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, northwest Michigan started cool with highs in the upper 60s. Scattered showers brought about a tenth of an inch of rain to the region Wednesday, Sept. 24. Temperatures increased over the weekend. High temperatures near 80 degrees Fahrenheit started this week and will continue through the weekend. A cool down back to normal is expected at the start of next week. No significant rain is expected in the near-term forecast.

Southwest Michigan also started last week cooler with highs near 70 degrees before climbing into the mid-80s over the weekend. No rain was reported last week. This warm trend should continue through the start of next week with dry, warm weather through at least the next 10 days. A small chance of a tenth to quarter inch of rain is possible in the middle of next week.

Southeast Michigan was warm and dry, much like its western neighbor. High temperatures started in the low 70s, climbing to the mid-80s. Again, no rain was reported. The forecast is for similar temperatures through the next week. A possibility of up to a half inch of rain is predicted later next week.

The medium range forecast is for a high probability of above average temperatures for the entire Great Lakes region. The precipitation outlook is mixed with northern Michigan expected to have equal chances of average precipitation, but below average predictions continuing for much of southern Michigan. The long-range forecasts for November through January do not show a continuation of this pattern. Northern Michigan should return to normal temperatures for the first half of the winter while southern Michigan is expected to stay above average. All of the Lower Peninsula of Michigan is predicted to be above average for precipitation for the end of 2025 and the start of next year.

Vine growth

In southwest Michigan, juice grape harvest has begun. Niagara harvest began Monday, Sept. 29. Concord harvest is slated to begin Oct. 9. Wine grape harvest continues. Tonnage continues to be lighter than expected with sugars accumulating on pace. Acidity is slow to drop in many varieties, potentially due to the dehydration most vineyards are seeing. The warm, dry weather is providing growers with the opportunity to extend hang time to continue ripening.

Southwest Michigan Research and Extension Center – September 29, 2025:

In northern Michigan , grape clusters are currently progressing through the modified E–L phenological stages 37. 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.

Northern Michigan, Old Mission Township – September 30, 2025:

In the Tip of the Mitt American Viticulture Area, harvest is underway for earlier ripening hybrid cultivars with some later ripening varieties scheduled for harvest next week. Marquette is at 23 brix, 2.98 pH. Early vinifera is ripening well, with Zweigelt at 18.5 brix, 2.99 pH. Riesling is at 15.5 brix, 2.87 pH. Botrytis and sour rot have been noted in some vineyards.

In southeast Michigan, Concord and Niagara harvest continues, with some growers wrapping up picking quickly. Most table grapes have been harvested with Vanessa and Mars harvest concluded in Lenawee County. In wine grapes, Petite Pearl was harvested over the weekend at Youngblood's, concluding their harvest in Ray Township.

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 the southwest, cluster rots are the primary focus. If powdery mildew is the only concern, there are a number of products that are effective (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.

Botrytis infections are continuing to develop. 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.

Remember that 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  

Monitoring traps for grape berry moth indicate very low activity in southwest Michigan, with two vineyards having caught zero moths, and the others having one to 10. This indicates the end of the fourth generation of this pest, with much lower activity than last week. We also found no evidence of egglaying or newly hatched larvae, which further shows the shorter day length has signaled that grape berry moth need to prepare for winter by pupating.

The only other pest species observed were a low count of vinegar fly adults and larvae, and some yellow jackets feeding on ripe berries. There were no other insect pest species reported.

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 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 a spot.

Upcoming events

2025 Great Lakes EXPO

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

Grape Session

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.

Spray Drone Symposium

In its second year, the all-day technology workshop will once again feature the use of spray drones in specialty crops. Three presentations will showcase spray drone usage in viticulture. Marco Grella and Eric Mozzanini will report usage opportunities being explored in Italy while MSU Extension's Michael Reinke will report on the two years of efficacy trials in grapes here in Michigan, comparing spray drones to conventional systems.

If you are interested in adoption of drones on the farm, whether specifically your vineyards or other crops as well, this is one of the best opportunities to get information in the country for spray drone information targeted to the specialty crop industry.

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

Register now

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• Using the MSU Enviroweather grape berry moth model in 2018