MSU Extension Grapes

Weather update 

Detailed seven-day forecasts for Michigan grape production regions are available below: 

• Southwest Michigan: Benton Harbor, Fennville, Lawton
• Southeast Michigan: Romeo
• Northwest Michigan: Old Mission, Petoskey, Traverse City

Agricultural Weather Outlook: Weekly in-season weather updates are provided to the Michigan State University Extension fruit team by Jeff Andresen, Michigan State University (MSU) agricultural meteorologist. These reports offer timely insights into weather conditions affecting fruit production across Michigan.  

Growing degree day (GDD) report 

Track vine development using the MSU Enviroweather Growing Degree Day (GDD) map. Visit the site to find your nearest weather station, create a free account, and monitor GDDs daily: MSU Enviroweather GDD Tool 

Weekly GDD summary (Base 50 F) 2612.8

The table below summarizes current GDD accumulation, last week's totals, and the weekly increase for each monitored location across Michigan: 

Weather forecast 

The last week was cooler and wetter than we have seen recently in much of the state. High temperatures were in the 50s and 60s, climbing into the 70s by the start of this week. A cold front that passed through on Wednesday, Sept. 3, brought rain to the entire state. Drought-stricken southwest Michigan saw the most rain at 1 to 1.5 inches from that system. That is close to 10% of the entire amount of precipitation that this region has seen this growing season. Northwest Michigan saw about a half inch from the same system and an additional half inch of rain from following systems at the start of the weekend. Southeast Michigan saw about a quarter inch from the Wednesday/Thursday system as well.

Over the next week, the state will see a continuing warm up. Southern Michigan will get into the 80s, 70s further north. Light, scattered rains are possible over the weekend, but no significant accumulations are expected. The two-week forecast is for above average temperatures and equal chances of average precipitation.

Vine growth

In southwest Michigan, we are in the second week of harvest. Early hybrids like Marechal Foch, Itasca and Marquette have already been harvested. Early Pinot Gris is also being harvested for sparkling uses. The cool weather seems to be slowing sugar development. Sour rot is starting to show up as well. Isolated crop loss due to birds is showing up in advanced varieties.

Table 1. Southwest Michigan Research and Extension Center - September 10, 2025

In northern Michigan, grape clusters are progressing through the modified E–L phenology stages 33 and 34. Development varies by cultivar:

• Early ripening varieties are entering the onset of veraison (Stage 34), where berries begin to soften and sugar accumulation has just started.
• Later-ripening varieties remain at Stage 33, with berries still hard and green and no visible signs of softening.

This variation highlights the natural spread in ripening across cultivars in the region, with early varieties moving into the initial stages of ripening while later varieties are still in the lag phase of berry growth.

For a precise visual reference, consult MSU Extension’s Grape Growth Stages chart.

Table 2. Northern Michigan, Old Mission Township – September 10, 2025.

In the Tip of the Mitt American Viticulture Area, hybrid cultivars are slowly progressing through ripening considering the cool, wet weather. Marquette measured 17 brix with a pH of 2.71, while Itasca measured 18 brix with a pH of 2.76. Veraison in vinifera cultivars is progressing slowly as well. The risk for powdery and downy mildew remain low to moderate.

In southeast Michigan, Concord continues to add color while more varieties undergo harvest. The table grape Vanessa was picked in Britton, Michigan, with Marquis up next while Frontenac Blanc was harvested at Youngblood’s in Ray Township, Michigan. Growers are letting wine grapes like Marquette hang while weather conditions are good.

Table 3. Grower provided grape chemistry data in southeast Michigan.

Horticulture

End-of-season strategies to accelerate grape ripening in cool, humid conditions

As harvest nears in Michigan, grape growers often face the challenge of achieving full fruit maturity amid the cool, humid conditions typical of fall. Once temperatures dip below 50 F (10 C), sugar accumulation slows dramatically, leaving clusters vulnerable to rot and stalled ripening. While no silver bullet exists in the final stretch of the season, several practical interventions can help accelerate ripening and protect fruit quality.

Targeted leaf removal

One of the most effective short-term strategies is selective leaf removal around the fruit zone. Removing a modest number of leaves—particularly on the sun-exposed side of the canopy—enhances light penetration and airflow. This not only promotes sugar accumulation but also reduces humidity, lowering the risk of Botrytis and sour rot. In Michigan’s late-season climate, the risk of sunburn from increased exposure is minimal.

