MSU Extension Grapes

Weather

Last week’s weather was much like the week before. For most of the week, highs were in the 70s in northern grape areas of Michigan and in the mid-80s in southern areas. Sunday, July 26, was hot and humid with highs in the low 90s in southern Michigan and mid-80s further north. Much of the state was dry all week except for a set of storms that crossed the northern part of Michigan on Wednesday, bringing up to 2 inches of rain. The next week will be cooler and dry, with highs in the mid- to upper 70s statewide. Little to no rain is predicted for the next week.

With the variable week, we still picked up an average number of growing degree days (GDD) last week: 145-170 GDD base 50. The southwest region is 395 GDD base 50 ahead of the northwest region.

Note: Wild grape bloom dates are estimates. See “Mid-season management of grape berry moth” for bloom dates.

Vine growth

Grapes in southern Michigan are at berry touch or bunch closure. Early hybrids such as Marquette are beginning veraison. Many growers have been doing cluster zone mechanical leaf removal, continuing shoot positioning on most vinifera varieties and starting vine hedging. This allows sprays to reach the clusters more easily, increases cluster temperature and air circulation and improves sunlight exposure.

By directly reducing leaf layers in the fruiting zone, leaf removal improves the microclimate around the clusters, improving fruit quality. Hedging removes excessive shoot growth from the top and sides of the canopy preventing shading and entanglement of shoots between rows, making it easier for tractors to drive through the vineyard.

Now is the time to accurately estimate vine yields. With a good estimate you can determine if, when and how much fruit you should remove at thinning time. The Michigan State University Extension bulletin “Predicting Harvest Yield in Juice and Wine Grape Vineyards (E3186)” provides tools to reduce both annual yield and quality variability among years.

In northern vineyards, most vinifera are at berry touch or bunch closure. Growers are performing leaf removal while they are tucking shoots in the catch wires. See this chart for grape growth stages.

This is the time of year to collet petiole samples for nutrient analysis. Petiole samples for foliar tissue analysis should be collected from mid-July to mid-August.

Diseases

For an overview on the general diseases and management options, see “Early season vineyard disease management.” At this time of year, the disease focus for most grape growers around Michigan is on downy mildew and powdery mildew management. Powdery mildew leaf and cluster infections are becoming common statewide. Downy mildew infections, which have been visible for several weeks, are beginning to make leaves appear necrotic. Black rot and Phomopsis cluster damage have been seen for several weeks throughout the state. Unfortunately, the window to control these diseases has closed for most varieties in Michigan.

Downy mildew requires wet leaves to develop. Rain is the most common source for leaf wetness. As little as 0.5 inch of rain is enough, but warm nights with high humidity and heavy morning dews such as those most have experienced in the last few weeks are also good for development. Fungicides applied later than one week post-bloom protect against defoliation but have limited control over fruit infections. As we get later into the season, please review this article for information on late season downy mildew control.

Unlike the other grape fungal diseases, powdery mildew doesn’t need leaf wetness to infect, just humid conditions. Even with low ambient humidity, the humidity inside the shaded canopy is higher. Opening up the grape canopy for air movement helps reduce powdery mildew and allows better spray penetration. Shoot positioning and cluster zone leaf removal can help. For more information on how weather conditions affect powdery mildew development, see “Why some seasons are worse for powdery mildew” from Good Fruit Grower.

With several grape varieties at bunch closure, now is an excellent time to control botrytis bunch rot. This is particularly important in vinifera and hybrid grapes. Several strategies contribute to good botrytis bunch rot management. These include opening up the canopy, properly applying fungicides and, using resistant cultivars when possible. Good botrytis control depends on getting good coverage. Fungicide resistance management is important for botrytis control. The most effective products for botrytis are site specific and prone to resistance development. A new Michigan Grape Fact Sheet is now available for managing botrytis bunch rot.

