Michigan grape scouting report – August 10, 2022

Esmaeil Nasrollahiazar<nasroll2@msu.edu>, Michael Reinke<reinkem3@msu.edu>, Paolo Sabbatini<sabbatin@msu.edu>, Timothy Miles<milesti2@msu.edu>, Rufus Isaacs<isaacsr@msu.edu>, Jacquelyn Perkins<albertj9@msu.edu>, Sushila Chaudhari and Cheyenne Sloan<sloanche@msu.edu>, Michigan State University Extension - August 10, 2022

Veraison is here. Harvest is coming!

A grape vineyard. — Photo 1. Photo by Esmaeil Nasrollahiazar, MSU Extension.

Weather

Click below for detailed seven-day forecasts for various grape production regions

Southwest

Southeast

Romeo

Northwest

Last week was pretty typical for the second week of August in Michigan’s grape-growing regions. On average, Michigan picked up 504.5 growing degree days (GDD) base 50 degrees Fahrenheit, which was 95 GDD more than the previous week. Southwest AVAs were the warmest grape-growing areas in Michigan, picking 178, 37.5 GDD more than the previous week. The southeast AVAs were the second warmest regions on average and picked up 176 GDD, while northwest Michigan picked up 150.5 GDD. The Tip of the Mitt AVAs in Michigan, as usual, was the coolest region and picked up 11 GDD more than the previous week.

Southwest Michigan GDD Summary from March 1 through Aug 08, 2022 Southeast Michigan GDD Summary from March 1 through Aug 08, 2022 Northwest Michigan GDD Summary from March 1 through Aug 08, 2022
Enviroweather station	Current GDD 50 F	GDD 50 F last week	Collected the past week
Benton Harbor (SWMREC)	2002	1809	193
Fennville	1807	1633	174
Lawton	1973	1795	178
SW Average	1946	1768	178
Romeo	1848	1673	175
SE Average	1895	1719	176
Old Mission	1486	1342	144
Petoskey	1442	1303	139
Traverse City (NWMHRS)	1553	1405	148
NW Average	1470.5	1328	150.5

Vine growth

In southern Michigan, grape clusters are bunch closure, and hybrid and some vinifera wine varieties are at veraison (Marquette, Geneva Red, Golubok, and Vanessa).

In the northwest AVAs, wine grape cultivars’ clusters are developing and berries are at buckshot berries and are approaching bunch closure (Photo 2). Some locations in the Leelanau peninsula, some early varieties such as Chardonnay, show some berry softening (Photo 3). In the tip of the mitt, hybrid cultivar berries are at bunch closure on most cultivars. Berries are beginning to soften.

See this chart for grape growth stages.

Vinifera grape at bunch closure. — Photo 2. Vinifera grape at bunch closure in northwest Michigan. Photo by Esmaeil Nasrollahiazar, MSU Extension.

Closeup of a grapes growing. — Photo 3. In northwest Michigan, berry softening and sugar accumulation are starting. Photo by Esmaeil Nasrollahiazar, MSU Extension.

Horticulture

Hedging is going to begin in vineyards in Michigan soon. The main objectives of hedging are to cut back on extra primary and lateral shoot growth on the top and sides of the canopy to avoid shading and shoot entanglement between vine rows to make the vineyard more accessible to workers and tractors. Hedging can further encourage growth by promoting lateral shoot growth in robust vines, even though it indirectly reduces the canopy by removing primary and lateral shoots. Hedging is best done in the early to mid-summer.

Hedging is typically done between fruit set and veraison, when primary and lateral shoots begin to bend (at a 45-degree angle), and it is critical to keep leaves, fruit, and developing buds exposed to enough light in dense canopies with excessive vegetative growth. Hedging too early in the growing season should be avoided because it can lead to increased lateral growth and canopy density.