Cluster thinning

Even late in the season, cluster thinning can improve ripening uniformity. Removing underdeveloped or diseased clusters allows the vine to concentrate its limited resources on the remaining fruit. While sugar gains may be modest, thinning helps ensure marketable maturity and reduces the risk of off-flavors in wine. Research shows that as little as 5% rot-affected fruit can negatively impact wine aroma and flavor, making this practice essential for preserving sensory quality.

Water management

Managing water inputs is critical during the final ripening phase. In irrigated vineyards, reducing or withholding water near harvest can curb vegetative growth and concentrate sugars and secondary metabolites. In regions with frequent rainfall, improving soil drainage is key. Techniques like deep ripping or subsoiling between rows can break up compacted layers, enhance infiltration and reduce waterlogging—ultimately protecting fruit integrity.

Selective harvesting

Staggered harvests may offer a practical solution. Picking compromised clusters first allows healthier fruit to remain on the vine for additional ripening. This approach helps salvage quality while minimizing disease spread.

Advanced techniques

In challenging seasons, some growers may consider pre-pruning—cutting fruiting canes before harvest—to redirect assimilates and concentrate sugars through partial dehydration. Applied to roughly half the fruiting canes, this technique can yield a mix of fully ripe and dehydrated fruit, enhancing wine complexity through blended chemical profiles.

Mechanical airflow enhancement

In especially humid sites, tools like compressed air leaf removers, sprayers, wind machines or fans can improve airflow and delay rot onset by drying out the fruit zone. Ultimately, while cooler temperatures limit the vine’s natural ripening capacity, a thoughtful combination of light management, cluster selection, water control and harvest timing can help growers make the most of the final weeks and safeguard fruit quality.

Preparing for harvest in Michigan wine vineyards

The final weeks before harvest are pivotal for Michigan wine grape growers. Fruit composition shifts rapidly, disease pressure intensifies, and coordination with wineries becomes essential. Strategic preparation ensures grapes are harvested at peak quality and delivered in optimal condition for winemaking.

Monitoring fruit maturity

Harvest decisions rely on both chemistry and sensory cues. Sugar levels typically range from 19-25 Brix, with sparkling wine bases harvested earlier at 18-20 Brix. Titratable acidity and pH should be tracked closely, as cool nights can shift acid balance quickly. Sensory indicators, such as seed and skin color, tannin texture and berry weight, offer critical insight into ripeness. Sampling should be representative, with about 100 berries or 20 clusters per block. Yeast-assimilable nitrogen is also vital, especially for aromatic whites and high-sugar reds, as low yeast-assimilable nitrogen can lead to sluggish fermentations.

Canopy and disease management

Maintaining an open canopy is essential. Leaf removal should leave no more than one or two layers around clusters to enhance light exposure, airflow, and spray coverage, reducing the risk of Botrytis and sour rot.

Harvest sanitation and food safety

All harvest tools and containers should be cleaned and sanitized before use. Damaged equipment must be replaced, and sanitation records maintained for compliance. Diseased or insect-damaged clusters should be culled in the field or at sorting tables.

Winery coordination

Clear communication with wineries is crucial. Confirm target maturity, container needs and delivery schedules. Wineries should ensure all equipment is sanitized and ready. Early morning harvests help preserve freshness, while shaded field storage limits oxidation. Efficient transport and clear block labeling streamline winery operations and maintain traceability.

Bird damage and ripening risks

Bird damage is a serious concern during the ripening period, especially from veraison through late fall. Without control measures, crop losses can reach up to 95% in red varieties and 60% in white. MSU Extension outlines a range of mitigation strategies—from netting and acoustic deterrents to robotic lasers and environmentally safe chemical repellents. Bird injury also increases susceptibility to fungal diseases like Botrytis and sour rot. For more on this topic, see “Bird damage reduction strategies utilizing viticultural practices.”

Diseases 

In southwest Michigan, disease management of downy mildew, powdery mildew and botrytis fruit rot are the primary focus. The morning dews we will begin to experience are an important reminder to maintain protection against downy mildew. With tight clustered varieties, this is the last chance to get botrytis bunch rot sprays into the tight crevices of the clusters.

