Michigan grape scouting report – September 6, 2023

It's time for harvest, and growers are looking forward to a bountiful yield.

Wine grapes on a 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




It's important to note that the GDD data in this report is as of Sept. 5, 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, harvest is ongoing for early wine grape varieties for sparkling wine. 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 will 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, grape varieties are ripening. Sugar accumulation is accelerating in grape varieties in northern Michigan, and the berries are now approximately at their mature size. In the Tip of the Mitt growing region, some early cultivars will begin harvesting this week.

Grape growers in northern Michigan are optimistic about the upcoming harvest. The weather has been favorable so far, and the grapes are looking healthy. The sugar content is increasing at a good rate, and the berries are reaching their mature size.

Some early cultivars, such as Frontenac Gris and Marquette, are expected to be ready for harvest this week. These grapes are typically used to make white wines. Later-season cultivars, such as Cabernet Franc and Pinot Noir, will not be ready for harvest until later in the season.

The 2023 grape harvest is expected to be a good one. With favorable weather conditions and healthy grapes, growers are anticipating a bountiful harvest.

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 véraison. 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 1,200 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 particular 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.

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

Most of the vineyards we are scouting have controlled berry moth well this summer. Continued control of this pest may only be needed at the borders of high risk vineyards or if there’s a significant fourth generation in southwest Michigan. This seems unlikely from the timing of egglaying of the third generation this season since that was in mid-August, but since we don’t fully understand how these extremely hot September conditions affect berry moth, we will continue scouting and looking to see if this pest comes back in mid-September.

The grape berry moth model on Enviroweather indicates a fourth generation would start egglaying in southwest Michigan during the week of Sept. 11. Michigan State University Extension will alert growers via the Grapes website and newsletter if that is seen to be picking up. Beetle and leafhopper activity is low too, so crop quality is generally looking very good for the 2023 vintage.

Brix levels are getting into the region of 14-15 Brix and above where sour rot becomes more of a concern in susceptible winegrapes. If you have susceptible cultivars with risk of sour rot developing prior to harvest, keep a close eye on those as we go through September. The hot, dry weather with warm nights can increase insect development, and we are receiving reports of significant yellowjacket wasp activity. This can be a vector for sour rot pathogens plus they can be painful for pickers. Baited traps at vineyard borders is one non-chemical strategy for collecting wasps, but since the colonies are now quite large at the end of the summer, it will be tough to collect all the wasps.

Yellowjackets also have a large flight range, so vineyards will likely be drawing in wasps from miles around. Still, if you can locate the nests, these can be removed or controlled. This is dangerous though, so a professional exterminator is recommended.

Finally, we thank the growers who responded to the recent survey on grape erinium mite. We will share results from that during the winter Extension meetings.

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 Dec. 5, 2023, at the DeVos Place Convention Center in Grand Rapids, Michigan.

The topic for the morning session is “All About Grapes.” Connect with juice and wine grape enthusiasts and explore grape cultivation, pest management and sustainability. The topic for the afternoon session is “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.






9.00 - 9:30 a.m.

Protecting berries against insect pests, from bloom to harvest

Rufus Isaacs


9:35 - 10:05 a.m.

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

Timothy Miles


10:10 - 11:00 a.m.

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

Brook Wilke

Kellogg Biological Station

Michael Person

Great Lakes Biochar Network

2:00 - 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


2:45 - 3:20 p.m.

Advancing the Clean Plant Program for Enhanced Grapevine Seedling Standards

Tefera Mekuria

Wonderful Nurseries

3:25 - 4:00 p.m.

Shielding Our Vineyards: Strategies for Frost Protection

Cain Hickey


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