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 degrees Fahrenheit)

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

Weather forecast

Michigan saw another week of average temperatures last week. Southern Michigan grape growing regions saw high temperatures in the 70s with a couple days in the mid-80s in the middle of the week. Northern regions reported temperatures 5-10 degrees cooler. Many grape growing areas saw very little rain over the last week. Scattered rain at the end of last week resulted in less than a tenth of an inch of rain. The Tip of the Mitt was the one exception. The system dropped almost a half-inch for the grape growers in that American Viticultural Area (AVA).

Michigan started seeing a warmup on Monday, with temperatures climbing 10 degrees from last week’s averages. This warmup will continue through the first part of next week. Southern Michigan is expected to see highs in the mid-90s Sunday and Monday, and upper 80s in the northern AVAs. A storm system on Wednesday evening is expected to bring strong winds and up to an inch or more of rain to much of Michigan.

Vine growth

In southern Michigan, grapes are blooming. Some early varieties are completing bloom. Later varieties and most vinifera are either just starting to bloom or immediately prebloom. Shoot positioning and tucking are continuing. Some growers are talking about leaf removal around the clusters.

In northern Michigan, Vitis vinifera continue to develop, though cooler-than-normal temperatures last week have slowed progress. GDD accumulation is approximately 130 units behind last season, and vine development is noticeably delayed. This week, shoots are measuring 7-15 inches, and clusters are expanding; however, shoot elongation and overall canopy growth remain subdued due to the recent cool weather. Despite the slower growth, this remains an ideal time to perform canopy management tasks such as shoot thinning, particularly at E-L stages 15–17, to enhance light penetration and air circulation.

Nutrient scheduling should also be prioritized to support vine health, although the cooler conditions may slightly delay nutrient uptake. In summary, while vine development is ongoing, it continues at a slower pace than usual, and timely canopy and nutrient management remain essential. Tracking phenology at this stage enhances decision-making for pest management, fertilization and canopy work. For detailed staging, refer to MSU Extension’s Grape Growth Stages chart.

In the Tip of the Mitt AVA, hybrid cultivars are in the 10-16 inch shoot stage with clusters in the immediate prebloom stage. Vitis vinifera cultivars are in the 4-8 inch growth stage with some shoots entering the 10-16 inch stage. Wild vines are in early stages of bloom. Rose chafers have been noted in some vineyards. Phomopsis and black rot pressure has been light thus far.

In southeast Michigan, bloom has started in juice grapes, table grapes and hybrid wine grapes. Grape berry moth counts in Lenawee and Macomb counties are lower than last week. Powdery mildew risk is high this week from June 18-19 and 22-25, though fruit is not present for infection. There is some risk for downy mildew and grape black rot infection from June 18-20.

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.

The accuracy of petiole analysis depends heavily on sampling the correct leaf position. At full bloom, the petiole should be collected from the leaf that is directly opposite the basal grape cluster, typically the fourth to sixth leaf from the base of the shoot. This leaf is considered physiologically mature and reflective of the vine’s nutrient reserves at bloom. At veraison, petioles should be collected from recently matured leaves, generally located around the fifth to seventh leaf from the shoot tip.

A representative sample should include petioles from a sufficient number of vines across the block or variety, ideally 60 to 100 petioles collected from at least 20 to 25 randomly selected, healthy vines. Leaves chosen for sampling should be free of visible disease, pest damage or physical injury. To avoid contamination, petioles should be collected with clean tools and placed in a paper bag (not plastic) to allow for air drying. Samples should be clearly labeled with block, variety, date and phenological stage, and should be dried or delivered promptly to a testing laboratory.

When properly conducted, petiole analysis provides a clear snapshot of vine nutritional health and supports informed management decisions that contribute to balanced growth, fruit development and overall vineyard sustainability.

Soil tests evaluate nutrient content, pH levels and other soil characteristics, offering insights into the soil's fertility and potential nutrient deficiencies. Petiole analysis, which involves testing the leaf of grapevines, directly measures the nutrients absorbed by the plant, providing a more immediate understanding of the vine's nutritional status. Regular monitoring through these methods allows for timely adjustments to fertilization practices, ensuring optimal nutrient availability throughout the growing season.

During the early stages of vine growth, particular attention should be given to the application of nitrogen, zinc and boron, as these nutrients are vital for shoot development and flowering. Potassium and magnesium are also important during this period, supporting various physiological processes within the vine.

Nitrogen plays a crucial role in vegetative growth and fruit development. However, its application must be carefully managed:

• Excessive nitrogen: Can lead to delayed flowering and excessive vegetative growth, potentially compromising fruit set and quality.
• Insufficient nitrogen: May result in reduced vine vigor, leading to lower yields and diminished fruit quality.

Therefore, precise nitrogen management, informed by soil and petiole analyses, is essential to balance vegetative growth and fruit production effectively. Shoot thinning will begin in the southern vineyards in the next week or two. Shoot thinning is an important canopy management tool to improve air circulation, minimize disease pressure, reduce shading, and improve spray penetration. The right time for shoot thinning time is when the shoots are 5-12 inches long.

The period from pre-bloom to fruit set presents a narrow yet crucial window for cultural practices that directly influence fruit quality, vine balance and long-term productivity. The following are key strategies recommended by leading viticulturists, including the Michigan State University’s viticulture team, with a focus on early leaf removal, shoot thinning and trunk cleaning as part of a precision canopy management approach.

Effective canopy management begins in the pre-bloom phase when the vines start to direct energy toward flowering and initial fruit development. The primary goals at this stage are to:

• Optimize sunlight exposure.
• Improve air circulation.
• Balance vegetative growth with reproductive load.

