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

Last week was cool for the end of May. Statewide, Michigan high temperatures remained largely in the mid-60s, if not cooler. Extreme inversions surprised many areas around the state on Sunday and Monday mornings, with some locations waking up to damaging frosts in low lying areas. Monday saw the start of a quick warmup. Southern Michigan hit the low 80s, upper 70s in the northern lower peninsula. The only rain recorded last week was a short-lived system that moved through the area on Wednesday of last week, bringing a tenth to a quarter inch for the state.

A storm front will again bring a line of storms on Wednesday that will drop Michigan back down into the 60s and 70s for the remainder of the week. This storm system should drop a half to an inch of rain. The system will exit the region later on Wednesday, but remnants will return into southern Michigan on Friday, bringing another chance of scattered rain with lower amounts of accumulation.

With the warmer weather this week, evapotranspiration needs of the grapevines will be higher. The state is expected to see up to an inch of rain, but the models are indicating plants will be wanting over an inch of water. Parts of southern Michigan are still seeing abnormally dry soils, according to the U.S. Drought Monitor. As we get to bloom, the vines will potentially need some additional water. For those that have irrigation installed, testing the systems ahead of the need can save emergency repairs later down the road.

Vine growth

In southern Michigan, little movement occurred last week. Juice grapes and early hybrid wine grapes are very close to bloom. Early bloom has been seen on Brianna cultivar at the Southwest Michigan Research and Extension Center. late hybrids and Vitis vinifera cultivars are around 12 inches of growth.

In northern Michigan, Vitis vinifera varieties have advanced one stage further in their development, with growth continuing despite the slow start caused by last week’s cool weather. Currently, most vines have newly grown shoots ranging from 3 to 5 inches in length, depending on the variety, and initial clusters are visible. Although growth is progressing, it remains early in the shoot development stage, leaving vines somewhat susceptible to environmental stresses, particularly late spring frost. While the likelihood of a frost event is low, the cooler spring conditions warrant vigilance. Growers are encouraged to closely monitor weather forecasts and implement frost protection measures if needed. Tracking phenology at this stage helps inform timely decisions for pest management, fertilization, and canopy work. For reference, see MSU Extension’s Grape Growth Stages chart.

In the Tip of the Mitt American Viticultural Area (AVA), finally some heat and sunshine this week has most hybrid cultivars in the 4-8 inch shoot stage, with Vitis vinifera varieties in the 1-3 inch growth stage. Unfortunately, there was a significant frost event the night of May 31 into June 1, which has resulted in some frost damage to young vines and vines planted in lower lying vineyards.

In southeast Michigan, Concord’s grape shoot extension has been observed as long as 24 inches in Hartland. Measurements taken on May 28 at Youngblood’s in Ray Township showed 12 inches shoot growth in Frontenac and Frontenac Blanc, 12 inches shoot growth in Itasca, and 10 inches shoot growth in Marquette. Wild grape bloom was just starting in Ray Township on May 28, but not near 50%. At Kapnick’s Orchard in Lenawee County on May 29, however, full bloom was observed in wild grape. Grape berry moths have been caught in traps at Kapnick’s and Youngblood’s.

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.

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

Thrips activity has been observed in vineyards in southwest Michigan this week, with activity mostly on clusters. These are tiny insects that can best be sampled by tapping clusters over a phone screen or sheet of paper. 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 increasing across the state, and we are scouting for larvae in clusters in southwest Michigan. With bloom approaching or underway in that region there may be 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. Look for the wild grape vines with a shiny underside to their leaves to distinguish them from summer grape (left side in the image below).

Activity of grape cane gallmaker was detected this week in southwest Michigan, where swollen shoots have larvae developing inside. Grape tumid gallmaker infestation is starting on leaves and clusters in susceptible varieties. If potato leafhopper is detected, protect susceptible cultivars especially in 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.