Over-cropping has been reported in some juice grape vineyards. Crop load management is critical to achieving your desired Brix level. In order to assess your cropload, use the MSU Concord model on Enviroweather and conduct a cluster thinning at 1500 GDD. For more information on this topic, see MSU’s Crop Control in Grapevines - A report from the Southwest Michigan Research and Extension Center .

Diseases

In the southwest, disease management of downy mildew, powdery mildew and botrytis fruit rot are the primary focus currently. The heavy dews we are experiencing means it is important to maintain protection against downy mildew. With tight clustered varieties, this is the last chance to get botrytis bunch rot sprays into the tight crevices of the clusters. In the northwest, disease pressure is increasing and a high pressure of powdery mildew has been reported from Leelanau County (Photo 4). In specific areas, black rot is becoming more apparent as well (Photo 5).

Photo 4. Powdery mildew infection on cluster and basal leaves and Phomopsis infection on shoots. Photo by Esmaeil Nasrollahiazar, MSU Extension.

Black rot on grapes. — Photo 5. Nearly all of Michigan's grape-growing regions have black rot infested grapes at this time of year. Photo by Esmaeil Nasrollahiazar, MSU Extension.

In this growing stage, you should consider choosing fungicides that control all the foliar and fruit diseases. For example, with downy mildew we are most concerned with foliar infection at this time and sprays should be timed regularly throughout the season when we experience heavy dews 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.

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.

As we approach bunch closure in the southwest, it is important to consider botrytis management. Several strategies contribute to good botrytis bunch rot management including opening up the canopy, properly applying fungicides, and using resistant cultivars when possible. Good botrytis control depends on getting good coverage. Just before bunch closure is the last chance to apply a fungicide to the inner part of the developing cluster. Fungicide resistance management is also important. 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 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

At vineyards in southwest Michigan scouted this week, there was a wide range of grape berry moth (GBM) infestations (Photo 4). The numbers of GBM-infested clusters were higher this week as the third generation of GBM picks up. Egg hatch was predicted to occur on Aug. 7 in the Lawton area, so initial cover sprays for protecting against the early half of the third generation should already be applied.

At high pressure vineyards, or areas of farms, it is also critical that these applications are followed up with another cover spray to hit the peak of GBM egg laying and larval pressure that will happen 10–14 days later. This should prepare growers for the harvest season with vineyards that can have a lower risk of damage in the pre-harvest window. Always be aware of the predicted rain showers over the coming week and avoid application just before rain.

After the 2021 experience, we are carefully watching vineyards with weekly scouting and growers should be doing the same to make sure that cluster rots and vinegar flies do not develop to be too much of a challenge. Monitoring traps are being added to vineyards in the SW and we will report those trap catches in the coming weeks. We are already primed for seeing some late-season grape berry moth pressure in SW Michigan given the early start to the third generation, and the long-lead weather forecast also calls for warm evenings. Continue to monitor this as we approach harvest and be prepared to control rots and their vinegar fly vectors with treatments once the berries reach 14 Brix. A future extension article will review our recent results on this topic and recommendations for the post-veraison part of the season.

Upcoming events

Register for Dirt to Glass 2022: Elevating Michigan Wine from the Ground Up. Michigan State University Extension is hosting a new conference for the Michigan grape and wine industry. Dirt to Glass 2022: Elevating Michigan Wine from the Ground Up is the first ever of its kind in Michigan and this year it will take place in Traverse City on August 25-26. This year's Dirt to Glass conference will focus on soil health, soil identification and soil fertility. Each year, the conference chooses a specific area of study based on the results of an annual educational needs assessment survey.

Tile Drainage Field Day 2022. Linking Soil Health, Nutrient Management, and Water Management for Improved Water Quality. The event will be hold on Thursday, Aug. 25, 2022 at 13000 Bird Lake Rd., Camden, MI 49232

This work is supported by the Crop Protection and Pest Management Program [grant no 2021-70006-35450] from the USDA National Institute of Food and Agriculture.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).