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From the Vineyard (#1): Post-veraison berry shrivel and discoloration – symptoms, causes, and conditions

by Cain Hickey, Viticulture Extension Educator, Penn State Extension

This is the first of a series of blog posts called “From the Vineyard.” This series will highlight stakeholder reports and questions. If one or two growers have a similar report or question, it is possible that others may as well. The timing of the second post in this series will be dependent on feedback, reports, and questions we hear from stakeholders. In other words, there is no predetermined, “set” frequency for “From the Vineyard” posts.

In the last few weeks, three different growers have sent me pictures of shriveled and/or discolored grapes within clusters. Two growers noticed pink berries (sometimes accompanied by shriveling) with green rachis tissues in their Cabernet franc vineyards located in Lancaster County, PA. Another grower noticed shriveled berries with necrotic, brown rachis tissue in their Petit Verdot vineyard located in Chester County, PA. See the photos, below (thanks to these growers for sharing the photos and giving permission to post!)

Cabernet franc in Lancaster County, PA (top, middle) and Petit Verdot in Chester County, PA (bottom).

When hearing about the observed symptoms over the phone, my immediate reaction was that “bunch stem necrosis” (BSN, also called “late season bunch stem necrosis”) was the issue in all cases. However, upon receiving photos and sharing them with Penn State Wine and Grape Team colleagues, I realized that BSN may have been the issue in only one of the cases. After doing a bit of research, it became evident that berries shrivel and become discolored after veraison due to different factors. The resources, below, collectively describe four different scenarios where berries may become shriveled and/or discolored after veraison: berry shrivel (also known as sugar accumulation disorder, SAD), dehydration, bunch stem necrosis, and sunburn. Note that sunburn in white-berried cultivars often manifests in skin browning, hardening, and cracking on the outside face of the cluster; the below resources show sunburn can result in pink berries in red-berried cultivars.

https://escholarship.org/uc/item/1931r74j#main

http://wine.wsu.edu/2006/11/06/berry-shrivel-all-the-same/

http://enology.umn.edu/news/tracking-down-causes-bunch-stem-necrosis

https://fruit.wisc.edu/2019/09/13/berry-shriveling-in-grapes-late-bunch-stem-necrosis/

The goal of this post is not to reiterate the information provided in the above resources (but I would encourage everyone to review those resources). The goal is to diagnose the symptoms in the above photos so that growers can be aware of these issues, scout their own vineyards, and share future observations Based on the information provided in the above resources, my hypotheses are that the top photo of Cabernet franc is a function of SAD (sugar accumulation disorder, or “berry shrivel”), the middle photo of Cabernet franc is function of sunburn, and the bottom photo of Petit Verdot is due to BSN. Here are some additional observations that resulted in my diagnoses…

Differentiating SAD and sunburn. The grower that provided the top photo of Cabernet franc also said that the fruit tasted very tart and “wasn’t ripening” – low Brix is a hallmark of berry shrivel, or SAD. Further, in the top photo, the pink berry color appears to exist in berries throughout the clusters and there does not appear to be a pattern where only berries highly exposed to sunlight are pink – a pattern we would expect to see if the issue was sunburn (and as seen in the middle photo). The grower that provided the middle photo of Cabernet franc also said that Brix levels were similar between pink and purple berries – maintained, or increased, Brix is a hallmark of sunburn (see top two reference links, above).

Differentiating sunburn and BSN. Though the start to the growing season was cool and “slow” in PA, June through August was relatively dry, warm, and sunny. Verbal reports suggest that some vineyards in Chester County received greater rainfall than some vineyards in Lancaster County. Based on my knowledge of the sites, it is likely that the Petit Verdot vineyard in Chester County has greater water holding capacity (relatively flat; high organic matter and clay content) than the Cabernet franc (middle photo) vineyard in Lancaster County (sloped, convex landform). Anecdotally, sunburn occurs more often under hot and dry conditions where water may be limiting (e.g. Lancaster County vineyard) whereas BSN has been linked to excessive water availability during and after veraison (e.g. Chester County vineyard). A symptomatic difference between BSN and other conditions that cause berry shrivel/discoloration is that the rachis tissue becomes dried and necrotic with BSN (see Petit Verdot photo) whereas rachis tissue remains green with the other conditions (see top two photos of Cabernet franc).

What does this mean practically? This is more just an “FYI” in case you have wondered about the cause or condition of similar symptoms in your vineyard. It is prudent to collect as much data as possible to identify causes of your own vineyard symptoms. Using a different example unrelated to berry shrivel… are those red canopy leaves due to nutrient imbalances or systemic disease? Observing the patterns of red canopy leaves throughout the vineyard and their relation (or lack thereof) to vineyard topography and soil type can aid in determining the cause of red canopy leaves. Nutrient imbalances are often seen where the land is highly convex and sloped (e.g. where “lean,” rocky soils exist). However, lab tests with vine tissues can objectively determine the cause, which may be an imbalance in potassium or magnesium, or the systemic disease known as “red blotch” (or possibly something else altogether).

Please note that the above diagnoses are only hypotheses; I documented my reasoning for these hypotheses based on the photos, grower feedback, and knowledge of weather patterns and sites. Also note that the above-mentioned berry shrivel conditions are not caused by pathogens (to our knowledge). Thus, spraying a fungicide will not manage these issues. In fact, since little is known about the cause of BSN and SAD, there are limited management recommendations for their control. Below is some further discussion regarding sunburn and BSN.

Sunburn. From the perspective of reducing sunburn, the obvious answer is to be judicious with leaf removal magnitude. However, since rot management is of utmost importance in our climate, refraining from fruit zone leaf removal altogether is unlikely a viable option for vinifera production, particularly in many rot-sensitive cultivars that are popularly grown in PA (e.g. Gruner Veltliner, Pinot gris, Pinot noir, Riesling, Sauvignon blanc).

Our research has shown that fruit zone leaf removal to the point where all clusters are completely exposed can increase wine quality potential and reduce bunch rot in Bordeaux reds; however, leaf removal to lesser extents (where an average of one to two fruit zone leaf layers exists around the clusters) has also been shown to improve wine quality potential and rot management. Thus, considering the constraints of time and labor resources, it would seam that fruit zone leaf removal to an average of one to two fruit zone leaf layers may be a commercially viable option to achieve a balance between wine quality, rot management, and sunburn avoidance. Removing two to three fruit zone leaves per shoot can produce an average of one to two fruit zone leaf layers; removing four leaves per shoot produces a fruit zone that is generally devoid of leaves (see photos, below).

Chardonnay fruit zones with two leaves per shoot removed (top) and four leaves per shoot removed (bottom).

We need to gain a better understanding of how the combination of leaf removal magnitude and timing effects sunburn development in both red- and white-berried cultivars. Our observations suggest that sunburn is minimized when leaves are removed early in the season (e.g. at BB-sized berries or earlier). However, we have observed sunburn on the external-facing side of clusters, regardless of when leaves were removed; this was especially observed in white-berried cultivars. Weather patterns complicate the issue (they always do with field experiments) and will likely result in difficulty in drawing concrete conclusions for optimal leaf removal practice to abate sunburn development.

