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 was a return to “almost” normal. July started with temperatures close to long-term averages this time of year. Southern Michigan regions saw highs in the mid-80s most days. Northern Michigan was a little more variable with high temperatures varying from the low 70s to high 80s through the week. High humidity kept temperatures in the mid-60s in southern regions, causing some powdery mildew infections to build. Northern Michigan was decidedly cooler at night over the last week with several nights experiencing low 50s.  

Rain has been less consistent over the last week. The higher humidity and occasional warm temperatures have corresponded with some unsettled air masses. The result was some pop up, scattered showers, usually in the afternoon and overnight. Some areas of the state saw scattered rain at the beginning of the week. Most locations recorded less than a quarter inch of precipitation.  

Another unsettled air mass covered much of the state over the weekend and start of this week. Totals from this second wave of precipitation was between a tenth and a half inch. Totals for the week for most grape producing areas of the state were less than an inch of rain. This is below the models that are indicating many plants this time of year need between 1.25 and 1.5 inches of rain a week for full growth potential. 

The forecast for the next week is much of the same. Northern Michigan is starting out with high temperatures in the mid-70s warming to the mid-80s by Friday. Other parts of the state should be in the mid or upper 80s for the entire week. Lows will stay in the mid to upper 60s, indicating continuation of the humid weather. Precipitation will be scattered again. Some parts of the state are looking at possible rain over the next day or two then another wave of scattered chances over the weekend. No significant amounts of precipitation are predicted.  

Vine growth 

In southwest Michigan, grape cultivars with vitis labrusca in their parentage (Concord, Niagara, Brianna) are sizing quickly many are at berry touch. Other hybrids are at pea size berry with some tight clustered varieties near berry touch as well. Vinifera cultivars are at buckshot with some large fruited varieties at pea size berry. Leaf removal is progressing well. Most locations doing removal are focused just on the East side of the vines to prepare the vines before they remove the leaves from the west side of the plants. This helps protect the clusters from sunburn, something southwest Michigan has seen more of the last few years.  

In northern Michigan, recent warm and humid weather has triggered rapid growth in Vitis vinifera cultivars. Over the past week, shoot elongation has accelerated significantly, with shoots now nearing the top wire. Most varieties are in bloom or just past bloom. According to the modified E-L system, the vines are currently at Stage 23 (approximately 50% capfall). 

This rapid phase of both vegetative and reproductive growth makes it a critical time to focus on nutrient management. Earlier in the season, cooler soil conditions may have slowed nutrient uptake, but the recent warming has reactivated root function and improved nutrient absorption, supporting vine development. 

Accurate phenological tracking is essential during this window—timely pest management, fertilization and canopy adjustments can significantly affect yield and grape quality. For a precise staging guide, consult MSU Extension’s Grape Growth Stages chart. 

In the Tip of the Mitt American Viticulture Area, hybrid cultivars are in buckshot berry to approaching berry touch stage. Vitis vinifera cultivars are in the buckshot berry stage of development. Rose chafer pressure has eased, however, Japanese beetle pressure is beginning. Phomopsis, black rot and mildew risk has eased.  

In southeast Michigan, we’re seeing good fruit set around the region, with many varieties at pea size and some approaching berry touch. Grape berry moth counts at sites in Lenawee and Macomb counties dropped to 0. There is severe powdery mildew risk all week from July 2- 9. There is negligible risk for downy mildew and black rot infections this week. 

Horticulture 

Accurate assessment of vineyard nutrient requirements is essential for optimizing vine health, fruit quality and yield. Implementing soil testing and petiole analysis provides critical data to inform nutrient management strategies. A quality soil test or a previous year’s petiole analysis is important in understanding which nutrients the vineyard needs. 

Petiole analysis should be done at bloom and veraison, and at specific stages of vine development when nutrient concentrations in the petiole are most stable and representative of the vine's overall status. The most widely recommended timing for petiole sampling is at full bloom, which occurs when approximately 70% of the flowers on a cluster are open. This stage provides the most reliable data for key nutrients such as nitrogen, phosphorus, potassium and micronutrients. 

An alternative or complementary sampling period is at veraison, when about half of the berries on a cluster begin to change color or soften. Sampling at veraison can be useful for monitoring nutrients like potassium, magnesium and boron later in the season or for tracking changes in nutrient dynamics over time. 

Cluster zone leaf removal: Timing and technique 

To regulate grape quality in cool-climate regions, growers employ various canopy management practices to counter the challenges of a short growing season. One of the most impactful techniques is cluster zone leaf removal, which plays a key role in reducing disease pressure and enhancing fruit quality. 

This practice improves spray coverage and airflow around the fruit, lowering humidity and minimizing the risk of harvest season cluster rot complex, including Botrytis and sour rot. Increased sunlight exposure also encourages development of a thicker wax cuticle on the berry skin, providing natural resistance to fungal infections and enhancing grape quality. 

Leaf removal can be performed manually or mechanically, and the industry is rapidly transitioning toward mechanization to improve both efficiency and economic sustainability. The effectiveness of this practice depends heavily on timing, particularly in relation to grapevine phenology. 

Leaf removal is typically applied between fruit set and veraison to improve cluster zone microclimate and increase disease resistance. When implemented around bloom or shortly before, it can also reduce fruit set and result in looser clusters, a valuable outcome for tight-clustered cultivars like Pinot noir and Riesling, which are prone to rot due to berry compression. 

