Michigan grape scouting report – June 21, 2023

Most of Michigan will experience a hot and dry week.

A grape vineyard.
Photo by Esmaeil Nasrollahiazar, MSU Extension


Click below for detailed seven-day forecasts for various grape production regions.




Report on growing degree days (GDD)

Southwest Michigan GDD summary from March 1 through June 20, 2023
Southeast Michigan GDD summary from March 1 through June 20, 2023
Northwest Michigan GDD summary from March 1 through June 20, 2023

The following table summarizes the GDD 50 degrees Fahrenheit for the current week and the previous week, as well as the accumulated GDD for each location:


Current GDD 50 F

GDD 50 F last week

Collected the past week

Benton Harbor (SWMREC)












Average for southwest Michigan








Average for southeast Michigan




Old Mission








Traverse City (NWMHRS)




Average for northwest Michigan




Based on the new data collected, the following report provides an update on the GDD values for grape growing regions in Michigan: Southwest Michigan, including Benton Harbor, Fennville and Lawton, Michigan, experienced a significant increase in GDD values compared to the previous week, indicating favorable growing conditions for grape cultivation. The average GDD for this region increased from 671.5 to 779.2, with a weekly increase of 107.7 GDD. Specifically, Benton Harbor had a GDD increase of 112, Fennville had an increase of 97, and Lawton had an increase of 115.

Southeast Michigan, represented by Romeo, Michigan, also saw an increase in GDD values, indicating conducive conditions for grape growth. Romeo's GDD increased from 581 to 683, with a weekly increase of 102 GDD. The average GDD for southeast Michigan increased from 603.3 to 713.5, with a weekly increase of 110.2 GDD.

Northwest Michigan, including Old Mission, Petoskey and Traverse City, Michigan, experienced slightly lower GDD values compared to the previous week but still provided reasonable conditions for grape growth. The average GDD for this region decreased from 488.8 to 569.8, with a weekly decrease of 81 GDD. Old Mission had a GDD decrease of 77, Petoskey had a decrease of 82 and Traverse City had a decrease of 88.

These updated insights can assist grape growers in Michigan in making informed decisions regarding planting, growth and management strategies tailored to localized GDD conditions. Monitoring GDD values remains crucial for grape growers to maximize their chances of successful grape production.

Vine growth

Southwest Michigan is currently observing the progress of grape growth stages. Juice grapes and some hybrids in the region are reaching the end of their bloom stage. At the Southwest Michigan Research and Extension Center, early hybrid varieties like Marquette and Brianna are approaching the buckshot berry stage. Simultaneously, other hybrid wine grapes and several vinifera varieties are currently in early bloom.

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.

See this chart for grape growth stages.


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.


During this time of the year, the primary diseases of concern for grape growers are phomopsis, black rot, anthracnose 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.


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.

Grape tumid gallmaker infestation on leaf.
First sightings of grape tumid gallmaker infestation in southwest Michigan. Photo by Rufus Isaacs, MSU.

Upcoming events

Optimize grapevine health and productivity at Michigan grape pre-veraison meeting
Enhance grapevine health and productivity with valuable insights from industry experts at our July 14 meeting focused on optimizing grape cultivation. Don't miss out, register now!

Southwest Michigan Viticulture Field Day
This all-day event with steak fry and wine tasting will be held Wednesday, July 26, at the Southwest Michigan Research and Extension Center in Benton Harbor, Michigan. Stay tuned as more details become available.

Register for Dirt to Glass 2023: Elevating Michigan Wine from the Ground Up taking place Aug. 24-25. Dirt to Glass aims to connect growers and producers each year and provides information to support the Michigan grape and wine industry in understanding the critical relationship between better farming and world class wine. Experience the pinnacle of collaboration at this groundbreaking event, where MSU, MSU Extension and industry innovators and educators come together. Join us as we share the latest research findings and cutting-edge practical information to elevate your knowledge and expertise in the field.

Related articles

Did you find this article useful?