Bunch stem necrosis (BSN). From the perspective of reducing BSN, we first must become more familiar with the causes of this disorder. The above resources mention that excessive water availability near and after veraison likely increases the development of BSN. Since we cannot control the weather, proactive decisions to reduce water status (e.g. planting on sites that have less clay, are sloped and well-drained; installing drain tile) may reduce the incidence of BSN. When I was working in Virginia, anecdotal observations suggested that Cabernet Sauvignon vines of lower vigor and water status (grown within root restriction bags) had less BSN than vines of greater vigor and water status (grown without root restriction). In time, we will hopefully become more familiar with the exact causes of BSN so we can refine recommendations to avoid its development.

I hope that many growers are not observing berry shrivel or discoloration in their vineyards. My hope is that many have benefitted from the recent dry weather patterns experienced across the state and that harvest is off to a great start. In general, reports suggest that harvest of white-berried cultivars is well underway and may be coming to an end in the coming week or so. Pinot noir is a red-berried cultivar that has likely been harvested in many instances; some reds have also likely been harvested for sparkling or rose wine production. Harvest of other red-berried cultivars for red wine will likely commence this week and next.

My very best to all PA growers and winemakers for continued success in the 2020 vintage!

Cain

Harvest decisions and the complexity thereof

by Cain Hickey, Viticulture Extension Educator, The Pennsylvania State University

Harvest is an exciting part of the season. It is the culmination of the long growing season. The harvested crop is taken into “safer” realms in tanks, bins, and barrels to eventually produce wines that will be enjoyed. Harvest is also a stressful time of the season. Schedules change several times and logistics need adjusted accordingly. The reward from the last five or six months of hard work (not counting dormant pruning) is at stake. And, among other variables that will be further discussed below, the outcome of harvest is dependent on weather patterns which are out of our control. Such is the life of farming and, in our case, growing grapes. But it is worth it. And, grape growing challenges are not unique to Pennsylvania and humid growing regions in the US; grape growing and harvest challenges abound in growing regions worldwide, from the western US to New Zealand, to European countries, and regions in between.

Cabernet franc ready to be harvested.

Harvest decisions are the result of knowledge that is applied to the unique combination of cultivars and growing conditions of each vintage.

“Harvest” is a timely topic in Pennsylvania. From now through the end of October, grapes will be harvested and processed into wine throughout the Commonwealth. Some grapes may even be harvested into November and beyond in regions where ice wine or “late harvest” production is a goal. After Dr. Molly Kelly and Dr. Gill Giese presented their “A Balanced Harvest” webinar (https://bit.ly/32ohHPR), I was inspired to try and document several considerations that go into harvest decisions. And not necessarily doing so by citing literature. But, rather, by summarizing and reflecting on what I have learned from being involved in viticulture research, education, and extension and working with members of the eastern US wine industry over the past decade. I will paraphrase previous experiences and conversations with growers. And try to make points through the use of fictional case studies to exemplify circumstances that may be encountered in commercial vineyards.

Do we have an extensive enough literature base to cite how the combination of site, cultivar, management decisions, post-veraison weather patterns, and primary and secondary metabolites manifest in the resultant wine? We do have pieces of this story. For example, in humid growing regions, we generally understand that exposed grapes have potential to have less rot, lower acidity, and greater aroma and color development relative to shaded fruit. However, it would be a lofty goal to write a review on all the factors that can affect wine quality potential and thus dictate harvest decisions. Conclusions from such a review would be highly speculative in order to account for the multitude of variables that influence harvest decisions. Further, it would be difficult to apply any conclusions across the unique terroirs in grape growing regions, even just in the “humid growing regions of the US” which encompasses southeastern, midwestern, mid-Atlantic, and northeastern US states. Harvest decisions can be nebulous and are difficult to research and draw conclusions from unless “harvest date” is a factor in the experimental design. There are scenarios that limit the ability to make an objective statement about harvest decisions. Take, for example the recent and highly divergent weather patterns throughout the post-veraison periods of 2018 and 2019. All other variables standardized, these vintages will likely result in different wines.

To further make a case that harvest and related decisions are difficult subjects for objective statements, let’s consider whether the vineyard or cellar is more impactful for determining harvest. Is harvest a viticulture-based or enology-based decision? The answer: “yes” (as in, “both”… but with a weak attempt at humor). As a viticulturist, I am biased. I could argue the harvest of grapes is only possible because of the vineyard and its judicious management in the current and previous seasons. Which is true. However, I know that Molly Kelly, Penn State Enology Extension Educator, would justifiably argue that we harvest grapes with winemaking goals in mind. Which is also true. And I know that, in commercial situations, harvest date is often a mutual decision between the vineyard manager and winemaker; sometimes one person occupies both roles and, in other situations, the vineyard manager and winemaker work together to decide the optimal harvest date. I am not sure if the decision is easier to make as an individual or as a collaborative decision… this opens up psychological debate which goes far beyond my expertise. I’d ask enologists and winemakers to be forgiving of the text written herein, as it may be biased toward the vineyard and I may show ignorance about many important enological considerations for harvest decisions.

Destemmed Cabernet franc.

Harvest decisions are a consequence of several factors, some predictable and many not, that are unique to each commercial situation.

Harvest decisions are a consequence of experiencetheoretical knowledgeseasonal vineyard management practices, and numerical and sensory-based measurements that are put into action under the constraints of site, cultivar, post-veraison weather patterns, labor availability, stylistic winemaking goalsvineyard acreage and winery tank space, and current inventory of wines. Read that previous sentence again; it is a lot. And, while thorough, it still falls short of exhausting the number of potential factors that may impact harvest decisions. So, what can we predict for sure about harvest decisions and timing in Pennsylvania? With rare exception (site limitation, vintage effect, wine stylistic goal), Chardonnay will be harvested before Petit Verdot. And, most grapes will be harvested between late August and late October. These are the limited number of “predictable outcomes”. “Unpredictable outcomes” may become more predictable with an increase in breadth of knowledge and experience, especially with the same cultivars grown on the same site. I wish there were more objective statements that could be made about ideal harvest decisions and parameters. But, “ideal” is dependent on several factors, including those words/phrases that are emboldened/italicized above. To read more about how each emboldened and italicized word/phrase from the first sentence of this paragraph may impact harvest decisions, see this file (I kept this text separate so as to reduce the length of this blog post):

Chardonnay ready to be harvested (photo courtesy of Rachael White).

Practical vs. abstract approaches to harvest.

Note that the use of the terms “practical” and “abstract” is not meant to impose partiality to one approach or the other, nor to suggest that approaches to harvest only exist in these two forms or only in contrasting extremes. But rather to highlight how harvest decisions can be vastly different and dependent on values and goals. The practical-minded grower may error on the “safe side” by picking early to get rot-free fruit in the winery that also may have a lower pH and is likely to be microbially stable; the “risk” here may be picking fruit at less than optimal maturity for a targeted wine style. In contrast, the abstract-minded grower may error on the “riskier side” by picking based on sensory observations to make wine with fruit at peak maturity; the “risk” in this case may be that fruit rot and pH have increased as a consequence of “extended hang time”. Can one be practical and achieve optimal fruit maturity? Absolutely; theory and practice can come together in the real world. And, “optimal fruit maturity” is highly subjective and dependent on winemaking style goals. However, the combination of variables mentioned above (site, cultivar, post-veraison weather patterns) will affect the chances of theory and practice coming together in the form of rot-free fruit that has balanced chemistry and flavors such that little amelioration of the must is necessary before fermentation commences. The more odds that are against us (lack of experience, poor vineyard management, poor cultivar match with site, adverse weather patterns, labor scarcity, etc.), the more difficult it will be to achieve targeted wine styles, regardless if one is more abstract- or practical-minded. Most stakeholders likely have a balance of practical and abstract approaches to harvest in an attempt to achieve fiscal sustainability (e.g. by avoiding extreme bunch rot and crop loss) and to produce wines with recognition that round out the offerings of various wine styles in the tasting room. Winemaking “intervention” is necessary when harvested fruit is over-ripe, under-ripe, or falls short of the ability to achieve the intended wine style.

