MSU Extension Grapes

Weather 

Click the following links for detailed seven-day forecasts for various grape production regions.   

Southwest  

• Benton Harbor   
• Fennville   
• Lawton   

Southeast  

• Romeo   

Northwest  

• Old Mission   
• Petoskey   
• Traverse City   

See the latest agricultural weather outlook from Jeff Andresen, Michigan State University (MSU) state climatologist.   

Check out the MSU Enviroweather Growing Degree Days (GDD) Michigan map. Find your closest weather station, create an account on the website, and track GDDs daily. 

Report on growing degree days (GDD) 

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

Vine growth 

Southwest Michigan 

Veraison is expanding into more vinifera varieties. Concord juice grapes also started showing color a week ago in several locations. Bird damage is noticeable on advanced varieties. Early ripening hybrid wine varieties have sugar levels around 16-20 degrees Brix. Cluster thinning continues for crop load management and to clean out powdery mildew-infected clusters infections.

Southeast Michigan 

Growers in warmer parts of the region near the Ohio border continue harvesting some seedless grape varieties, including continued harvest of Reliance and Canadice. Soluble solids on these varieties ranged from 16–18 Brix. Concord observed in Romeo on Aug. 19 was at veraison, but those observed the same day in Fenton were still green. Fredonia grapes in Romeo are about a week away from harvest.

Growers of cold hardy winegrapes are starting to monitor soluble solids and will start analyzing other juice chemistry attributes soon. Measurements done by Youngblood Vineyards on Aug. 15 have Marquette at 20.5 Brix and Prairie Star at 18 Brix. Itasca measured 23 Brix on Aug. 20 with harvest anticipated for the end of this week. Some growers have put bird netting up in valuable varieties that have hit veraison. More information about bird damage reduction strategies can be found in this 2020 Michigan State University Extension article.

The fair dry weather over the next week should make for good harvesting conditions. RimPro models for Romeo indicate little risk of black rot. Downy mildew risk is similarly low with a small peak of infection risk predicted Aug. 26. Powdery mildew risk, on the other hand, remains high for the entire forecasted period from Aug. 21-29.

Northern Michigan

In the vineyards of northern Michigan, grapes are getting close to the veraison stage. The berries remain firm as sugar accumulation begins, and they have reached approximately 75% of their final size. The clusters are in the phase of bunch closure, a critical stage where the berries begin to tighten (E-L numbers 34 -35).

Tip of the Mitt region

Hybrid cultivars continue through veraison with some nearing completion. Early Vinifera cultivars are beginning veraison, while we are still waiting on veraison in later season cultivars. Powdery mildew and downy mildew have been noted in several vineyards. Growers have begun preventative measures to protect grapes from bird predation.

Horticulture

Vineyard hedging has been going on for a couple weeks in southern Michigan and should begin soon in northern vineyards. The primary focus for hedging is on curtailing excessive primary and lateral shoot growth on the canopy's top and sides to prevent shading and entanglement between vine rows. By doing so, the vineyard becomes more accessible for workers and tractors. Interestingly, hedging also stimulates growth by promoting lateral shoot development in vigorous vines, despite the reduction of the canopy due to the removal of primary and lateral shoots.

It is recommended to perform hedging during the early to mid-summer period. Ideally, hedging is carried out between fruit set and veraison. This timing is crucial as it ensures adequate exposure of leaves, fruit, and developing buds to sufficient light, especially in dense canopies of hybrid cultivars with excessive vegetative growth. To avoid potential issues, it is essential to refrain from hedging too early in the growing season, as it may lead to increased lateral growth and canopy density.

However, we consistently perform early-season hedging due to late bud break and elevated spring temperatures, which promote vigorous growth early in the season. We always avoid hedging during bloom or shortly thereafter to prevent increased cluster fruit set that consequently increases cluster compactness, making the clusters more prone to harvest season cluster rot complex diseases. Growing grapes in Michigan can be likened to a "100-meter sprint," as Craig Cunningham, an experienced vineyard manager in Michigan, describes it, compared to the marathon-like growing season of other viticultural regions. Typically, we hedge twice: once before veraison, and once after. Hedging three times in a season is unusual and indicates a problem. If your vines require a third hedging, it likely points to issues such as inappropriate rootstock selection, excessive nitrogen levels, or incorrect planting density.

Cluster zone leaf removal is underway depending on the region. Cluster zone leaf removal at the pre-veraison stage (mechanical or manual) is a critical viticultural practice with significant benefits for grape quality. By removing leaves around the grape clusters before veraison (the onset of ripening), growers can improve sunlight exposure and air circulation within the canopy. This increased exposure to sunlight enhances the development of desirable phenolic compounds and promotes even ripening, leading to better color, flavor, and aromatic profiles in the grapes. Improved air circulation reduces humidity around the clusters, which lowers the risk of fungal diseases such as powdery mildew and botrytis bunch rot, contributing to healthier fruit and reduced need for chemical treatments.

