MSU Extension Grapes

Weather update

Winter 2024-2025 cold hardiness report: Overview and observations

The 2024-2025 winter presented typical northern-tier challenges for Michigan vineyards, with several periods of extreme cold but relatively limited ice coverage on Lake Michigan. While East Grand Traverse Bay briefly froze in mid-February, the West Bay did not, reducing lake-effect moderation in some vineyard sites. This may have increased cold exposure risk, especially in inland and less protected locations.

Grapevine cold hardiness overview

Field evaluations conducted from November 2024 through March 2025 tracked lethal temperature to 50% of primary buds (LT50) for 11 Vitis vinifera and one high-tunnel Cabernet Franc block. Findings revealed significant varietal differences in cold tolerance:

• Most cold-hardy varieties: Pinot Noir, Pinot Gris and Cabernet Franc (especially high tunnel) maintained lower LT50 thresholds, indicating strong resilience to freezing conditions.
• Moderately hardy varieties: Chardonnay, Riesling and Pinot Blanc showed steady acclimation but increased vulnerability in mid-winter.
• Least cold-hardy varieties: Refosco and Teroldego exhibited higher LT50 values and slower acclimation, particularly early in the season.

By March, most varieties began deacclimating, increasing susceptibility to late winter or early spring frosts. This year, bud winter damage is relatively low (less than 20% bud mortality) in the best sites and does not require any specific pruning adjustment due to bud loss.

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 should expect to see a continuation of the warm weather for the next few days. Most grape producing areas are expected to see small chances of scattered rain showers over the next few days with the best chances over Thursday night into early Friday morning. Most portions of the state are predicted to see total accumulations of a half an inch of rain or less. Parts of southwest and central Michigan might see up to an inch of rain. The next chance of rain comes Tuesday and Wednesday.

A cool down will follow the rain system. Temperatures will drop into the upper 60s in southern Michigan, mid-50s in northern locations. These temperatures should continue for the next couple of weeks. The medium-term predictions are for a cooler and wetter than average end to May in Michigan.

Vine growth

Juice grapes and early hybrid wine grapes are at 4-8 inches of shoot growth in southern Michigan. Clusters are separating from the shoots and beginning to elongate. Early hybrids and vinifera cultivars are between first leaf and 1-inch of growth.

In northern Michigan, most Vitis vinifera are transitioning from late bud swell to early bud break—a key shift from dormancy to active growth triggered by warming temperatures. Bud break, marked by emerging green shoot tips, is a vulnerable stage due to frost risk, which can impact yields. Monitoring forecasts and applying frost protection are essential. Recording bud break also helps guide timely decisions for pest management, fertilization and canopy work. For reference, see MSU Extension’s Grape Growth Stages Chart.

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 what nutrients the vineyard needs.

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, pay particular attention to the application of nitrogen (N), zinc (Zn) and boron (B), as these nutrients are vital for shoot development and flowering. Potassium (K) and magnesium (Mg) 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 year, the primary diseases of concern for grape growers are phomopsis, black rot, anthracnose and powdery mildew. If you're seeking detailed insights into prebloom fungicide options and the effects of rain on disease spread, we recommend referring 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

First generation grape berry moth larvae have been detected in grape clusters in recent weeks in southwest Michigan. Based on the degree day model for this pest, the start of the second generation egglying is expected in late June/early July in southwest Michigan, proceeding later into July at further north locations. Also, the first sightings of grape tumid gallmaker infestation and the first rose chafers were detected last week in southwest Michigan.

Potato leafhopper has also been found in grape vineyards and is active across the state from recent spring invasion on warm winds from the south. Protect susceptible cultivars, especially in young vineyards where potato leafhopper 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.

Begin your 2025 scouting, spray planning and auto-recordkeeping with the SAM Tool

One user shared, “It took me only 53 seconds to create the first task, and less than 30 seconds for each one after that.” Imagine the efficiency and simplicity this tool can bring to your daily vineyard operations! The Sustainable Agriculture Management (SAM) Tool now supports both spray and non-spray task management with a more intuitive, user-friendly interface. It’s easy to use and can help you save time and money.

The latest version includes:

• Streamlined task planning
• Easier task duplication and editing
• Improved tracking for both spray and non-spray activities
• Last-minute task changes and timely communication with workers
• A redesigned mobile interface with a clearer layout
• Support for grower participatory research
• Many other management assistant functions

SAM Tool is free to use and developed specifically with Michigan grape growers in mind. We support small growers and large growers. Whether you’re returning to the platform or trying it for the first time, now is the perfect time to explore how it can support your work this growing season.

The SAM Tool team is offering personal Zoom assistance through May 30, 2025, for anyone who would like a walkthrough or help getting started. Visit Sustainable Agriculture Management or email Karen Chou directly at chouk@msu.edu to schedule a time.

Upcoming events

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.

35th annual Michigan Viticulture Field Day and inaugural Michigan Enology Experience

Michigan State University’s famous field day and steak dinner is back on July 31. We are now adding a second day just for the winemakers. Stay tuned for more details and the registration link.

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