Destemmed Petit Manseng.

Take home.

Harvest decisions are complex and sometimes difficult. And justifiably so – lots is at stake. You want the best possible outcome (balanced, pleasant wines) from the crop you have worked hard to cultivate and maintain free from diseases and other pests all season long. Aim for the greatest quality each vintage. The successful approach to achieving quality starts during the dormant and growing season by practicing good viticulture. Use every advantage you have to make judicious harvest decisions. Be able to predict your harvest schedule but allow for some flexibility and be prepared to change plans as circumstances change. There are constraints in every vintage. As the number of adverse situations (rainfall, bunch rot, poorly drained site, suboptimal vineyard management) increase, there is greater need to take a pragmatic approach to harvest. Be practical in dealing with constraints to make the best harvest decisions possible; this requires the use of past experiences, knowledge of viticulture and enology theory, and an understanding of how current season viticulture management, weather patterns, and numerical and sensory observations will impact crop quality and the ability to achieve your targeted wine style. Acknowledge limitations of your cultivars grown on your site and be prepared to be responsive to the weather patterns of the immediate past and future.

The Winemakers Research Exchange, funded by The Virginia Wine Board, has published a newsletter with several Harvest Reminders. You can access this newsletter by clicking here: http://www.winemakersresearchexchange.com/august-2020-harvest-reminders-1

Chambourcin ready for processing (photo courtesy of Rachael White).

Hopes for a great 2020 vintage.

My sincere best wishes to all for a wonderful 2020 harvest! May the weather be dry with just enough intermittent rainfall to keep canopies photosynthetically active yet limit downy mildew and rot development.

All the best to each of you!

Vineyard herbicide drift webinar – review

On August 12, 2020, Dr. Bruce Bordelon, Professor of Small Fruits and Viticulture at Purdue University, presented “Fundamentals of Herbicide Drift in Vineyards” as part of the PSU Wine and Grape Team’s Weekly Webinar Series.

Bruce discussed several aspects of herbicide drift in vineyards, including causative chemicals, detrimental weather patterns that exacerbate chemical drift and volatility, common vine herbicide injury symptoms, differences in cultivar susceptibility to herbicide injury, and productive actions after herbicide injury has been identified in the vineyard. Bruce was kind enough to provide his presentation for reference. The link to the presentation can be found here:

All the best to everyone for dry weather throughout harvest of the 2020 vintage!

A Balanced Harvest

On Wednesday, August 19, 2020, Molly Kelly, Enology Extension Educator, Penn State Extension, and Gill Giese, Assistant Professor and Extension Viticulture Specialist, New Mexico State University presented A Balanced Harvest live webinar.

Gill Giese covered applied physiology of grape maturation; vineyard factors that impact grape ripening; relating vineyard to winery (grapes to wine); practical aspects of harvest.

Molly Kelly introduced BSA (berry sensory assessment), a codified system of describing grape composition, providing a common language of grape ripening and grape maturity assessment.

Please see links to both presentations below, including a written transcript.

Fungicide resistance management – webinar review

by Michelle Moyer, Viticulture Extension Specialist, Washington State University, and Cain Hickey, Viticulture Extension Educator, Penn State University

On June 24th, Dr. Michelle Moyer gave a webinar presentation entitled Fungicide Resistance and the Acronyms. In this webinar, Michelle discussed the nuances of managing fungicide resistance through judicious application of fungicides. Fungicide resistance management is optimized through good fungicide stewardship. Good fungicide stewardship includes rotating fungicide modes of action (within season and between seasons), timely application of  fungicides (before disease symptoms appear), using tank mixes that effectively and broadly manage a targeted fungal disease, and proper calibration and maintenance of spray application equipment to ensure adequate coverage of the target (grapevine canopy and/or clusters).

It is important to know that some fungicide classes are at greater risk of It is important to know that some fungicide classes are at greater risk of resistant development than others. For example, Fungicide Resistance Action Committee, or FRAC, code 7 (SDHI) and 11 (QoI)  fungicides are “high risk”, while FRAC code 3 (DMI) fungicides are currently considered “moderate risk”; but fungicide resistance to these classes have been found in several grape fungal pathogens. While good fungicide stewardship (see above for resistance management considerations) is important for all fungicides, it is important to be judicious when applying fungicide FRAC codes with moderate to high risk of resistance development. Please see Michelle’s slides for more information about fungicide resistance and its management:

Remember that chemical management of fungal diseases is only one part of an integrated management system. Cultural practices, such as canopy management and appropriate cultivar selection are also major contributing factors to a successful fungal disease management program.  

For additional information on fungicide resistance management in wine grapes, please visit: https://framenetworks.wsu.edu/ 

Spotted Lanternfly Management in Vineyards 2020 – Webinar Recap

by Heather Leach, Extension Associate, Penn State Entomology 

Introduction 

Spotted lanternfly (SLF) populations are expected to be high in 2020 and some growers have already reported significant populations of nymph SLF in their vineyards. Remember that SLF feed on over 70 different plant species and will be present throughout the landscape, including areas surrounding your vineyard. All life stages of SLF except for the egg masses actively feed on plant sap (phloem). To date, grapevines have been the hardest hit agricultural commodity by SLF with ~30% of growers in southeastern PA reporting vine damage (dead vines, reduced yields) from this insect.  

Summary of 2019 Data 

In 2019, we learned more about the phenology and behavior of SLF in vineyards. Some of that work has recently been published, see here: https://link.springer.com/article/10.1007/s10340-020-01233-7  

The key takeaway points:  

-SLF are present in vineyards as nymphs, but typically not very numerous. The most problematic life stage is the adults, which are very numerous and most problematic in vineyards from late August-October.  

-SLF is an edge-pest with unequal spatial distribution within a given block. The “hot spots” with SLF typically stay that way for the majority of the season. Scout your vineyard regularly and learn these “hot spots”.  

– SLF flight activity is highest in the afternoon, and populations may increase on the vine throughout the day  

-After harvest, adult SLF begin feeding more heavily on the trunk (as compared to the shoots and cordon earlier in the season). When applying an insecticide, make sure you’re hitting the target.  

-Based on preliminary data, high levels of SLF feeding (200 per vine for ~5 weeks) reduce photosynthesis, transpiration, brix values, and cold hardiness (cv. ‘Riesling’). 