Diseases

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, we 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 northern vineyards. Southern vineyards have been seeing early infections of Phomopsis and are beginning to see black rot lesions on fruit and leaves. Powdery mildew symptoms are becoming more apparent, so vineyards should be scouted to check on the status of diseases.

With the exception of powdery mildew, these spring disease infections typically require rain events. It only takes 0.1 inches of rain above 50 degrees Fahrenheit 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.

Remember, as you choose a fungicide, check the Michigan Fruit Management 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).

Since some parts of the state are near veraison, this is also an important time to consider botrytis management. Botrytis has been spotted in many early ripening varieties like Marquette, 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.

Insects

Berry moth populations have generally been managed well by growers this summer in the vineyards we are scouting. There is the typical border pressure, but in sites with active management the combination of degree-day timed sprays and rotation of effective products seems to have kept populations relatively low. However, a few locations have high pressure still and should be scouted and actively managed through the time until harvest.

With veraison happening in southern Michigan, focus some scouting and management in rot-susceptible cultivars and implement a sour rot management plan. We recommend starting protection from pathogens and the insect vectors at around 15 Brix in these cultivars to delay the onset of sour rot. Vinegar flies have been observed in vineyards where there is some botrytis this week. We are in the window of the season in southern Michigan where vineyards that have had rot problems in the past should be managed to prevent this. The rainy weather through this weekend is another risk factor, so susceptible vineyards that reach 15 Brix next week should be protected.

MSU is working with state and federal agencies monitoring sites across the state for spotted lanternfly, including multiple vineyard locations. Growers can help out by reporting any suspect detections to the Department of Natural Resource’s Eyes In The Field.   

Become a legacy user of the SAM Tool and enhance your vineyard management!  

The Sustainable Agricultural Management (SAM) Tool is a comprehensive software application designed to streamline your vineyard management processes. We understand that adopting new technology can be daunting, but we believe the SAM Tool has the potential to transform your vineyard management for the better. This growing season, we are dedicated to offering personalized support. Thanks to the valuable feedback we've received, we continue to shape both the current features and the future of the tool. 

The SAM Tool can manage tasks for a small number of individual vines, as well as large vineyards spanning hundreds and thousands of acres. Your participation as a legacy user will not only help us improve the tool but also provide you with a unique opportunity to be part of an innovative community. Together, we want to ensure it meets your needs and the needs of other Michigan vineyards. 

Ready to get started? Let us help you migrate your spray plan to the SAM Tool and explore the many cool features we've created for you. The SAM Tool can automatically populate this year’s spray tasks into next year’s calendar, allowing you to edit each task to meet the following year’s needs. 

Contact Karen Chou for personalized help migrating your spray plan to the SAM Tool and to help us fine-tune the tool so that it truly meets the needs of growers like yourself. We welcome the opportunity to serve vineyards of all sizes.  

Upcoming events 

The Great Lakes Expo: Dive deeper into grapes this year. The Great Lakes Expo, held annually in early December at the DeVos Place Convention Center in Grand Rapids, Michigan, is the largest show of its kind in the country. It attracts a diverse audience of grape growers from the Great Lakes region. 

This year's Grape Section takes place on Tuesday, Dec. 10, 2024. We're offering the Grape Section in two sessions to provide a more in-depth exploration of both juice and wine grapes: 

Morning Session: Focuses on all things juice grapes, covering topics like varieties, cultivation practices and juice production. 

Afternoon Session: Delves into the world of wine grapes, exploring varietals and viticulture techniques. 

This expanded format allows you to tailor your experience to your specific interests, whether you're a juice grape grower, a wine grape producer or simply curious about both. Don't miss this valuable opportunity to connect with industry experts, gain valuable knowledge and explore the latest advancements in the grape growing industry. 

We look forward to seeing you there! 

To get a better sense of the Great Lakes Fruit, Vegetable, and Farm Market Expo, check out this video. 

Related articles 

• Early season vineyard management
• Early season control of grape berry moth in Michigan vineyards for 2022
• Fruit insecticide registration update for 2023
• Michigan Grape Scouting Report for July 31, 2024
• MSU Fruit Pest Management Guide (E-154)
• Grape growth stages
• Early season vineyard disease management
• Early season vineyard management
• A Mobile Guide for Grape IPM Scouting in North Central and Eastern U.S.
• Using the MSU Enviroweather grape berry moth model in 2018