From the Vineyard: Post-veraison Berry Shrivel and Discoloration

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."

Three different PA growers have sent pictures of shriveled and/or discolored grapes within clusters; all vineyards are located in southcentral or southeastern Pennsylvania. Two growers noticed pink berries (sometimes accompanied by shriveling) with green rachis tissues in their Cabernet franc vineyards. Another grower noticed shriveled berries with necrotic, brown rachis tissue in their Petit Verdot vineyard. See the photos, below (thanks to these growers for sharing the photos and giving permission to post!)

Cabernet franc. Photo credit: Zach Waltz

Petit Verdot. Photo Credit: Ben Cody

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.

• Distinctive symptoms differentiate four common types of berry shrivel disorder in grape (UC Agriculture & Natural Resources)
• Berry Shrivel – All the Same? (Washington State University)
• Tracking Down the Causes of Bunch Stem Necrosis (University of Minnesota)
• Berry Shriveling in Grapes: Late Bunch Stem Necrosis (University of Wisconsin)

The goal of this article 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. Based on my knowledge of the sites, it is likely that the Petit Verdot vineyard has greater water holding capacity (relatively flat; high organic matter and clay content) than the Cabernet franc (middle photo) vineyard (sloped, convex landform). Anecdotally, sunburn occurs more often under hot and dry conditions where water may be limiting (e.g. Cabernet franc vineyard) whereas BSN has been linked to excessive water availability during and after veraison (e.g. Petit Verdot 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. Note that, since the original writing of this article (on September 30, 2020), it became apparent that the middle photo of Cabernet franc was likely caused by BSN; symptoms can change over time – photos often capture only one time in symptom development. 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 seem 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.