2020 Research Plans 

-Feeding trials to evaluate the damage that SLF causes to vines will continue  

-Additional insecticide trials, including testing new active ingredients, evaluating residual efficacy in-field, and application methods will be looked at 

-Landscape-scale control of SLF utilizing the biopesticide containing Beauveria bassiana is being evaluated in large trials in natural areas and in smaller trials adjacent to vineyards 

-Exclusion netting, flight-intercept traps, and utilizing tree-of-heaven will also be investigated for their use in vineyards 

Management 

-Specific questions about management of SLF on your vineyard can be directed to Heather Leach, hll50@psu.edu. Additionally, please review this factsheet for more information: https://extension.psu.edu/spotted-lanternfly-management-in-vineyards  

-The best control of SLF to-date in vineyards is the use of repeated broad-spectrum insecticides. Key pre-harvest products that are often relied on by growers for SLF management include the active ingredients zeta-cypermethrin, carbaryl*, malathion, dinotefuran, and thiamethoxam. For post-harvest, the longest lasting chemical is bifenthrin (which has a pre-harvest interval of 30 days). *Note that carbaryl has been reported as ineffective against SLF by some growers – this is being investigated for 2020.  

-There are no established action thresholds for SLF. The “best guess” approximation is ~15-20 nymphs/vine and 5-10 adults/vine. Please keep in mind that this is observation-based, not science-based. This threshold likely depends on other factors including pre-existing stress of the vine (e.g. presence of disease), age of the vine, and many other factors.  

-To avoid resistance (with SLF and other important vineyard pests like fruit flies), rotate chemical classes.  

-This year, we are seeing flare-ups of secondary pests (leafhopper, mealybug, mites) which may be attributed to the increased use of broad-spectrum insecticides for SLF control. Keep an eye on other pests in your vineyard.  

Please see this link for a PDF copy of the presentation on SLF detection and management in vineyards (from the June 17th webinar):

Mid to late season control of downy and powdery mildew and bunch and sour rots in 2020

by Bryan Hed, Grape Pathology Research Technologist, Penn State, Lake Erie Regional Grape Research and Extension Center

Rainfall is always a key factor in any discussion of grape disease development in the eastern US. After all, most of the major pathogens we do battle with every year are fungal in nature, and fungi are favored by wet, humid conditions. For that reason, 2018 was a disastrous season for many grape growers in Pennsylvania, when boatloads of rain fell almost everywhere below Interstate 90. In that year, many growers suffered heavy losses to fungal disease, making it one of their worst crops ever. Last season began looking a bit like a replay of 2018, but a relatively dry harvest season renewed our faith in the possibilities for premium wine grape production in Pennsylvania. At the time of writing this post, a look at the twenty-something NEWA weather stations across PA, (newa.cornell.edu) shows it’s been shaping up to be a drier season in many places, cutting growers a much-needed break so far. In fact, rainfall was below average in many Pennsylvania locations during May, and June rainfall – though heavy in some locations – has been defined by just three rain periods (June 3-5, 10-11, and 19-20) with lots of dry weather, and opportunities for protective sprays to be applied, in between. But current rainfall amounts only tell part of the picture. Some vineyards may still harbor fair amounts of overwintering inoculum for diseases like downy mildew and black rot, requiring that growers remain vigilant with regard to their disease control sprays, particularly when invested in the ultra-susceptible varieties of Vitis vinifera in the southeastern part of the state. In those cases, fungal diseases are still a serious threat, even if conditions remain relatively dry.  

Most of us are past bloom and have already applied sprays for fruit protection at ‘immediately before bloom’ and ‘first post bloom’. If you applied these sprays diligently, not stretching intervals beyond 14 days (10 days is even better), using best materials, spraying every row with maximum coverage, you made your way through the most critical period for fruit protection. In this blog I’ll talk about sprays beyond that critical period; mid-late season, when we continue the battle to control downy and powdery mildew on leaves, and bunch and sour rot on fruit. As always, I’ll borrow information from past blogs so as not to try to reinvent the wheel.

Downy mildew

The pathogen that causes downy mildew is dependent on wet conditions; without a wet plant surface, no infection takes place. Fortunately, the fruit of most varieties are resistant to direct invasion by this pathogen by about 3 weeks after capfall. In other words, a developing berry is only susceptible to direct penetration of the pathogen from about the time the flower cap comes off (at the beginning of bloom) to about 3 weeks later. However, the cluster stems may remain susceptible for a couple weeks or more after fruit are resistant, and for this reason, fruit loss can continue to occur from cluster infections 4 or 5 weeks after capfall. Once we get past the critical sprays for fruit protection (which we always apply, rain or shine!), scouting for the distinctive white ‘downy’ sporulation on the undersides of leaves and on cluster stems is very important, and yields valuable information with regard to future need to spray (Figure 1). Growers of susceptible varieties do well to keep closely monitoring their vineyards for active sporulation and use that information in combination with the DMCast model on NEWA (http://www.newa.cornell.edu/) to determine if and when infection periods have occurred or will occur. 

09082000-0075
Figure 1. Late summer leaf symptoms of downy mildew (Niagara) showing ‘blocky’ lesion development and discoloration on the top side (above) and ‘downy’ white sporulation on the underside of a grape leaf (below). The late summer leaf blotches can differ dramatically from the yellow ‘oil spot’ symptoms that are observed in spring.
09082000-0079

Leaves will remain susceptible all season, though they do become less susceptible as they age. For this reason, the limiting or elimination of new shoot growth by veraison, through good nutrient and/or canopy management, can help to reduce the supply of susceptible tissue in the vineyard during ripening, and make post veraison control of this disease more manageable. I have gone into vineyards in late August-early September and observed that downy mildew was largely present on new shoot growth, but not on mature leaves at older nodes. There were two reasons for this: i) new shoot growth is more susceptible than older, mature growth, and ii) new shoot growth, unless just sprayed, is unprotected or less protected by previous fungicide applications. Symptoms on mature leaves in late summer may appear different from those on young leaves in early spring (Figure 1).  

The sight of active, white sporulation on green vine tissues means the downy mildew pathogen is capable of spreading quickly under wet conditions, and that sprays for downy mildew should continue, especially for susceptible varieties. Even humid nights that result in heavy dews by morning, can continue to fuel downy mildew development, generating fresh sporulation that can spread the disease rapidly when plant surfaces are wet. If you let downy mildew get out of control, it can strip vines of their leaves and in the worst cases, effectively end fruit ripening for the year, and shoot ripening for next year’s crop. Your grapevines go into winter dormancy in poor condition, and are more vulnerable to damage by severe cold, leading to crown gall and expensive trunk renewal the following season, with little or no crop to pay for it; all that stuff is connected, so you want to keep downy mildew under very tight control, especially on Vitis vinifera.

Chemical control: Your list of chemical control options will start to dwindle as we get within 66 (Mancozeb products, Ridomil MZ), 42 (Ridomil copper), 30 (Ranman, Reason), 21 (Ziram), and finally 14 (Revus, Revus Top, Zampro) days of harvest. In the end you’ll be left with Captan (which will also control ripe and bitter rot), copper, and phosphorous acid products (0-day pre-harvest interval), which have their own shortcomings, discussed below. 

Products like Ridomil (the mefanoxam component), Ranman, Reason, Revus/Revus Top, Phos acid products, and Zampro, are more rainfast than the surface protectants (like copper, mancozeb, ziram, and captan) but contain chemistries that are prone to the development of resistance. Therefore, they should not be used to put down an epidemic, which will only speed up the resistance development process. Even phosphorous acid products can be lost to resistance through repeated applications on a diseased vineyard, so keep downy mildew well under control. The resistance prone materials (Ridomil, Ranman, Reason, Revus/Revus Top, Zampro, Phos Acid products) are best used to maintain a clean vineyard, NOT to put down an epidemic. Conversely, the surface protectants would be least risky in terms of the development of resistance and can be an effective means of controlling downy mildew late into the growing season. Just be aware of seasonal limits, so plan ahead as best you can.  