Now is a good time to scout your vineyard for suspicious issues related to other stressors. Many of these problems can be abiotic including nutrient related issues and graft incompatibility. However, some of the biotic problems can be caused by grapevine trunk diseases or grapevine viruses. If you are interested in taking samples and testing, you can send them to MSU Plant & Pest Diagnostics. Please see these articles on grapevine trunk disease and sampling for grapevine viruses.

A complete list of fungicides for all these diseases with efficacy and other details can be found in the E154 Michigan Fruit Management Guide from Michigan State University Extension. Check the guide for potential phytotoxicity of certain sprays on Concord grapes especially (this has been particularly noted for fungicides like Revus Top). Phytotoxicity risk is higher with high temperatures and quickly growing vines.

Insects

Note: Thoroughly scout before assuming insects are an economic problem.

Grape berry moth populations are active this year with the usual variation among vineyards, highlighting the importance of scouting. In southwest Michigan, we are now finding mature larvae from the second generation in clusters. Egglaying of this generation should have started last week for much of northern Michigan. Third generation egglaying is expected to start next week for much of southwestern Michigan, and activity from this pest is expected to continue through the coming weeks.

Growers with problems from grape berry moth last year should scout vineyards and see how sprays applied in July have worked. For high pressure sites, apply an application to protect from the third generation. For selective insecticides such as Intrepid and Altacor, time applications for egglaying, which will occur at 1,620 GDD from wild grape bloom. For broad-spectrum insecticides, delay the next application until 1,720 GDD since the young larvae are the target and this gives the new eggs time to hatch. Since the third generation activity becomes more or less continuous through August and into September, scout to determine if a follow-up application is required approximately two weeks later.

For more detailed information on choosing the right insecticide, see: “Mid-season management of grape berry moth.” Since grape berry moth attacks berries in the clusters, good spray coverage is critical for control. Be sure to use enough water to penetrate the canopy and cover the clusters. A spreader/sticker can also help cover the confined spaces inside the canopy. Unless clusters are exposed, it will be essential that sprays get up under the layers of leaves, so sprayers should be calibrated and nozzles aligned appropriately.

Potato leafhoppers and grape leafhoppers are at low levels but starting to develop populations, and should be scouted during weekly vineyard visits.

Japanese beetles continue to be reported throughout the state, with some hot spots especially near irrigated turf. Japanese beetles feed between the leaf veins causing a lace-like skeleton on heavily damaged leaves. They seldom feed directly on clusters. Japanese beetles are generally only considered a pest of new vineyards where vines are more sensitive to leaf feeding.

Treatment recommendations for these insects can be found in the MSU Fruit Pest Management Guide (E-154).

Upcoming meetings

Grower meetings have moved online. MSU and many other organizations are using Zoom for meetings. Here is more information on Zoom.

This year’s Viticulture Field Day in southwest Michigan has moved online. Instead of a full day program, the event will include five days of lunchtime webinars, each highlighting one MSU researcher’s viticulture work. The event will be held July 27-31 from 12-12:45 p.m. each day. View details and registration information.

The grape grower organization, Parallel 45, is hosting a webinar series this summer. Registration for any/all webinars in their series can be found at 2020 Northwest Michigan Viticulture Webinar Series. At the next scheduled webinar on Aug. 7 from 12-1 p.m., a representative from Enartis will be presenting on "How to improve quality in underripe and compromised fruit with enological tools.”

Related resources

• Michigan grape scouting report – July 22, 2020
• Southwest Michigan Fruit Update – July 28, 2020
• East Michigan Fruit Update – July 28, 2020
• Michigan spotted wing Drosophila update – July 28, 2020
• A Mobile Guide for Grape IPM Scouting in North Central and Eastern U.S.
• Managing grapevine leafroll disease
• Michigan pesticide applicators receive temporary emergency license extension
• Agriculture support labs still open for business, with modifications
• Early season vineyard disease management
• Mid-season management of grape berry moth
• Late season downy mildew control
• Do you suspect grapevine trunk disease in your vineyard?