Shoot thinning is often the first intervention. By removing excess shoots, especially non-fruitful or weakly growing ones, growers reduce canopy density and reallocate vine resources toward stronger shoots. According to the MSU viticulture team, optimal shoot densities (15-18 shoots per meter of cordon) significantly enhance fruit exposure and uniformity.

One of the most impactful practices during this period is early leaf removal, specifically removing basal leaves around the fruit zone before bloom or shortly thereafter. Research conducted by the MSU viticulture tram across multiple vineyards and years in northwest and southwest Michigan have demonstrated that:

• Early defoliation improves fruit zone microclimate and reduces incidence of cluster rot (e.g., Botrytis).
• It modifies fruit set by reducing the number of berries per cluster, effectively decreasing compactness and improving airflow.
• Phenolic composition, especially flavonoids and anthocyanins, improves due to better light penetration.

Early leaf removal is particularly valuable in cool, humid climates like Michigan where tight clusters are prone to disease. Trials with Merlot and Pinot noir varieties reported an increased berry skin-to-pulp ratio and enhanced aroma potential when defoliation was implemented pre-bloom.

Often overlooked, trunk cleaning is critical for disease management and vine longevity. Trunks should be inspected and cleared of basal suckers and adventitious shoots that harbor pathogens and divert energy.  Coupled with sucker removal, these sanitation practices create a cleaner, more efficient vine structure, easing future operations such as spraying and harvest.

For more comprehensive information on shoot thinning and other early season vineyard management practices, please refer to the article, "Early season vineyard management."

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.

As bloom continues in southwest Michigan, start choosing fungicides that control all the fruit diseases. 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

Continued thrips activity has been observed in southwest Michigan vineyards, with activity mostly on clusters that are in bloom. These are tiny insects that can best be sampled by tapping clusters over a phone screen or sheet of paper. We have also deployed yellow sticky boards, and some of those had high densities of thrips when checked this week. We don’t have thresholds developed for these pests, but their feeding in spring can lead to reduced fruit set and “giraffe skin” scarring on clusters at harvest time.

Grape berry moth adult catches in traps are still high across southern Michigan sites where we are monitoring, but we are not yet finding grape berry moth eggs or larvae in clusters. With bloom underway, we expect first generation egglaying in the coming weeks. However, this early-season activity is usually very low and clusters can compensate for low levels of early season feeding. Getting a close read on the bloom timing of Vitis riparia (wild grape) is important for initiating the grape berry moth model that is posted on MSU’s Enviroweather system. Although there were some early sites with bloom in late May, there was widespread wild grape bloom in Berrien County around June 2. We should be checking the wild grape vines with a shiny underside which distinguishes them from summer grape.

Yellow sticky traps showing high thrips density at some vineyards where bloom is underway highlight the need for vigilant monitoring and control.

Potato leafhopper has been detected in southwest Michigan, which was predicted given the recent weather systems with airflow from the southern sections of the Mississippi Valley. Growers of grapevines that are stunted by this pest should scout their plants to check for leafhoppers and protect susceptible cultivars, especially young vineyards where this pest can stunt shoot growth. Neonicotinoid insecticides such as Assail and Admire Pro can provide protection against potato leafhopper, rose chafer and tumid gallmaker if present in the same vineyard site.

Make SAM tool your vineyard assistant this season

The Sustainable Agriculture Management (SAM) Tool helps you stay on top of vineyard tasks while streamlining records for decision-making and compliance. Here are some of the most helpful features growers are using right now:

Photo-integrated recordkeeping. Capture scouting notes, pest observations or other issues using photos tied to specific tasks, blocks and dates. One grower recently used SAM Tool to record petiole sampling by attaching photos of sample bags and noting geo-locations, all saved for future reference and easy sharing with vineyard team or advisors.

Task tracking. Keep track of completed, scheduled and upcoming tasks like fungicide applications, shoot thinning or irrigation. Task lists update automatically across desktop and mobile, so your team stays in sync.

Worker communication and coordination. Assign tasks to team members with clear instructions and deadlines. Assigned workers receive email reminders and see what needs to be done and when, reducing confusion during busy weeks.

Product usage log. Enter pesticide applications once—SAM Tool auto-generates compliant records. This feature helps ensure safety requirements are met and saves time for recordkeeping.

Mobile-friendly design. Whether you're scouting in the field or reviewing records in the office, SAM Tool is optimized for both desktop and mobile use. The SAM Tool goes where you go.

Interested in trying SAM Tool? Visit the SAM Tool website to create your farm and blocks or email sustainableag@msu.edu and we’ll help you get started.

Upcoming events

Third Thursday – Biochar and Drone Demonstrations

Join us Thursday, June 19, from 2-6 p.m. at Shady Lane Cellars in Suttons Bay, Michigan, for our June Third Thursday event featuring hands-on demonstrations and practical insights into sustainable vineyard management practices.

Schedule:

• 2 p.m. – Biochar production demonstration begins
• 3 p.m. – Biochar talk with Paul May
• 4 p.m. – Drone sprayer demonstration by Mike Reinke

A casual happy hour will follow at Shady Lane Cellars (glass and bottle purchases only; no outside alcohol permitted).

Please park at the tasting room—not at the winery facility.

This event offers 2 RUP credits.

Register

Don't miss this chance to learn, network and explore innovative vineyard technologies. We hope to see you there!

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.

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.

Related articles

• Early season vineyard management
• Early season control of grape berry moth in Michigan vineyards for 2022
• 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