Here are some precautions to consider with use of the ‘old standard’ protectants: 

  • Some insecticides and oils should not be applied with Captan.  
  • There is the concern for plant injury by copper applications, which will be exacerbated by application under slow drying conditions and application to wet canopies (for example, don’t make applications to dew covered canopies in the early morning). The addition of lime to the application raises the pH of the spray solution and reduces the chances for plant injury.  
  • Consider that copper is poisonous to yeasts and that excessive copper residues at harvest can interfere with fermentation, and wine stability and quality. Unfortunately it’s impossible to predict how high residues will be on fruit at harvest; that’s going to depend on the copper formulation (some of the newer coppers utilize lower copper concentrations), rate of material used, number and timing of applications made, spray coverage, and amount of rainfall from application to harvest. I am not aware of any information that establishes a nice, clean cut-off date or pre-harvest interval for avoiding excessive copper residues at harvest, but I have heard that cutting off copper use about a month before harvest may be sufficient in most cases.  
  • There is also evidence that late Captan sprays can delay fermentation and have negative effects on wine quality but the consequences seem less severe and irreversible than those associated with copper use.  For more on this, consider this online article by Dr. Annemiek Schilder, former fruit pathologist at Michigan State University: https://www.canr.msu.edu/news/late_season_fungicide_sprays_in_grapes_and_potential_effects_on_fermentatio 

If you are protecting a non-bearing, young vineyard from downy mildew (you’re not selling/harvesting a crop), you can continue to use mancozeb products to control downy mildew past the 66-day pre-harvest interval. You can also consider using mancozeb after harvest to keep canopies clean of downy mildew and ‘firing on all cylinders’ until that first frost. The longer your vines can continue to produce and store carbohydrates after harvest, the better prepared they’ll be to withstand winter cold without damage (and the crown gall that follows).  

Powdery mildew

In contrast to downy mildew, the fungus that causes powdery mildew is dependent on rainfall only for the initial release of spores in early spring. There is no requirement for plant wetness beyond that, which is why this disease is a problem even in dry climates like California. Once primary spores (ascospores) land on a susceptible grape surface, they germinate and form colonies that grow across the plant surface (Figure 2), sucking resources from the plant and producing secondary spores (called conidia) that are now spread by wind/air currents. The disease spreads rapidly under ideal conditions of high humidity, cloudy skies, and warm (but not hot) conditions. During the mid-late summer period, every day can be a powdery mildew infection period.  Fortunately, just as for downy mildew, fruit of most grape varieties becomes resistant to powdery mildew after about 3-4 weeks past capfall, although native varieties like Concord may be resistant just 2-3 weeks after capfall. Nevertheless, it’s especially important to tightly control the disease on fruit of V. vinifera and susceptible hybrids up to 4 weeks after bloom, as even tiny amounts of powdery mildew infection on fruit (amounts you can’t even see) can leave microscopic breaches in the berry skin, and increase the susceptibility of that fruit to bunch rots later near harvest.  

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Figure 2. Greyish-white colonies of powdery mildew on the upper surface of grape leaves.
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At four weeks post capfall the focus for powdery mildew control shifts to keeping leaves clean, especially on V. vinifera, for many of the same reasons expounded in the section about downy mildew (ensure optimal ripening of fruit and shoots/canes, ensure optimal cold hardiness, more effectively and more easily manage fungicide resistance, etc). But there is another very important reason, demonstrated by some excellent research conducted by Wayne Wilcox and others at Cornell University, who showed that controlling powdery mildew up to about Labor Day can reduce overwintering inoculum and disease pressure the following spring. Why Labor Day? When powdery mildew infected leaves die by that first hard frost in fall, the mildew on those leaves also dies…UNLESS it has had time to form fully mature, winter resistant structures called chasmothecia. These are the tiny, dark, period sized (.) structures you observe in heavily infected tissues in late summer. If the chasmothecia do not have time to fully mature before the leaf dies (as we would expect from infections that occur after Labor Day), they will not be tough enough to survive the dormant period (winter) and will not contribute to the bank of primary inoculum that infection periods draw upon the following spring.  Knowing this, a grower can continue their spray program up to Labor Day, to control the ‘size’ of the powdery mildew problems he/she will potentially face next spring. Once again, this is most important if you are growing Vitis vinifera and less important for growers of native varieties like Concord and Niagara.  

Chemical control:  Fungicides like Quintec, Vivando/Prolivo, Torino, Endura (note that the price of Endura is significantly lower in 2020), Luna Experience/Sensation, Miravis Prime, Cevya (V. vinifera only; read the label) and other sterol biosynthesis inhibitors (tebuconazole, tetraconazole, difenoconazole products) can be used for additional post bloom applications to control powdery in early/mid-summer in vineyards that have maintained tight control of the disease (‘clean’ vineyards). Remember to limit the use of any one FRAC group to 2 applications per season (the FRAC group designation is on the label). 

However, where disease control has been less than ideal, and especially for late summer applications (August and September), options for powdery mildew control should emphasize materials that are less risky in terms of the development of resistance, like formulations of sulfur on varieties that are not sensitive to it (which could also be tank mixed with the aforementioned synthetics to manage resistance). The more expensive, micronized formulations of sulfur will generally perform better (more effective per pound, more rainfast) than the cheaper, wettable powder formulations, especially for V. vinifera (you get what you pay for). Also, according to the 2020 New York and Pennsylvania Pest Management Guidelines for Grapes, “sulfur activity is strongly influenced by formulation, rate, frequency of application, and weather”; that is, higher rates and shorter spray intervals will provide better control than lower rates and longer spray intervals.  Do not apply sulfur within 14-21 days of an oil application.  

Sulfur residues on fruit at harvest have been correlated with increased hydrogen sulfide and sulfurous off-aroma formation during fermentation, and so growers will need to phase out sulfur applications at some point well before harvest. This is mostly a problem for wines that are fermented on the skins (reds) where most of residues at harvest are found. Work published by Dr. Misha Kwasniewski showed that “sulfur residues are likely of low concern in white wine making, especially when juice is clarified before fermentation. However, residue levels in red fermentations (fermented on skins) can exceed levels associated with increased hydrogen sulfide production when some sulfur sprays are applied within 8 weeks of harvest” (Kwasniewski et al. 2014). However, in that same study, ceasing sprays no later than 5 weeks before harvest, resulted in sulfur residues that were below a concentration consistently shown, in previous literature, to increase hydrogen sulfide production. Therefore, for reds fermented on the skins, one may need to end sulfur applications a month or two before anticipated harvest date, to avoid increased H2S production during fermentation. Another strategy can include early-mid summer sulfur applications with micronized formulations (that are longer lasting/more rainfast) and making the later sulfur applications with a wettable powder formulation that is more quickly reduced by weathering. 

Other late season options include coppers, potassium salt sprays like Nutrol, formulations of potassium bicarbonate, and horticultural oils. Keep in mind that the later oil applications are made, the greater the chance they will negatively impact ripening. Therefore, the current recommendation is to discontinue oil sprays as you near veraison. As for coppers, the same concerns that were mentioned in the section on downy mildew, apply here as well. These materials can be a good way to maintain decent control of powdery mildew on leaves, while minimizing the chances for the development of resistance to the riskier materials used earlier in summer. 

