MSU Extension Grapes

Weather

The last week started hot with highs in the upper 80s in northern areas and lower 90s in southern Michigan. A cold front came through on Friday, July 10, bringing storms to much of Michigan. Most areas saw a half to 1 inch of rain. It also brought cooler temperatures for the weekend and start of this week, with highs in the 70s and lows in the upper 50s. The heat is returning with temperatures again near 90 by the weekend. A weather front on Wednesday into Thursday will the best chance of rain this coming week.

With the variable week, we still picked up a lot of growing degree days (GDD) last week: 150-180 GDD base 50. The southwest region is 345 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 buckshot berry and many are at berry touch. Tight cluster varieties such as Vignoles are at bunch closure. Growers have been doing leaf removal and continuing to do shoot positioning on most vinifera varieties. This allows sprays to reach the clusters more easily, increases air circulation and improves sunlight exposure. 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 or near buckshot berry, depending on the variety. Growers are starting to perform leaf removal while they are tucking shoots in the catch wires. See this chart for grape growth stages.

While the rains over the weekend provided a brief break, the hot, dry weather the last several weeks dried many soils around the state. Southwest and southeast Michigan are the most impacted at this point. Growers that have irrigation systems in their vineyards should inspect them to be ready if they start seeing drought stress. With the hot weather continuing for at least the next week, irrigation might be necessary soon, especially on young vines and vineyards on lighter soils.

Symptoms of drought stress will follow a general progression. Tendrils will first start to dry out, followed by shoot tips falling downward and leaves bending. Under severe drought stress conditions, shoots stop, tendrils will dry up and shoot basal leaves will turn yellow.

Diseases

At this time of year, the disease focus for most grape growers around Michigan is on downy mildew and powdery mildew management. Powdery and downy mildew leaf and cluster infections have been observed statewide.

Downy mildew requires wet leaves to develop. Rain is the most common source for leaf wetness. As little as 0.5 inches 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. During buckshot berry, downy mildew infections that occurred in previous weeks may become visible, especially with enough moisture. Fungicides applied more than one week post-bloom protect against defoliation but have limited control over fruit infections.

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.

For more information on both powdery and downy mildew, see “Protecting young clusters from powdery and downy mildew” from MSU Extension. For an overview on the general diseases and management options, see “Early season vineyard disease management” from MSU Extension.

Black rot cluster damage is showing up in several research blocks and some commercial vineyards in southern Michigan. Black rot develops when rains trigger spore releases, typically in the spring. The timing for berry rot management is from immediate pre-bloom until three to four weeks after bloom. This window is closed for juice and hybrid grapes and rapidly closing for vinifera grapes in southern Michigan.

Several grape varieties are at bunch closure and now is an excellent time to consider controlling botrytis bunch rot, particularly in vinifera and hybrid grapes where it can be an issue. 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. Effective systemic fungicides for botrytis are in FRAC codes 2, 7, 9, 11, 12 and 17. A new Michigan Grape Fact Sheet is now available for managing botrytis bunch rot for more information about the disease.

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. Several vineyards scouted in southwest Michigan have had 10% of clusters infested during the first generation, some with multiple larvae per cluster. Moth catches are still low, but we are now finding young larvae in clusters, indicating that second generation has emerged and are laying eggs. First sprays for grape berry moth should have gone on last week or the start of this week for much of southern Michigan. Start of egglaying is expected within the next week for much of northern Michigan.

Wild grape bloom is used as biofix for the grape berry moth model predicting timing for insecticide sprays. This biofix is recorded as 50% of the clusters on wild grape are at 50% bloom. For For 2020, approximate wild grape bloom dates are June 2 in Berrien, June 4 in Van Buren, June 5 in Allegan, June 11 in Ottawa and June 12 in Grand Traverse counties. The date can be entered into the grape berry moth model in Enviroweather to predict when egglaying will start for the second generation of berry moth.

Using the approximate bloom dates above, the start of egglaying July 4 in Berrien, July 6 in Van Buren, July 9 in Allegan, July 9 in Macomb, July 13 in Ottawa, July 18 in Grand Traverse and July 20 in Emmet counties. This highlights the timing when growers should be starting to think about protecting clusters from the second generation of this pest.

Once egglaying has begun, there are a number of insecticides that can be used to prevent grape berry moth infestation, but their timing is critical to give the best performance. Insecticides that affect eggs should be applied at the beginning of egg laying at 810 GDD47 after wild grape bloom. Intrepid and Altacor are examples of these. Insecticides that are best against berry moth larvae should be applied when eggs start hatching. This is 100 GDD47 after the start of egglaying (910 GDD47). The first larvae are predicted to start hatching in the southwest around July 13 in Allegan County, and in the southeast around July 12 in Macomb County.

Keep a close eye on the grape berry moth model for your area to know when to make applications and to select products that will target the correct life stage. There are a number of chemical options that target larvae. For vineyards that saw significant damage last year or already have seen damage this year from grape berry moth, a second spray targeting larvae should be applied two weeks after your first spray. For more detailed information on choosing the right insecticide, see “Mid-season management of grape berry moth” from MSU Extension.

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.

Japanese beetles and potato leafhopper have both been reported throughout the state. Potato leafhoppers feed on leaves. Some sensitive grape varieties can get yellowed and cupped leaves, and stunt growth in young vines. Most mature vines are tolerant of this damage and control isn’t often warranted. 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.

Foliar infestations of grape phylloxera have been found in vineyards over the last month. Reducing root infesting phylloxera is the main reason rootstocks are used. Hybrid and juice grapes are significantly more tolerant of root infestations. Hybrid grapes, however, are more susceptible to significant foliar infestations of this insect. Vines can withstand a lot of leaf damage. If there is a high infestation, Movento is an effective insecticide for phylloxera. This should be applied to the canopy at the first sign of infestation. Movento needs to penetrate the waxy leaf surface, so application with a penetrating adjuvant is essential to get control.

Treatment recommendations for these insects can be found in the MSU Fruit Management Guide (E154).

Upcoming meetings

Our grower meetings have moved online. We are using Zoom for our meetings. Here is more information on Zoom.

In place of Viticulture Field Day, we are having Viticulture Virtual Field Days - Webinar Series. This will be a week of webinars from 12-1 p.m. during July 27-31. Registration is now open!

See also

• Michigan Grape Scouting Report – July 8, 2020
• A Mobile Guide for Grape IPM Scouting in North Central and Eastern U.S.
• Southwest Michigan Fruit Update – July 14, 2020
• Northwest Michigan Fruit Update – July 14, 2020
• East Michigan Fruit Update – July 14, 2020
• Managing grapevine leafroll disease
• Michigan pesticide applicators receive temporary emergency license extension
• Online review now available for private pesticide applicators
• Weekly fruit IPM meetings start April 13 and are now online
• Agriculture support labs still open for business, with modifications
• Early season vineyard disease management
• Disease control in grapes critical during and after bloom