Michigan grape scouting report – August 30, 2023

As September approaches, Michigan grape growers must focus on disease and bird management to protect their clusters. Sour rot is a major concern, and growers should be vigilant in their scouting efforts.

grape vineyard
Photo by Esmaeil Nasrollahiazar, MSU Extension


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




Watch the Agricultural Weather Outlook

Report on growing degree days (GDD)

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.

Michigan Grape Growing Regions

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




The latest GDD data for Michigan grape growing regions shows that conditions are favorable for grape growth. Southwest Michigan has the highest average GDD, followed by southeast Michigan and northwestern Michigan. All three regions are seeing an upward trend in GDD values, which is good news for grape growers.

It's important to note that the GDD data in this report is as of August 29, 2023. GDD values can change quickly, so it's important to stay up-to-date on the latest data. You can find the most current GDD report via Michigan State University Enviroweather.

Vine growth

In southwest Michigan, grapes are in the middle of or have completed verasion, depending on cultivar. Leaf pulling, shoot thinning, and hedging should be complete. Early varieties show sugar levels of 15-20 brix. Acidity levels are around a pH of 2.9-3.2. Due to the good weather over the next week, wineries doing early harvest are looking to push into next week before they start.

Several people have been reporting damage to grape clusters over the last few weeks. A series of rain/sun events occurred over the last couple weeks of July and early August resulting in wet berries with high sun load. This resulted in significant sun damage. In most cases the berries look sunken or shriveled and eventually completely dehydrate and harden. Symptoms can be confused with black rot infection or pesticide injury, but close inspection shows the damage to be mainly on the tops of exposed clusters, especially in leaf-pulled blocks.

In northern Michigan, early-ripening grape varieties are currently in the veraison stage, the point at which the berries begin to change color and soften. Sugar accumulation has also begun, and the berries are now approximately at their eventual mature size.

In the Tip of the Mitt Region, hybrid grape varieties are also at veraison. Hybrid grapes are a cross between vinifera grapes (the traditional wine grapes) and American grape varieties. They are often more cold-hardy than vinifera grapes, making them a good choice for growing in northern Michigan.

Grapes on the vine photo.jpeg
Photo 1. Photo by Derrick Vogel, Petoskey Farms Vineyard and Winery.

See this chart for grape growth stages.


Northwest region (Brix)

Southwest region (Brix)







Pinot noir



Pinot Blanc



Pinot Gris



Cabernet Franc






















Vineyard hedging is about to commence in Michigan, driven by key goals. The primary aim is to limit excessive primary and lateral shoot growth on the canopy's top and sides, preventing shading and entanglement between vine rows. This enhances accessibility for workers and tractors. Interestingly, hedging fosters growth by promoting lateral shoot development in robust vines, even as the canopy thins due to shoot removal. Performing hedging in early to mid-summer is advised.

The optimal time for hedging is between fruit set and veraison. At this stage, primary and lateral shoots begin bending at a 45-degree angle. This timing ensures proper exposure of leaves, fruit, and buds to sufficient light, critical in dense canopies of hybrid cultivars with vigorous vegetative growth. To avoid problems, refrain from early-season hedging, which could lead to excess lateral growth and canopy density.

Furthermore, some juice grape vineyards report over-cropping issues. Effective crop load management is crucial for desired Brix levels. The MSU Concord model on Enviroweather is a valuable tool for assessment, and considering cluster thinning at 1200 GDD is recommended.

During fruit maturation, bird damage poses a significant challenge. Losses can reach 95% and 60% for red and white varieties, respectively. Employ various protective measures, such as bird nets, streamers, scare-eye balloons, electronic distress calls, propane-fired cannons, and even Robotic laser bird repellent. Read “Bird damage reduction strategies utilizing viticultural practices” for more information.

Diseases scouting report

At this time of year, the main diseases to focus on are downy mildew, powdery mildew and botrytis bunch rot. Downy and powdery mildew is causing trouble for growers this year. The recent waves of rain and high humidity have created good conditions for repeated infection events. If they are permitted, fungicides that are broad-spectrum/contact like captan are effective products for resistance management. Viticultural practices that reduce canopy wetness such as good irrigation timing, leaf removal and good weed management can reduce these diseases in a vineyard.

The main concern with downy mildew is late season defoliation. So far this season we have observed downy mildew more frequently than normal due to the high relative humidity and longer dew periods. Defoliation will impact winter hardiness and survival, as well as health of the vine next year. Effective fungicides for downy mildew include products in FRAC codes 4, 11, 21, 40 and 45 as well as phosphorus acid salts, captan and some biologically based products.

Since many parts of the state are at veraison, this is also an important time to consider botrytis management. Botrytis has been spotted in many early ripening varieties like Marquette or Aurore, and sporulation can be seen on infected berries that were likely started by berry moth infestations. Several strategies contribute to good botrytis bunch rot management including opening up the canopy, effective insect control, properly applying fungicides, and using resistant cultivars when possible. Good botrytis control depends on getting good coverage. Just before bunch closure is the last chance to apply a fungicide to the inner part of the developing cluster. The most effective products for botrytis are site specific and prone to resistance development. A Michigan Grape Fact Sheet is available for managing botrytis bunch rot.

