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Michigan State University Grape Update

Southwest Michigan

Michigan State University Extension — Soils continue to be very dry. The U.S. Drought Monitor is indicating the entire southwest Michigan region is in a moderate drought condition. Even with the rain the last couple days, expect this drought to continue. In these conditions, small and young fruit crops are seeing stress and leaves are wilting. For larger trees with deeper roots, stress is less visible but nutrient uptake by the roots near the surface is still compromised. If this drought continues, expect small fruit and/or additional fruit drop.

Vinifera bloom is finishing, Juice grapes and early hybrid fruit are continuing to increase in size. Some juice grapes are nearing berry touch at advanced sites.

Rose chafer populations remain high. This beetle emerges from the ground this time of year. Last week’s rain event likely softened the soil enough for them to emerge at once.

Tumid gallmaker reports continue to come in. This is a fly that causes red splotching and galls on leaves, shoots and developing clusters. They are primarily a pest of hybrid wine grapes but have been seen on Concord grapes in the past as well.

The biofix for the second generation of grape berry moth should be next week. Predictions are for between July 4 and 6 depending on location and when wild grape bloom was recorded at your location. Michigan State University Extension recommends biofix as good timing for application of an insecticide with ovicidal activity to manage grape berry moth populations. These include insect growth regulators like Intrepid and diamides like Altacor and Verdepryn. Other products that have larvicidal activity should be applied 100 GDD base 47 later. This time of year, that is typically around a week after biofix.

Juice and early hybrid grape bloom ended around three weeks ago. This is when developing fruit start becoming resistant to many of the spring diseases. We start focusing on foliar infections of downy mildew and powdery mildew. Treatments for black rot should end soon. Protections against cluster rots like botrytis bunch rot should continue until bunch closure.

For vinifera grapes, fruit infections from all the major diseases should be managed for at least the next three to four weeks until the fruit become resistant. This is especially true around rain events like the last few days and the predicted rains later this week.

Photo by Esmaeil Nasrollahiazar, MSU Extension

Northern Michigan

Moving to northern Michigan, significant developments are seen in the vineyards. Clusters on all vinifera cultivars are expanding. The shoot length varies depending on the cultivar, with later budburst varieties like Cabernet Franc and Merlot ranging from 6 to 10 inches. In contrast, earlier budburst cultivars like Chardonnay, Pinot Noir and Sauvignon Blanc measure between 15 and 20 inches. In the Tip of the Mitt region, hybrid cultivars are in the immediate pre-bloom phase, characterized by a well-developed inflorescence and displaying approximately 30% flowering. Vinifera varieties in the region are experiencing growth between 10 to 16 inches.

Horticulture

Shoot thinning and trunk cleaning should be completed in the southern vineyards and underway in many varieties in the northern vineyards. Shoot thinning plays a crucial role in canopy management, offering multiple benefits such as enhanced air circulation, reduced disease susceptibility, minimized shading, improved spray penetration, and ultimately elevating fruit quality during the harvest period. It is advisable to perform shoot thinning when the shoots reach a length of 5-12 inches, ensuring they are still easily breakable from the woody tissues.

For more comprehensive information on shoot thinning and other early season vineyard management practices, please refer to the article “Early season vineyard management” from Michigan State University Extension.

Diseases

During this time of the year, the primary diseases of concern for grape growers are phomopsisblack rotanthracnose and powdery mildew. If you’re seeking detailed insights into pre-bloom fungicide options and the effects of rain on disease spread, I recommend referring to a grape scouting report from earlier or exploring an article on early-season disease management. It’s worth noting that some growers have recently observed isolated cases of downy mildew infections in vineyards.

As bloom continues in southwest Michigan, start choosing fungicides that control all the fruit diseases. For example, with downy mildew we are most concerned with fruit infection at this time, and sprays should be timed prior to bloom and at bloom for optimal control. Downy mildew is caused by a fungal-like organism, so many site-specific systemic fungicides that target other spring diseases do not work on downy mildew. Effective fungicides for downy mildew include products in FRAC codes 4, 11, 21, 40 and 45 as well as phosphorus acid salts and some biologically based products.

Except for powdery mildew, these spring disease infections typically require rain events. It only takes 0.1 inches of rain above 50 F to trigger a possible infection. Viticultural practices that reduce canopy wetness such as good irrigation timing, leaf removal and good weed management can reduce many of these diseases in a vineyard. Typically, DMIs (FRAC 3), captan and EBDCs (FRAC M3) are effective for phomopsis, black rot and anthracnose.

Insects

First generation larvae of grape berry moth have been detected in grape clusters in recent weeks in southwest Michigan. Based on the degree day model for this pest, the start of the second generation egglaying is expected in early July in southwest Michigan, proceeding later into July at further north locations. Also, the first sightings of a grape tumid gallmaker infestation and rose chafers were detected last week in southwest Michigan.

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