Botrytis bunch rot control; a recap

Hopefully premium wine grape growers have applied fruit-zone leaf removal to open their fruit to better sunlight and aeration and better pesticide penetration. Research has repeatedly shown that there are great benefits of this practice in our wet, humid climate. It is one of the most effective cultural treatments for reducing the susceptibility of the crop to disease of all kinds (especially bunch/sour rots (Figure 3)), and improving coverage, and therefore efficacy, of fruit protection sprays. Though the benefits of leaf removal may generally be reduced the later it is applied after fruit set, it’s not too late yet. However, there is a greater danger of sunburn on your fruit the later this leaf removal is applied. For this reason, mid-late summer leaf removal may be best confined to the east or north side of the trellis (depending on row orientation), especially in areas where very hot mid/late summer temperatures are expected. Fruit zone leaf removal can be mechanized and is probably performed most effectively on vines trained to a vertical shoot positioned (VSP) or some other two-dimensional trellis system with a relatively focused and narrow cluster zone.  

Fungicide sprays for bunch rot are mostly for Botrytis and there are many products to choose from: Vangard, Inspire Super, Switch, Rovral/Meteor, Elevate, Endura, Luna Experience/Sensation, Fracture, Miravis Prime, and the strobilurins (Flint, Pristine, Intuity). Flint and Pristine will also control ripe/bitter rot. Use them sparingly (once or twice per season) and always rotate FRAC groups with every application. Also, pay close attention to pre-harvest intervals on each label. A Botrytis specific fungicide at full bloom and pre closure, especially in varieties with very compact clusters, can be extremely important to reduce ‘latent’ Botrytis infections that continue to accumulate throughout the ‘green’ berry development period. The next fungicide application for Botrytis is made just before or at veraison, as fruit begin to soften and skins become thinner and more easily penetrated by fungal pathogens like Botrytis. After veraison, fruit are also more likely to become injured by birds, insects, excess moisture/humidity (rain cracking), and overcrowding of berries in tight clusters. Botrytis fungicides can protect intact fruit surfaces and may help to reduce the spread of Botrytis rot on fruit, even after they have become injured.  

Finally, a Botrytis fungicide application about 2-3 weeks after veraison, especially under wet weather conditions, can reduce further rot development during the last stretch of ripening.  However, Botrytis fungicides control Botrytis and will not provide protection against sour rot organisms that often destroy fruit of overly compact clusters, despite the application of a full Botrytis fungicide program. 

Figure 3. Botrytis bunch rot (above photo) and sour bunch rot (below two photos) in Vignoles.
A close up of a fruit tree

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A group of fruit and vegetables

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Sour rot

Work conducted by Dr. Megan Hall, a former graduate student of Wayne Wilcox at Cornell University, demonstrated how additional pesticide applications during the latter stages of ripening (beginning around 15 brix) can significantly reduce the development of sour rot. Her work has shown that there is a close connection between fruit flies and sour rot development, and that the presence of the flies is important to the accumulation/generation of acetic acid in rotting fruit. Treatments composed of weekly, tank mix applications of an insecticide (to control the flies) and an antimicrobial (to kill bacteria) have been found to reduce sour rots by 50-80% over unsprayed vines. The best results appear to occur when weekly sprays are initiated just before sour rot symptoms are observed (preventive sprays before about 15 brix).  

Good canopy and fertility management is essential to maximizing the effectiveness of your pesticide applications: make sure shoots are tucked and spaced within catch wires and that shoots have not flopped over to block spray penetration into the fruit zone. Also, as stated above, limiting shoot growth after veraison with good fertility management will limit the supply of new green tissue that is hyper susceptible to powdery and downy mildew and will make late season management of these diseases more effective.    

For further reading on this and many other disease management topics, refer to the 2020 New York and Pennsylvania Pest Management Guidelines for Grapes. If you don’t have a copy, you can get one through Cornell University press. Every commercial grape production operation should have one!  

References:

Kwasniewski, M. T., Sacks, G. L., Wilcox, W. F. 2014. Persistence of elemental sulfur spray residue on grapes during ripening and vinification. Amer J of Enol and Viticulture 65: 453-462. 

Vineyard nutrition – webinar review

by Tony Wolf, Professor and Viticulture Extension Specialist, Virginia Tech, and Cain Hickey, Viticulture Extension Educator, Penn State Extension

For a limited time, Penn State Extension is offering free online ...

On June 10, 2020, Dr. Tony Wolf presented Assessing and Managing Vine Nutrient Status in Mature Vineyards as part of the PSU Wine and Grape Team’s weekly webinar series. Tony discussed perspectives on why vineyard nutrition is important, how vineyard nutrition can be assessed, ways to optimize vine nutrient status. Tony reviewed nitrogen management considerations in vineyards that maintain cover crops on the entire vineyard floor and also discussed the popular topic of potassium nutrition (as it relates to wine pH issues). What follows is a brief review of the points discussed. Use this blog post, the information in Tony’s slides, and the Wine Grape Production Guide for Eastern North America as information resources to guide nutrient management in your own vineyard. Here are Tony’s slides from his presentation on June 10:

Assessing vine nutrient status. Soil tests reveal mineral nutrients status of the soil, which may be available for vine uptake. Plant tissue tests reveal mineral nutrient status of the vines themselves and are indicative of the mineral nutrients that are, or are not, effectively taken up from the soil. While plant tissue analysis is most revealing of actual vine nutrient status, both soil and plant tissue tests are important to assess in order to understand and effectively manage vineyard nutrition. Visual observations can also be used to help diagnose mineral nutrient imbalances; however, plant tissue tests should be used to objectively confirm those visual diagnoses. As an extension publication from Rutgers points out, leaf reddening can be caused by numerous issues, including nutrient imbalance or systemic infections. Note that it is not uncommon for young vines to display foliar mineral nutrient deficiency symptoms as their root systems are shallow and small and are thus ineffective at accessing some of the soil nutrients, particularly during drought conditions. Similarly, foliar mineral nutrient deficiency symptoms may also be more common in vines planted on vineyard sections with “lean, rocky soils” (e.g. on highly sloped hillsides, or at the apex of a convex landform).

The two common growth stages to sample grapevine tissues for nutrient analyses are at bloom and “mid-summer,” or veraison. Assessment of vine nutrient status is timely right now as many PA vineyards are at bloom, or will be at bloom, in the near future. It is important to notify the analytical laboratory about the stage at which tissues were sampled as tissue nutrient concentrations change over the course of the season. For example, petioles at bloom generally contain greater nitrogen concentration compared to petioles at veraison.

Bloom (top) and veraison (bottom) are two common growth stages at which grapevine tissues are sampled for nutrient analysis.

Petioles (leaf stems), leaf blades (lamina), or whole leaves (petioles + blades) are all examples of grapevines tissues that can be sampled and submitted for mineral nutrient analysis. It is thus also important to notify the analytical laboratory about the specific tissues (e.g. petioles, blades, whole leaves) being submitted as sufficiency ranges vary between tissues. Historically, petioles have been the most common grapevine tissue used for nutrient analyses in the United States; thus, interpretation of petiole analyses are based on a comparatively larger database relative to leaf blades or whole leaves (petioles + blades). There are several analytical laboratories that will quantify tissue mineral nutrient contents, including the Penn State Agricultural Analytical Services Lab.

