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)

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

Weather forecast 

Last week, southwest Michigan saw another week of above-average temperatures with highs in the mid to upper 80s and lows in the mid-50s. A cold front entered the area on Friday, Sept. 19, bringing with it rain and humidity. High temperatures dropped into the upper 70s, but low temperatures stayed up in the 60s. The southern tier of Michigan also saw about an inch of rain between Friday and Monday.

Southeast Michigan was decidedly cooler than the western side of the state. The week started similarly, but the cool weather that entered on Friday did not contain the same moisture. High temperatures dropped into the low to mid-70s and lows stayed in the low 50s. No rain was recorded until late on Sunday. Total accumulations through Monday have been less than half an inch.

Northwest Michigan saw weather conditions somewhere between its southern neighbors. The week started with highs near 80 degrees Fahrenheit and lows in the upper 50s. A cool front moved through on Thursday, dropping high temperatures into the low to mid-70s, but increasing overnight lows to near 60 F. Rain started Saturday and continued through Monday. Total accumulation is around three quarters of an inch.

The cooler weather is expected to continue. Daytime highs are expected to be around 5 degrees above average, overnight lows approximately 10 degrees above. This translates to 70-75 F during the day, 55-60 F at night. Scattered rain is expected through the week with accumulations of around a quarter inch in northern Michigan, up to an inch possible in southern areas.

Vine growth

In southwest Michigan, Bordeaux reds are beginning to be picked this week as are mid-season hybrids. Some grapevines appear to be showing water stress, especially those that have been mechanically pruned. This is likely due to the lack of critical decision making with mechanical pruning in trimming location on the plant during winter pruning. Concord and Niagara grape growers are reporting sugar levels that indicate those grapes are ready to be picked. Welch’s has confirmed Niagara harvest to begin next Monday, Sept. 29. Concord harvest is slated to begin Oct. 9. 

Table 1. Southwest Michigan Research and Extension Center - September 22, 2025.

In northern Michigan, grape clusters are progressing through the modified E–L phenological stages 36 and 37. Stage 36 corresponds to berries with intermediate sugar levels while stage 37 represents berries that are approaching full ripeness but are not yet mature. Progression varies by cultivar, with early-ripening varieties entering the initial phases of maturity, while later-ripening cultivars remain in the lag phase of berry development. This variability underscores the asynchronous nature of ripening across cultivars in the region and the importance of close monitoring to guide harvest decisions.

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

Table 2. Northern Michigan, Old Mission Township - September 23, 2025.

In the Tip of the Mitt American Viticulture Area, hybrid cultivars are ripening nicely with the sunshine and heat. Marquette grapes are at 22.1 brix and 2.91 pH, while Itasca are at 23 brix and 2.95 pH. Harvest for early varieties such as Brianna began this week. Veraison in vinifera cultivars is progressing. Botrytis and sour rot has been noted in some vineyards.

In southeast Michigan, Concord and Niagara harvest has begun, with some grapes still hanging on vines. Most table grapes have been harvested, but there is ongoing harvest of Vanessa and Mars in Lenawee County. In wine grapes, Frontenac was harvested at Youngblood’s in Ray Township on Sept. 23 with Petite Pearl expected to follow immediately.

Horticulture

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

As the 2025 harvest season advances across Michigan, regional differences in timing are becoming clear. Vineyards in southwest Michigan are now nearly a month into harvest, while those in the southeast are about three weeks along. These early progressions highlight the strong influence of heat accumulation and seasonal variability on fruit development across the state.

In cooler northern regions, harvest is just beginning, and growers face the familiar challenge of achieving full fruit maturity under declining fall temperatures and persistent humidity. Once daily averages dip below 50 F (10 C), sugar accumulation slows sharply, leaving clusters more vulnerable to disease pressure and delayed ripening. Understanding where each region stands in harvest progression helps guide timely management decisions to accelerate ripening and protect fruit quality in the final stretch of the season.

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  

In our weekly scouting at vineyards with high pressure from grape berry moth, we again found a mix of situations. Some sites still have clean clusters with low infestation from berry moth and rots. Others are showing poor control of the fourth generation of grape berry moth, with increasing numbers of larvae infesting clusters at vineyard borders.

Understandably, most growers are focused on harvest preparations or harvest this month, but recent rains have also increased splits in berries and there are more vinegar flies this week than last week in the scouting. These can help move the pathogens around for sour rot, and we are also starting to detect sour rot berries in some clusters as berry quality declines. Based on your predicted harvest dates and cluster condition, it is worth checking regularly through the upcoming warm weather to make sure rot levels remain low. Consult the MSU recommendations for options to prevent or stop further rot development.

SAM Tool: Easy access to your spray and fertilizer records

One of the valued features of the Sustainable Ag Management (SAM) Tool is the ability to download pesticide and fertilizer application records anytime you need them. With just a few clicks, you can select the time period you want and export your records in a spreadsheet format. This makes it simple to review, share or submit reports when needed — no extra typing required.

The report includes key details such as farm location, block information, product sprayed, worker, weather conditions, REI, FRAC/IRAC Codes and more. All of these items are automatically generated to save you time and reduce errors. (See the picture insert for the list of report fields.)

As the growing season winds down, don’t forget that the SAM Tool also makes it easy to record your harvest. Storing these records now will keep your farm history complete and set you up for a smooth start next year.

If you’d like help getting started, please email Karen Chou atchouk@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 a 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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