SAM Tool new mobile app for vineyard scouting & recordkeeping

Whether you’ve used the Sustainable Agriculture Management (SAM) Tool before or are just getting started, this year’s updates are designed to make recordkeeping and task management easier and more efficient—right from your smartphone or desktop.

New mobile app – Please delete the old one

If you have the older version of the SAM Tool mobile app on your phone, please delete it and download the new version to ensure full access to the updated features. Use the QR codes below to install the latest version:

• Download mobile app from the Google Play
• Download mobile app from the Apple Store

Roles and access

• Managers and workers can access their associated vineyards on both the desktop and mobile apps.
• Managers can create, edit, and perform tasks.
• Non-managerial workers can view and perform assigned tasks only.
• To add workers to your digital workforce:
  1. Ask your workers to download the app and sign up using their email.
  2. As a manager, use the "Add User" function and enter the worker's email to bring them into your team.
  3. Once added, workers can use their phones to view and complete their assigned tasks.

Desktop or mobile – Use what works best for you

While the mobile apps offer powerful on-the-go access, some users may still prefer the desktop version for creating and managing tasks. You can use both—the SAM Tool is designed to support your workflow in the field and in the office.

To learn more or get help getting started with the SAM Tool, visit the SAM Tool website or contact Karen Chou at chouk@msu.edu. For this growing season, we are offering one-on-one assistance to help growers migrate their management plans into the SAM Tool.

Upcoming events

Michigan Grape Society’s June twilight grape growers social

The June growers’ social will be at St. Julian’s Mountain Road vineyard on June 18. In addition to the great wine and pig roast, Paolo Sabbatini will be leading a discussion on canopy management. Activities begin at 6 p.m. Register online.

Third Thursday – Biochar and Drone Demonstrations

Join us Thursday, June 19, from 1-4:30 p.m. online or at Shady Lane Cellars in Suttons Bay, Michigan, for our June Third Thursday hybrid event featuring two hands-on demonstrations:

• Biochar Production and Vineyard Applications: Learn about biochar production and its potential to improve soil health and sustainability in vineyards.
• Drone Spray Demonstration: Explore the latest in aerial spray technology and strategies to reduce chemical use in vineyard pest and disease management.

This informal, educational event is open to all and will include restricted use pesticide (RUP) credit opportunities (pending approval).

We’ll be sharing more details and the registration link soon.

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.

MDARD seeks wine grape grower input

Advisory committee nominations accepted through June 11, 2025

The Michigan Department of Agriculture and Rural Development (MDARD) is seeking input on the potential establishment of a legislatively enacted Michigan wine grape commodity group after growers submitted enough signatures to pursue organization. The department is currently taking nominations for a temporary advisory committee composed of industry members, which will oversee the process moving forward.

“MDARD is committed to maintaining a diverse food and agriculture industry within Michigan and highlighting unique agricultural opportunities,” said MDARD Director Tim Boring. “The potential establishment of a wine grape commodity group would support Michigan craft beverage producers and businesses to continue to grow their product, expand their knowledge and improve their economic impact.”

In Michigan, Public Act (P.A.) 232 provides an avenue for commodity growers to organize and vote on whether or not to establish a legislatively enacted commodity group. Two hundred signatures or 25 percent, whichever is lower, from growers are first needed to start the process, which are then submitted to MDARD as required by the statute. MDARD currently has legal, fiscal and program oversight responsibility for 15 legislatively established commodity boards.

Michigan wine grape growers, and businesses affiliated with the purchase of Michigan wine grapes, are encouraged to submit their contact information to receive updates about the P.A. 232 process, including information on committees, public hearings, and referendums.

Using this form, grape growers may also nominate themselves to participate in the temporary advisory committee. Survey responses are due June 11, 2025 to be considered for the committee.

The responsibilities of a temporary advisory committee will include developing program recommendations to be considered by fellow wine grape growers at future public hearings. If the feedback at those hearings is positive, a referendum date will be set for qualified producers of the commodity. A successful referendum is the final step in enacting a wine grape P.A. 232 group.

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