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Bunch Grape Chores – Summer 2019

UGA Extension Viticulture Blog

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Veraison is the transitional period of grape development when grapes begin to soften, and sugars and secondary flavor, color, and aroma compounds begin to accumulate. Grapes are said to be “ripening” starting at veraison. Knowing veraison may help you predict harvest date – especially if you have kept good records over time at your site. Since post-veraison weather patterns can drastically change ripening rate, we must monitor maturity and not strictly rely on records (see chore 4). Most vineyards in the southeastern U.S. are at veraison now or will be going through veraison in the next few weeks. The timing of veraison is largely dictated by region, the meso climate of your specific vineyard site, and cultivar. These grape chores will cover veraison through harvest.

My best wishes to all for a warm, dry post-veraison period and a successful finish to the 2019 vintage!

Canopy management

Canopy management is mostly left to remedial fruit-zone leaf removal and hedging at this point. It is never too late to do some selective leaf removal around the clusters to aid in drying and improve pesticide coverage on the fruit. However, leaf removal does little if the canopy is shading out the canopy and fruit-zone. Thus, hedging should continue in the vineyard until strong vegetative growth and canopy self-shading subsides, which is often not until several weeks after ripening has started.

Crop load management

Crop load management is commonly practiced before veraison starts. If you are going to thin your crop, focus your efforts on regions of the fruit-zone that are densely packed with clusters; touching clusters will often develop bunch rots due to the lack of pesticide coverage as well as reduced air movement between them. A past blog post on the UGA Extension Viticulture Blog went in to a bit more depth on when and why one might thin crop in their vineyard. I have seen what appears to be some heavy crops out there this year, likely due in no small part to full clusters as a function of the dry weather experienced during bloom and fruit set throughout much of the region.

Vine tissue sampling

Petioles and leaves are often sampled at bloom, but vine tissue sampling can be conducted again at veraison. While petioles and leaf blades are sampled from opposite the clusters at bloom, these tissues are sampled from the youngest fully expanded primary shoot leaves – often near the top of the now-hedged shoot. Sampling tissues at veraison allows one to target “nutritional problem areas” of the vineyard that were otherwise not evident at bloom, when the soils were at field capacity and nutritional symptoms were not evident in unstressed vines. Newly-planted vineyards will often show nutrient deficiency symptoms at this time of year; this is typically due to their shallow root systems having limited access to soil mineral nutrient resources. What follows is a brief commentary on the timing of tissue sampling follows, hopefully answering: “What would guide my decision for vine tissue sampling at one stage over the other?”

There are two phenological (growth) stages for which grapevine nutrient guidelines have been developed: bloom and veraison. Bloom, also called “flowering”, is the stage at which flower caps fall from the flower clusters. Veraison is the stage at which grape berries soften (white cultivars) or become colored (red cultivars) and transition to ripening. Since mineral nutrient concentrations fluctuate in vine tissues over the growing season, leaf blades or petioles should be sampled at bloom or at veraison to routinely monitor vine nutrient status. Sampling at bloom may allow time for nutrient adjustments to be made within the current growing season, although the success of post bloom fertilization depends on many factors including method of application, properties of material used and weather conditions. Sampling at veraison is better for diagnosing macronutrients such as nitrogen (N), phosphorus (P), and potassium (K) due to the high concentrations of these nutrients during bloom and their relative mobility within the plant.

Monitoring grape maturity

Monitoring grape maturity is important to determine the optimum harvest date. I would encourage that a combination of chemical and sensory evaluations be used to help determine when to pick. The chemical and sensory benchmarks that define maturity will vary based on cultivar and winemaking goals. For example, if I wanted to make a sparkling rosé out of my Merlot, I would aim for lower sugars and higher acids; the opposite would be true if I wanted to make a full bodied red wine. Soluble solids (Brix) is a ubiquitously used maturity benchmark, likely due to its relative low cost/ease of measurement. Barring special winemaking goals of sparkling wines rosés, and late harvest/dessert-style wines, most will look to harvest somewhere between 19-25 °Brix, and the accompanying acidity and pH will greatly vary by cultivar. Bear in mind that high pHs (i.e. > 3.8) can put the wine at higher microbial spoilage risk unless post-harvest amendments (e.g. acidulation) are made in the winery. As far as sensory evaluation of maturity goes, many will use the color of the seed, the flavor/aroma of white grapes, or the taste and “mouthfeel” of red grapes to aid in maturity evaluation.

Please consider the integrity/sanity of the crop as you monitor the quality. Keep in mind that rot tends to increase in severity as fruit maturity advances, and this is particularly true in some cultivars over others. For example, Blanc du Bois (BdB) is very rot prone, but tends to have good varietal character at relatively low Brix levels (in fact, BdB is a cultivar that does not generally attain high Brix levels). I just spoke with a BdB grower today. They picked their BdB at 17.8 Brix as rot was starting to come in. Chaptalization (sugar addition to unfermented grape must) will be used in the winery to achieve desired sugar concentration before starting fermentation. Please make educated decisions and balance the risk/reward of hanging your crop for extended periods, especially if persistent rain is forecasted.

It is important to monitor maturity separately for each cultivar. It is also important to sample grapes in an unbiased fashion by sampling from all parts of the cluster, and across a section of vineyard that represents a separate vinification. For example, samples from blocks A and B should be kept separate if they will be harvested and vinified separately – even if they are the same cultivar. However, care should be taken to sample across each block to best characterize the average maturity. This may best be done by creating a randomized sampling scheme for each block.

Scouting for fungal diseases and insects

Downy mildew, botrytis, and late-season rots often make unwelcomed appearance from now until harvest. It is important to protect your vines from downy mildew, powdery mildew, botrytis, and ripe, sour and bitter rot by using appropriate pesticides – especially when weather patterns are conducive to disease development. The post-veraison period is also a good time to scout for Pierce’s disease symptoms in your vineyard. Please reference the UGA Extension Bulletin entitled “Pierce’s disease of grape: Identification and management.”

For insects, Japanese beetles, grape root borers, grape berry moths, and spotted wing drosophila are of primary concern in the post-veraison period. I would encourage everyone to scout for and manage these insects accordingly. Read labels and take head of long pre-harvest intervals (especially products containing mancozeb).

Please reference the viticulture management poster (English) or (Spanish version)  for reminders about what diseases and insects are important to manage and scout for throughout the ripening period. This poster will be made available at regional grape and wine-related workshops and conferences and can also be obtained by contacting your local county extension agent.

Take inventory of, and deploy, wildlife deterrence equipment

You know your vineyard and are thus the best judge of historical wildlife pressure. I have personally seen both birds and raccoons take more than their share of crop from vineyards – please do not underestimate the amount of crop that can be lost to wildlife. Bird netting and other scare devices should be going up in vineyards all over the southeastern US over the next month or so. Netting is the best strategy to manage bird pressure. Traps, exclusion tactics, and other vertebrate control measures should be deployed if necessary.

Take inventory of harvest shears, lugs / bins, and other harvest supplies

Harvest will be here before we know it; in fact, it has started in some early cultivars in the piedmont and coastal plain regions. Please be prepared to pick your entire crop exactly when you or your customer wants. Don’t be caught short handed with supplies when you need them most. Make sure tank space is ample to accommodate the amount of crop that will be vinified.