Frühjahrsfröste schädigen Weinberge in Michigan

Michigan grape growers are heading into the season with a mixed picture after two spring freeze events damaged vines in parts of the state, while cooler weather has slowed development in the north and kept disease and pest pressure under close watch.

Michigan State University Extension said in its latest scouting report that southern vineyards have already seen frost and freeze damage, including losses of primary shoots in some locations. In southwest Michigan, a freeze on April 7 brought low temperatures in the mid-20s, with primary bud damage ranging from 0% to 90% depending on site. A second frost on May 2, with temperatures in the upper 20s, caused additional injury in some low-lying areas, where more than 75% of primary shoots were lost in certain blocks. Other vineyards were not affected.

The report said the impact is still being assessed, especially after the May 2 event. In damaged areas, secondary shoots have begun to grow, but growers are still determining how much crop loss they will face this year.

The cold weather has also slowed vine growth across northern Michigan. In northwest Michigan and the Tip of the Mitt region, most Vitis vinifera vines remain dormant or at very early stages, with little or no visible bud swell. Some pruned vines have begun to bleed sap, a sign that dormancy is ending, but MSU Extension said bud break may be delayed if cool conditions continue.

In southern Michigan, where some vineyards are farther along, juice grapes and early hybrid wine grapes are generally at 3 to 6 inches of shoot growth. Clusters are separating and beginning to elongate. Late hybrids and vinifera varieties are mostly between first leaf and about 1 inch of shoot growth. In southeast Michigan, juice grapes are moving into shoot elongation near the Ohio border and reaching bud burst in cooler areas. Hybrid varieties around Romeo are at stages ranging from full swell to bud burst.

The report said early-season disease risk is rising in southern Michigan, especially for phomopsis cane and leaf spot and black rot. Anthracnose, downy mildew and powdery mildew are also concerns. MSU Extension said rain events can trigger infections for several of these diseases, with black rot needing only 0.1 inches of rain above 50 degrees Fahrenheit to create a possible infection period. The agency urged growers to keep protective fungicide programs in place and to reduce canopy wetness through practices such as careful irrigation timing, leaf removal and weed control.

Insect activity has also started. The first grape berry moth males were trapped last week in southwest Michigan vineyards, though MSU Extension said that finding does not yet require action because females have not emerged to lay eggs. Flea beetle damage has been seen at some sites, and growers were reminded that the action threshold is 2% of buds damaged. Bud swell is also the key time to consider control for grape erineum mite in northwest Michigan vineyards that have had problems in past seasons.

The report emphasized that management decisions now should focus on early disease protection, shoot thinning once growth reaches about 5 to 12 inches, and nutrient planning based on soil tests and petiole analyses. In southern Michigan, nitrogen applications may begin in split doses where vines are already growing. In northern regions, MSU Extension advised waiting until active growth is established before applying nitrogen.

The agency also pointed growers to its Enviroweather growing degree day map for tracking vine development. As of the report date, southwest Michigan had accumulated 327.4 growing degree days on a base of 50 degrees Fahrenheit, compared with 97.8 in northwest Michigan and 97.9 in the Tip of the Mitt region.

MSU Extension said it has moved this year to a biweekly scouting schedule after feedback from growers who wanted more site-specific observations and practical guidance. The report is part of a broader effort by the university’s grape team to provide updates on weather trends, vine development, pest pressure and disease risk across Michigan’s wine regions as the season unfolds.