In northwest Michigan, disease pressure is still low but continues to increase in specific areas where black rot is becoming more apparent. Black rot symptoms are beginning to show up throughout the state, but it is largely too late to control these infections and fruits will begin to become resistant.

In this growing stage, consider choosing fungicides that control all the foliar and fruit diseases. For example, with downy mildew we are most concerned with foliar infection, and sprays should be timed regularly throughout the season when we experience heavy dews 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.

If powdery mildew is the only concern, there are a number of effective products (FRAC codes 3, 7, 11, 13, U8, 50, and U13 as well as sulfur). Combining fungicides from different FRAC classes should also be effective while helping with resistance management.

As we approach bunch closure in southwest Michigan, consider botrytis management. Several strategies contribute to good botrytis bunch rot management including opening up the canopy, 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. Fungicide resistance management is also important. 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.

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).

Insects  

Several weeks ago, a grower reported spotted lanternfly on their vines in a small vineyard in the Detroit area. This has been confirmed by an MSU Extension staff member visit to the site, where they found a low level of spotted lanternfly adults (fewer than 10 found during a 1-hour visit). These were found on the upper canopy of the grapevines and in the nearby tree of heaven saplings at the site. No egglaying was observed, but that is expected to start in the coming weeks.

This find is not unexpected given that spotted lanternfly has been reported in this region of the state previously, but it is the first confirmed detection on grapevines. We hope all grape growers are being vigilant for this pest. Please report sightings to the Michigan Department of Natural Resources Eyes in The Field online reporting system or your local MSU Extension educator if it is detected in your area, on your vines or on nearby trees.

There are multiple effective controls for spotted lanternfly, but since the levels are still very low, MSU Extension encourages following the advice from the state to See it, Squish It, Report It. For more on this pest, visit the MSU Extension Spotted Lanternfly page.

Scouting vineyards at borders with high pressure from grape berry moth indicates a mix of situations. Some sites have very clean clusters with evidence of effective control through the season so far. Others are showing the start of a fourth generation, with evidence of recent moth flight, egglaying and larval activity. Most of the larvae seen on Monday, Aug. 25 were first instars, indicating recent hatch from eggs on the berries. A walk deeper into these vineyards showed that the pressure was mostly on the edge, so border sprays could be considered as a way to maintain control while reducing costs.

With veraison increasing across southwest Michigan, be alert to the risk of sour rot. Check vineyards for the development of rots in high-risk vineyards, sites where berry splitting has been happening from recent rains and berry moth infestation. The cooler night conditions will limit vinegar fly activity, but as it warms up again, we may see more activity of vinegar fly and yellow jackets that can vector sour rot pathogens. Research by MSU over recent seasons shows the benefit of controlling both the pathogens and the insects during the late-season window before harvest of tight clustered cultivars that are susceptible to sour rot.

GPS, flexibility and ease: What’s new in SAM Tool

Working with our current users, the Sustainable Ag Manamgent (SAM) Tool is now even smoother. One valuable update is the ability to view GPS tracks of completed tasks on smartphones while managers are on the go. This gives confidence that sprays are applied correctly and on time. The feature provides a clear record of what was done.

Another grower-friendly improvement is how vineyard blocks are handled. The SAM Tool is designed to be very forgiving; you can set up your blocks however you like, whether by variety, clone, vine age or your preferred management style. If you decide later to redraw your blocks, the SAM Tool can backtrack spray records for you. Your records remain intact and linked to both the original and updated block layouts, giving you flexibility and confidence in managing your vineyard.

If you’d like help getting started, please email Karen Chou at chouk@msu.edu to schedule a time to create your virtual farm and migrate your spray plans and recordkeeping into the SAM Tool. One-on-one assistance is available over the next three months, so reach out early to reserve your spot.

Upcoming events

2025 Great Lakes EXPO – Grape Session

Join us at the 2025 Great Lakes Fruit, Vegetable & Farm Market EXPO, happening Dec. 9–11 in Grand Rapids, Michigan.

This year’s grape session will bring together growers, researchers and industry experts to explore the latest in vineyard management, disease control, varietal updates and innovative technologies tailored for the Great Lakes region.

Whether you're new to grape growing or a seasoned professional, this session offers practical insights, valuable connections, and tools to help you grow smarter.

Don't miss it—visit the Great Lakes EXPO website to register and join us in December!

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