We have also been seeing sour rot in some vineyards in southwest Michigan. Sour rot is caused by a variety of microorganisms (e.g., yeasts and bacteria) and is often used as a catch-all for a variety of cluster problems. It is usually characterized by clusters that smell like vinegar, juice that drips over the fruit surfaces, a melting-type decay with skin slippage, and vinegar flies and fly larvae typically present.

We have also been seeing sour rot in some vineyards in southwest Michigan. Sour rot is caused by a variety of microorganisms (e.g. yeasts and bacteria) and is often used as a catch-all for a variety of cluster problems. It is usually characterized by clusters that smell like vinegar, juice that drips over the fruit surfaces, a melting-type decay with skin slippage, and vinegar flies and fly larvae typically present. Recent research has shown a significant correlation between insect activity and sour rot movement. These insects have been primarily vinegar flies, but at MSU we have observed yellow jackets, honey bees, and bald-faced hornets on sour rot clusters. Unfortunately, since sour rot is a disease complex involving many different yeasts and bacteria, we are still working to identify fungicides that are effective against sour rot. Recent research into a well-timed insecticide at 14 Brix along with a sterilant (e.g., ProBLAD Verde or Oxidate) prior to sour rot symptoms is an effective method of control.

Remember as you choose a fungicide, check the guide for potential phytotoxicity of certain sprays on Concord grapes especially (this has been particularly noted for fungicides like Revus Top). Phytotoxicity risk is higher with high temperatures and quickly growing vines. Also there is a significant phytotoxicity risk with specific contact products such as copper and sulfur for Labrusca type grapes (Concord and Niagara).

It is important to remember to manage fungicide resistance and avoid applying similar products back-to-back. This is particularly important with site-specific systemic fungicides. To reduce the development of resistance with systemic fungicides:

  • Do not make more than two applications per season of the same FRAC code.
  • Do not make two consecutive applications of the same FRAC code.
  • Rotate with unrelated fungicides in a different FRAC code that have efficacy on the target pathogen.
  • Include a contact multisite fungicide into a program (e.g., sulfur, captan, oils or biological fungicides).

Insect scouting report

This week’s vineyard scouting in southwest Michigan indicates that while berry moth has been active through early August, growers are generally keeping decent control of this pest and sprays for generation three have been effective. Borders of vineyards with infestations from grape berry moth can be found but we noticed this week that those were also not as far into the vineyard as in recent years. Beetle and leafhopper activity is low too, so crop quality is generally looking very good for the 2023 vintage.

The brix levels in some southwest Michigan vineyards are starting to reach the 14-15 Brix mark this week, where sour rot becomes more of a concern. If you have susceptible cultivars with risk of sour rot developing prior to harvest consider protecting the clusters by reducing pathogen development and limiting insect vector activity. In recent field trials with Miles lab group, programs initiated at 14 Brix that included Fracture for pathogen control combined with either Delegate, Mustang Maxx, or Leverage 360 significantly reduced sour rot incidence. Weather conditions during the ripening period can also strongly influence the risk of sour rot, so with the warm dry weather in the forecast this may not be a particularly high risk season. However, it is something to remember to watch for as you’re scouting vineyards in the coming weeks.

Important notice

We're conducting a thorough evaluation of grape erineum mite infestations across the state, with a focus on northwest Michigan. Should you encounter grape erineum mite issues, kindly report them via this survey.

Your prompt reporting is highly appreciated.

Upcoming events

Announcing the Great Lakes EXPO Grape Section: A Must-Attend Event for Grape Growers

Save the date for the Great Lakes EXPO: Grape Section on December 5, 2023, at the DeVos Place Convention Center in Grand Rapids, Michigan.

Event Details:

  • Date: December 5, 2023
  • Venue: DeVos Place Convention Center, Grand Rapids, Michigan

Morning Session: All About Grapes

  • Connect with juice and wine grape enthusiasts. Explore grape cultivation, pest management, and sustainability.

Afternoon Session: Elevating Wine Grapes

  • Dive into wine grape production, canopy architecture, clean plant programs, and frost protection strategies.
  • Stay tuned for registration details and join us for an enlightening day dedicated to the world of grapes.






9-9:30 a.m.

Protecting berries against insect pests, from bloom to harvest

Rufus Isaacs

Michigan State University

9:35-10:05 a.m.

Disease Management Strategies for Grape Production in Michigan: Current Challenges and Best Practices

Timothy Miles

Michigan State University

10:10-11 a.m.

Elevating Sustainability in Grape Cultivation: Integrating Biochar for Enhanced Agroecological Practices

Brook Wilke

Kellogg Biological Station

Michael Person

Great Lakes Biochar Network

2-2:40 p.m.

Influence of Canopy Architecture and Fruit Microclimate on Flavonoid Accumulation and

Antioxidant Capacity in  Merlot (Vitis vinifera L.) Berry Components.

Paolo Sabbatini

Michigan State University

2:45-3:20 p.m.

Advancing the Clean Plant Program for Enhanced Grapevine Seedling Standards

Tefera Mekuria

Wonderful Nurseries

3:25-4 p.m.

Shielding Our Vineyards: Strategies for Frost Protection

Cain Hickey

Penn State University

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