Wholes leaves (top left), petioles (top right), and blades (bottom) are grapevine tissues that can be sampled and submitted for nutrient analysis. Note: petioles have historically been the most common grapevine tissues submitted for nutrient analysis in the United States.

Correcting vineyard nutrient imbalances. Optimal soil pH for grapevines ranges from 5.5 to 6.5, which corresponds to the soil pH at which many soil macronutrients and micronutrients are available for plant uptake. Correction of soil pH is more effective before planting, when amendments such as lime can be disked and incorporated into the soil. After a vineyard is established, several commercial fertilizers, lime, and other amendments are available to correct grapevine nutrient imbalances. Soil mineral nutrient analysis, grapevine tissue mineral nutrient analysis, soil type, and vine age should all be taken into consideration when developing a plan to correct vineyard nutrient imbalances.

Nitrogen and under-trellis cover crops. The use of cover crops and grasses in vineyard alleyways is a common practice in order to prevent soil erosion and accommodate tractor and ATV traffic. However, more recently, growing cover crops underneath the vine trellis is becoming common in an attempt to combat vine vigor in humid regions that promote vine vigor through supra optimal water and nutrient availability. Further, as vineyards are established on steeply sloped sites (15 to 20%), under-trellis cover crops provide a means to prevent soil erosion on these particularly prone sites. One potential drawback to under-trellis cover crops is their competition with vines for available nitrogen, and potentially other soil mineral nutrients. Too much resource competition could result in greater vigor reduction than desired, which could ultimately result in poor perennial vine health, decreased crop yields, and premature vine decline. To be brief, the importance of monitoring vine nutrition may be heightened when under-trellis cover crops are used; while nitrogen may be particularly important to monitor, cover crops may also compete with other essential mineral nutrients such as phosphorous. Find more details on cover crops and nitrogen nutrition in Tony’s slides (attached above in this post).

Under-trellis cover crops in a Cabernet Sauvignon vineyard in Virginia. Under-trellis cover crops can help regulate vine vigor. However, under-trellis cover crops may increase the importance of monitoring vineyard nutrition as they can compete with vines for mineral nutrients such as nitrogen.

Potassium and wine pH. With some exceptions (e.g. young vineyards, high pH soils), soil potassium is generally in sufficient quantities and readily available for uptake in eastern US vineyards. Potassium is an important macronutrient and is needed for many physiological and metabolic processes in plants. Thus, potassium deficiency should be avoided. Potassium has a (somewhat weak) positive correlation to grape, must, and wine pH. Due to potential drawbacks of high wine pH (color and microbial instability), excessive vine potassium concentration is undesirable. Limited management tools are available to moderate potassium uptake. However, using rootstocks with V. berlandieri heritage (e.g. 420-A, 100-R), limiting canopy self-shading, and avoiding planting on sites with excessive soil potassium are a few ways to limit potassium concentrations in vine tissues, including harvested grapes. See Tony’s slides (above) for more detail on potassium nutrition. Also, see previous outreach literature on potassium nutrition from Tony Wolf and Michela Centinari.

Please contact Tony (vitis@vt.edu) or Cain (viticulture@psu.edu) if you have further questions about vineyard nutrition. Best wishes for dry and warm weather during bloom and fruit set… and beyond!

Canopy fruit zone management – fundamentals and webinar review

by Cain Hickey, Viticulture Extension Educator, Penn State Extension and Michela Centinari, Assistant Professor of Viticulture, Penn State Department of Plant Science and Penn State Extension, and

For a limited time, Penn State Extension is offering free online ...

On June 3, Cain Hickey and Michela Centinari reviewed the fundamentals of canopy fruit zone management as part of the PSU Wine and Grape Team’s weekly webinar series. Here are the slides from that webinar.

Bloom has started in southeast PA which means bloom will commence in northwesterly PA regions in the coming week or so. Fruit zone leaf removal is timely when implemented during late bloom into the BB-size or pea-size berry growth stages. Below is just a brief review of fruit zone management fundamentals. Use the words below in tandem with the attached slides for a review on how fruit zone management can be effectively implemented in your own vineyard management program.

Well-exposed Merlot clusters at pre-harvest.

Bunch rots. Leaf removal is unnecessary to manage bunch rots in arid/dry regions like eastern Washington and California, but fruit zone leaf removal aids in bunch rot management in PA and other humid growing regions in the eastern US. Leaf removal is more critical to manage rots in rot-prone cultivars relative to those that are less prone (see below about “prioritization of leaf removal”). The slides provided (above) show several cases in which bunch rot was reduced in exposed relative to shaded clusters. The photo of the Cabernet Sauvignon cluster, below, was taken after harvesting that cluster from a deeply shaded canopy; this is a good visual that shows excessive shade can result in rot development.

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This Cabernet Sauvignon cluster has poor color development and Botrytis bunch rot. It was harvested from a shaded canopy. This photo summarizes the effects that shaded fruit zones can have on rot and wine quality potential in humid regions.

Fruit composition. In general, exposed clusters have enhanced “varietal character,” meaning that increased radiation and temperature enhance some of the “positive” sensory impact compounds associated with a particular cultivar. Exposed clusters can have reduced “negative” sensory compounds, such as those associated with undesirable, herbaceous aromas in some Bordeaux reds (e.g., Cabernet Franc). Well-exposed clusters also tend to have lower acidity than those ripened in shade. Thus, one must consider winemaking goals and use fruit zone management to target grape compositional attributes that can increase the chances of achieving the desired wine style. As an extreme example, acid preservation is key for sparkling wine production while color and aroma enhancement, in addition to lower acidity, may be desirable for the production of some white and red wines. In these two unique cases, fruit zone management (in tandem with harvest date) can be used to target the desired compositional attributes.

The shaded (left) and exposed (right) sides of the same two clusters. This photo exemplifies how radiation can change the color (and likely the primary and secondary metabolite profile) of grape berries. Musts composed predominantly of grapes similar to those in the left photo will likely be good for sparkling or “fresh” wine styles while those made primarily with grapes similar to those in the right photo are predicted to produce less acidic, more aromatic wines.

As with bunch rot management, leaf removal may be more critical to enhance color in red grapes grown in the eastern US compared to drier, warmer regions of California, eastern Washington, and southern Oregon. In fact, there are several cases in the attached slides (above) where exposed red grapes had greater anthocyanins and phenolics than shaded red grapes; those studies were conducted in humid growing regions. Studies conducted in hotter and drier regions in the western US have shown that well-exposed red grapes had reduced anthocyanin levels compared to shaded grapes, and this was particularly true on the “hot” or “afternoon sun-exposed” canopy side (e.g. on the west side of north/south-oriented rows). The variable cloudiness experienced in humid, eastern US growing regions attenuates the radiant heating of exposed grape clusters; clouds act as “large leaves” that preclude radiation penetration to the fruit zone. Fruit zone leaf removal thus may be considered a less “risky” practice in the eastern US, where radiation, heat, and air movement around the fruit zone can result in rot-free grapes with a desirable and balanced primary and secondary metabolite profile.

When, how, and to what extent? Common questions regarding optimal leaf removal practice are “when should I remove leaves?”, “how should I remove leaves?”, and “how many leaves should I remove?”.

