MSU Extension Grapes

Weather

This past week was cooler. Southwestern Michigan had highs in the mid- to upper 70s. Southeastern and northern Michigan were 5 to 10 degrees cooler. The remnants of Tropical Storm Cristobal brought rain and wind across the state on Tuesday night and Wednesday, June 9 and 10. A second system brought scattered but severe storms on Thursday and Friday. Total rainfall amounts were from a half-inch to an inch and a half with the heaviest amounts in the west central portion of the state.

Warmer temperatures will return this week. Expect highs in the 80s and lows in the 60s for much of the state. Winds should be light most of the week. The best chance of rain will be on Saturday, June 20, into Sunday.

With the cooler week, we still picked up a decent number of growing degree days (GDD) last week: 130-170 GDD base 42, and 80-115 GDD base 50. The southwest region is 375 GDD base 42 and 250 GDD base 50 ahead of the northwest region.

Vine growth

Early hybrid varieties such as Marquette, Frontenac and Chancellor are past bloom and moving to buckshot berry stage in Berrien County as of Monday, June 15. Juice grapes in southwestern counties are at or past full bloom. Vinifera grapes are starting to bloom, especially early varieties such as Chardonnay and the Pinot family.

In northern vineyards, shoots are at 10-16 inches of growth, depending on variety. Clusters are elongating and separating. Many are at immediate prebloom. Wild grape is in bloom. See this chart for grape growth stages.

Diseases

At bloom, choose fungicides that control all the fruit diseases. These include downy mildew, phomopsis, powdery mildew and black rot. These diseases all infect new green tissue as it is exposed. Protecting clusters from downy mildew and black rot is critical at this time. With the rain expected on Saturday and Sunday, the risk for infection is high. The first few fungicide applications until three weeks post-bloom are the most important.

Phomopsis damage on canes and leaves has been reported by several scouts around the state. For phomopsis on canes, look for necrotic lesions that are dark brown to black. Black rot has just been seen this week at the Michigan State University Plant Pathology Farm in East Lansing, Michigan. Early signs of downy mildew on leaves (oil spots) were seen in Niagara at the Southwest Michigan Research and Extension Center and the Trevor Nichols Research Center.

Downy mildew can cause fruit infection and late season defoliation. During this time of year, we are most concerned with fruit infection and sprays should be timed prior to bloom and at bloom for optimal control. Fungicides for downy mildew include some strobilurins (e.g., Abound), phosphorus acid salts (e.g., Phostrol) and CAA-fungicides (e.g., Revus), among others.

In warm, humid weather, powdery mildew is a major concern. Infection can occur at temperatures from 59 to 90 degrees Fahrenheit, but 68 to 77 F are optimal. Unlike the other fungal diseases listed above, powdery mildew doesn’t need leaf wetness to infect, just humid conditions. Even if the ambient humidity is low, in the shaded canopy it is likely higher. Opening up the grape canopy to air movement helps reduce powdery mildew while also allowing better spray penetration. Shoot positioning and cluster zone leaf removal can help once bloom has finished.

Chemical products are also important tools for managing powdery mildew. Some of these include demethylation inhibitors (FRAC 3), succinate dehydrogenase inhibitors (FRAC 7), quinone outside inhibitors (FRAC 11), quinolines (FRAC 13), phenyl acetamide (FRAC U6) and benzophenone (FRAC 50). Also, biologicals and contact materials show efficacy against powdery mildew if applied before the infection occurs.

For an overview on the general diseases and management options, see early season vineyard disease management and the importance of bloom applications. 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. 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 catches have gone up sharply in the past week in southwest Michigan. Spraying for this first generation is generally not recommended because the populations are low. However, growers that had a difficult time managing grape berry moth last year should be scouting their vineyards to look for larvae in clusters. Some sites in southwest Michigan already have larvae feeding in the clusters, some with multiple larvae per cluster. At sites like this, growers might think of adding an insecticide with one of their post-bloom sprays. If you had a difficult time with grape berry moth last year or are seeing high levels of cluster infestation already, contact your local Extension educator to discuss whether an early management program is right for you.

Grape berry moth scouting can also be used to prepare for the later spray timings for generation two and three in July and August. Wild grape bloom is used as biofix for the grape berry moth model predicting timing for these applications. This biofix is recorded as 50% of the clusters on wild grape are at 50% bloom. For 2020, approximate wild grape bloom dates are June 2 in Berrien County, June 4 in Van Buren County, June 5 in Allegan County and June 11 in Ottawa County. 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.

Rose chafer and potato leafhopper have both been reported in southern and northern vineyards. Rose chafers can be found feeding on leaves and young clusters. Potato leafhoppers feed on leaves. Some sensitive grape varieties can get yellowed and cupped leaves and stunted growth in young vines. Most mature vines are very tolerant of their damage and control isn’t often warranted. Treatment recommendations for these insects can be found in the MSU Fruit Pest Management Guide (E-154).

Tumid gallmaker symptoms have been apparent in southern Michigan for a couple of weeks. It is now becoming more noticeable at sites in northern regions. Some emergence has been seen in these galls in southern Berrien County. They are more common in hybrids, but have also been found in Concord in Berrien and Van Buren counties. For small vineyards, hand picking and removing infestations before the flies have emerged can be an effective management technique.

Grape erinium mite has been observed on young leaves at a few vineyards in the Traverse City area. Established infestations are moving slowly onto the new leaves. These mites rarely reach economic infestation levels but can be confused with disease symptoms. Grape plume moth larvae have also been found on Old Mission Peninsula. These light yellow, hairy caterpillars fold leaves and create webbing between leaves and stems. Grape plume moths are rarely an economic pest, but localized infestations can occur when near a lot of wild grape.

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 Field Week online. This will be a week of webinars from 12-1 p.m. during July 27-31, 2020. More information to come.

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• Agriculture support labs still open for business, with modifications
• Early season vineyard disease management
• Disease control in grapes critical during and after bloom