Early leaf removal, at pre-bloom or pea-size stage, offers additional benefits under climate change conditions: 

• Promotes flavonol synthesis, protecting berries from UV damage. 
• Reduces incidence of cluster rot, potentially lowering fungicide use. 

Mechanical early leaf removal systems (Collard leaf remover) use compressed air to remove leaves and dead flower tissue from the fruit zone. Removing dead ovaries is critical, as they serve as an entry point and nutrient base for Botrytis spores. The pathogen can remain latent until berry sugar levels rise and acidity drops, after which rot symptoms develop rapidly in warm, humid conditions. 

Read more in the MSU Extension article, “Cutting to the core: Michigan State research leads a canopy innovation in cool-climate viticulture.” 

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, refer to previous grape scouting reports 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 developing fruit. 

As bloom continues in southwest Michigan, start choosing fungicides that control all the fruit diseases (i.e., powdery mildew and botrytis bunch rot), such as chemicals in FRAC codes 7, 9, 11 and 12 . For example, with downy mildew we are most concerned with fruit infection at this time and sprays should be timed prior to bloom and at bloom 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. 

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

Insects  

We continue to see evidence of grape berry moth larvae feeding in clusters at scouting sites in southwest Michigan. Some of these are evident from berries being bored into by the larvae, others have webbing around the feeding sites and some have both. These detections have mostly been along wooded borders, and are easy to detect by checking clusters at this time of year.  

Treatment for berry moth at this time of the season can overlap with populations of leafhoppers, rose chafers and tumid gallmakers, so a thorough check of the vines is useful in early July to confirm they have low levels of any insect pests. Check the MSU Fruit Pest Management Guide (bulletin E154) for recommendations. A hard copy or a PDF version can be ordered from shop.msu.edu.  

Now that we are further into the 2025 season, the dates for biofix of wild grape bloom in late May and early June can be used to predict timing of when the second generation starts egglaying using the grape berry moth Enviroweather model. The table below reports what you can see in the Enviroweather model based on the biofix dates listed. Use local values from your own farm. Insecticides targeting eggs such as Intrepid could be applied starting later this week in the far southwest of Michigan. Next week would be a good timing for broad spectrum products targeting egg hatch. 

We have received reports of grape cane girdler and grape cane gallmaker in recent weeks. The girdler cuts two rings of holes in the developing shoots whereas the gallmaker lays eggs in the cane, which then swells. Both pests are small beetles that are usually not noticed until the damage is done and the symptoms are seen when shoots start breaking off. They both have one generation per year and tend to be most abundant along wooded borders, so this can be an area for focused management to break the life cycle and stop the insects from developing. 

When the damage is observed, cut off the shoot just below the damage and destroy the material to prevent the beetle’s development. In extreme cases, an immediate post-bloom insecticide application can help reduce populations. 

Make collaboration seamless with SAM Tool 

The Sustainable Agricultural Management (SAM) Tool makes it easy for vineyard managers, field workers and research collaborators to stay organized, whether you’re in the office or out in the field. Using the SAM Tool from your phone or desktop, you can: 

• View scouting photos and notes 
• Assign and complete tasks 
• Add task-completion comments 
• Track restricted-entry areas 
• Access the farm calendar 

Get started with the mobile app 

To add workers or collaborators to your digital farm team, they’ll need to create a free SAM Tool account on the SAM Tool website or directly in the mobile app: 

• Android phones: SAM Tool on Google Play 
• iPhones: SAM Tool on the App Store 

Once workers or collaborators have signed up, farm managers can add them to the digital workforce using the “Add User” feature on the SAM Tool website. If you have suggestions or would like help getting started, please contact us at sustainableag@msu.edu or reach out to Karen Chou, PhD, directly at chouk@msu.edu. We’d be happy to show you how SAM Tool can support your vineyard’s recordkeeping, compliance and research efforts. 

Upcoming events 

Third Thursday – Gibberellic Acid Vineyard Trial 

Join us for an on-site vineyard presentation and tour focused on the ProGibb trial. We’ll explore the use of Gibberellic Acid in Pinot Blanc and Riesling, testing its efficacy alone and with two surfactants (Embrece-EA and Talavera) to reduce cluster compaction and mitigate Botrytis risk. 

Date: Thursday, July 17, 2025 
Time: 3:00 PM – 5:30 PM 
Location: Hawthorne Vineyards, 1000 Camino Maria Dr, Traverse City, MI 

Register 

The 36th annual Michigan Viticulture Field Day and 2nd annual Michigan Enology Experience 

Michigan State University’s famous Viticulture Field Day on July 30 is a staple for the state’s grape growing community. The latest local viticulture research will be showcased. New last year for the winemakers, MSU partnered with Lake Michigan College to add a second day for wine making education. The Michigan Enology Experience on July 31 provides hands-on education for new and established enologists alike. View more information and registration details. 

Register 

2025 Dirt to Glass Conference: Elevating Michigan Wine from the Ground Up 

Join grape growers, winemakers, researchers and industry leaders Aug. 21-22 in Traverse City, Michigan, for two days of insight, collaboration and innovation. From site selection to sustainable practices and wine marketing, this event is designed to strengthen Michigan’s wine industry—one vine and one voice at a time. Register now and be part of the movement from intention to impact. 

Register 

Follow us on Facebook and Instagram for the latest industry news, educational programs and viticulture resources. Stay connected and informed with timely updates and expert insights. 

Related articles 

• Rose chafer management for northwest vineyards 
• Fruit insecticide registration update for 2023 
• Southwest Michigan fruit update – June 4, 2024 
• Michigan grape scouting report – June 8, 2022 
• 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