When? In general, leaf removal is ideally implemented between late bloom and pea size berries. Implementing leaf removal between these growth stages will aid in spray penetration to, and air movement through, the fruit zone at the beginning of the critical period for cluster disease protection (bloom through bunch closure). Pre-bloom (or “during bloom”) leaf removal to excessive amounts (e.g. removal of greater than 4 leaves per shoot) can cause a precipitous decrease in crop yield; therefore, it is judicious to moderate the extent of leaf removal when it is implemented before or during bloom. Implementing leaf removal before pea-size berries will allow berries to acclimate to ambient radiation outside of the canopy starting at an early stage of berry development. That said, some remedial leaf thinning between bunch closure through veraison, and even after veraison, will create a microclimate that is less favorable to rot development and one that will improve wine quality potential. In many cases, a “follow-up” leaf removal pass, or two, will be needed after the initial effort in order to maintain a favorable fruit zone microclimate as lateral shoots and leaves continue to grow throughout the growing season in a humid climate.

Pea-size berries, an arbitrary the growth stage terminus for the implementation of “early season leaf removal”.

How? Leaf removal has been traditionally performed manually by removing leaf blades and petioles and lateral shoots from the fruit zone by hand. Manual leaf removal is still more common than mechanized leaf removal. However, “air pulse” and “cutting” types of tractor-mounted, mechanized leaf removal machines are becoming popular in eastern US vineyards. In fact, a few vineyard and winery enterprises in PA employ the use of a mechanical leaf removal machine. Mechanical leaf removal offers the advantage of practice efficiency, which results in leaf removal implementation across sizable acreage within a narrow time-frame. While speculative, and variable on a case-specific basis, mechanical leaf removal may be worthwhile in vineyards larger than 10 or 15 acres, as upfront cost of the machine may be offset in only a few growing seasons (assuming that leaf removal is viewed as a meaningful practice and is consequently manually implemented across the whole vineyard). It is safe to say that upfront costs will be recouped faster with increasing vineyard size; it would take several growing seasons to offset the upfront costs of a mechanized leaf removal machine in a 5-acre vineyard.

To what extent? In general, leaf removal to an average of one or two leaf layers surrounding the fruit zone should offer some rot management and fruit composition benefits relative to unmanaged fruit zones. In general, removal of two or three leaves per shoot from the fruit zone region will produce an average of one or two fruit zone leaf layers. Leaf removal to greater extents (as in the Merlot photo, below) can reduce rots, increase anthocyanins and phenolics, and reduce acidity, collective traits that may be desirable in the production of some red and white wine styles. Fruit zone leaf removal to such magnitudes (zero leaf layers) may not be commercially feasible due to the amount time and labor required to do so.

A Merlot fruit zone with zero leaf layers from a RESEARCH (not commercial) plot. While such fruit zones may improve spray penetration and rot management and produce less acidic grapes with desirable primary and secondary chemistry, commercial implementation to such extents would be difficult.

Refinement for fruit zone leaf removal prioritization. Fruit zone leaf removal is a vineyard practice used to manage cluster diseases, fruit composition, and crop yield. Like other vineyard practices, the effectiveness of fruit zone management is optimized when it is refined by region and cultivar. Since manual leaf removal (by hand) can be laborious, refining practice by prioritization may help define situations where leaf removal is most needed. Consequently, leaf removal implementation across several acres may not seem overwhelming. As an extreme example, and as mentioned above, fruit zone leaf removal should be a greater priority for fruit quality production in humid relative to arid/dry climates. The climate in southeast PA is humid, subtropical while the climate in northeast and northwestern PA is warm-summer humid, continental. Thus, while rot management is important across PA, one could make a case that rot is particularly pervasive in southeast PA and therefore leaf removal for rot management may be of greater importance in southeast PA relative to northwestern PA. However, leaf removal is still an important tool for improving fruit composition under the cooler conditions and shorter growing seasons typical of north and central PA regions.  

Leaf removal may be “high priority” in white-berried, tight-clustered, thin-skinned, vinifera cultivars, that are highly susceptible to rot. Leaf removal may be “lesser priority” in many hybrid cultivars but “greater priority” in vinifera cultivars. Leaf removal could be prioritized based on phenology. For example, Chardonnay breaks bud and blooms early; therefore, it may be judicious to perform leaf removal earlier in Chardonnay (or Merlot) relative to Petit Verdot or Cabernet Sauvignon. Leaf removal may be unnecessary in cultivars like Norton, Chambourcin, and Charodnel. This is not to say that fruit zone leaf removal should be completely avoided in “low priority” situations. But developing a plan based on cultivar rot susceptibility and/or wine quality potential “gained” will help determine where to commence seasonal leaf removal and where it may be okay to wait for a couple weeks.

While some PA regions are warmer than others, all are humid and are characterized by variable rainfall and cloudiness throughout the growing season. Source: https://upload.wikimedia.org/wikipedia/commons/c/c3/US_50_states_Köppen.svg

Practical “pros and cons”. A practical disadvantage to fruit zone leaf removal is that it requires labor, time, and/or money, regardless if manually or mechanically implemented. Bird and vertebrate pest pressure is variable across sites. However, when clusters are exposed, pests can easily target their bounty. Therefore, the need for pest management and bird netting may be heightened when leaf removal is implemented to the point where clusters are easily visualized. Harvesting visible clusters is much easier and efficient than harvesting clusters surrounded, and hidden, by foliage. Therefore, a benefit to leaf removal is the ability to efficiently harvest; some may make a final “leaf removal pass” nearing the harvest date for this reason.

Due to leaf removal and the subsequent ability to visualize clusters, harvest will be more efficient from the forefront fruit zone relative to the background fruit zone.
Bird netting and other pest management measures may be important to prevent depradation and crop yield loss.

Please contact us (Michela – mzc22@psu.edu; Cain – viticulture@psu.edu) if you have questions related to fruit zone management or other vineyard management practices.

Best wishes on finding a suitable and effective fruit zone management plan. And best wishes for warm and dry weather throughout bloom and fruit set… and beyond!

Grape Disease Management Reminders: Bloom Through Bunch Closure – webinar review

by Dr. Mizuho Nita, Virginia Tech Grape Pathology Extension Specialist and Cain Hickey, Penn State Viticulture Extension Educator

Clusters are within the critical period for cluster protection at the pea-size berry stage

On Wednesday, May 27, Dr. Mizuho Nita presented the Grape Disease Management Reminders: Bloom Through Bunch Closure webinar as part of the Penn State Wine and Grape Team’s weekly webinar series. The webinar focused on optimal grape fungal disease management practices between bloom and bunch closure.

It is important to manage for Botrytis bunch rot during bloom and berry touch stages

The critical period for grape cluster fungal disease protection is between bloom and bunch closure, when grape clusters are highly susceptible to many pervasive fungal diseases. Powdery mildew, downy mildew, black rot, botrytis, ripe rot, and other fungal diseases can infect flowers and young berries. Integrated management options to optimize the control of these diseases during the early grape cluster developmental stage were covered. Please find a PDF version of Mizuho’s presentation here:

And access and follow Mizuho’s grape pathology blog here: http://grapepathology.blogspot.com.

Thanks for the presentation, Mizuho!

Michela Centinari and Cain Hickey will present the Canopy Fruit Zone Management webinar on Wednesday, June 3. This webinar will outline fruit zone management fundamentals in a humid climate and share data to help growers refine their practices. Register for this webinar here: https://bit.ly/2TruM7H.

Best wishes for dry and warm weather in the coming weeks to limit fungal disease pressure and optimize fruit set and crop potential in your vineyards!