Michigan grape scouting report – August 22, 2023

Michigan's vineyards are where grapes thrive and ripen with promise.

A sun setting over 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




Vine growth

In southwest Michigan, grapes are in the middle of or have completed veraison, 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 90% of 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.

A cluster of grapes hanging from a branch.
Photo by Esmaeil Nasrollahiazar, MSU Extension

See this chart for grape growth stages.


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 Concord Model on Michigan State University’s Enviroweather is a valuable tool for assessment. Michigan State University Extension recommends cluster thinning at 1,200 GDD.

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.

A cluster of grapes that had been eaten by birds.
Photo by Esmaeil Nasrollahiazar, MSU Extension

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.

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 Tim Miles’ lab, 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. With the warm, dry weather in the forecast, this may not be a particularly high-risk season. However, it is something 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

Dirt to Glass 2023: Elevating Michigan Wine from the Ground Up is the annual Michigan grape and wine conference created to connect growers, winemakers, scientists and stakeholders and provide information to support the Michigan grape and wine industry in understanding the critical relationship between vineyard practices and world class wine. This pioneering event is a second-year collaboration involving MSU, MSU Extension, Intentional Agriculture and esteemed industry leaders. The program will cover crucial wine stages from vineyard planning to uncorking a bottle for examination. With a commitment to advancing the grape and wine industry, this year's focus is soil origin, functions and management. Our distinguished speakers and blind tastings promise valuable insights.

This sold-out conference unites colleagues statewide, fostering collaboration and knowledge exchange. Thanks to the dedicated committees and financial support, this event is realized. Your involvement remains vital for future conferences; please consider volunteering in any capacity for next year's conference.

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