Michigan grape growers face frost risk in the north

Michigan grape growers are moving into one of the most important periods of the season, with vines in the south advancing quickly toward bloom while northern vineyards remain behind and face a lingering frost risk, Michigan State University Extension said in its latest scouting report released Wednesday.

The report, based on field observations from May 20, said southwest Michigan has accumulated 472 growing degree days, with a forecast of 556.6, and that many vineyards there are now in early shoot growth, cluster separation and inflorescence development. In southeast Michigan, vines are at shoot elongation with flower clusters developing. In northwest Michigan, most Vitis vinifera vines are still pushing buds or just breaking dormancy, while in the Tip of the Mitt region hybrid grapes are at bud push and vinifera vines are only beginning to move.

That uneven progress matters because bloom is the key window for disease control, especially for black rot, powdery mildew and downy mildew. The extension team urged growers to scout carefully and protect open flowers with fungicides during this stage, while rotating FRAC groups to slow resistance. The report pointed to FRAC groups 3 and 11 as important tools for powdery mildew and black rot, along with multi-site protectants such as captan and mancozeb. For downy mildew, it cited FRAC groups 40, 45, 21 and 11, as well as phosphorous acid products, when weather favors infection.

The timing is especially sensitive in northern vineyards, where two cooler nights could bring frost or freeze conditions. Growers in those areas were advised to hold off on nitrogen applications and canopy work until active growth is established. In southern Michigan, by contrast, split nitrogen applications may begin soon, along with planning for shoot thinning in the next one to two weeks.

The report also said grape berry moth activity is high in southern Michigan after warmer weather, though egg laying by females is not expected yet. Growers were told to record the bloom date of wild grape species used in pest models later in the summer. MSU Extension stressed that the correct species is Vitis riparia, which blooms before Concord and has glossy leaf undersides.

Early-season pest pressure from flea beetles was described as generally tolerable, even though “window-pane” feeding damage is visible on new leaves. Tumid gallmaker symptoms were found on hybrid grapes this week, and the report said isolated infestations can be managed by removing affected tissue, while broader outbreaks may require treatment.

Weed control remains another priority as soils warm and winter annuals begin flowering or setting seed. The report recommended scouting vineyard floors now for winter annuals, summer annuals and perennial weeds, then choosing residual herbicides or burndown products based on whether weeds have already emerged. It also warned growers to follow label restrictions closely and avoid herbicide contact with green shoots and young vines.

Beyond crop protection, the extension team emphasized soil health and recordkeeping. It pointed growers to cover crops, compost use and reduced compaction as ways to improve long-term vineyard performance. It also highlighted the Sustainable Agriculture Management Tool, a free planning platform developed for Michigan grape growers that helps organize spray schedules and other vineyard tasks.

The report came with a calendar of industry events across the state, including P45 Third Thursday sessions beginning June 18 at Mawby Vineyard on compost tea applications, a July 16 session at the Northwest Michigan Horticulture Research Center on sap analysis and an Aug. 6 spray efficacy workshop there focused on coverage and drone-based sprayers. Registration is also opening for the Michigan Viticulture Field Day and Enology Experience and for the Dirt to Glass Conference in Traverse City on Aug. 20–21.