson M. 2005. Lower petiole potassium concentration at bloom in rootstocks with Vitis berlandieri genetic backgrounds. Am. J. Enol. Vitic. 56:163-169.
  13. Rojas-Lara BA, Morrison JC. 1989. Differential effects of shading fruit or foliage on the development and composition of grape berries. Vitis 28, 199–208.
  14. Dokoozlian N, Kliewer MW. 1996. Influence of light on grape berry growth and composition varies during fruit development. J. Am. Soc. Hortic. Sci. 121, 869–874.
  15. Hepner Y, Bravdo B. 1985. Effect of crop level and drip irrigation scheduling on the potassium status of Cabernet Sauvignon and Carignane vines and its must and wine composition and quality. J.Enol. Vitic. 36, 140–147.
  16. Freeman BM, Kliewer WM. 1983. Effect of irrigation, crop level and potassium fertilization on Carignane vines II. Grape and wine quality. J.Enol. Vitic. 34, 197–207.
  17. Coniberti A, Ferrari V, Fariña L, Disegna E. 2012. Role of canopy management in controlling high pH in Tannat grapes and wines. Am. J.Enol. Vitic. 63:554-558.