Michigan grape scouting report – July 15, 2020

Bloom has ended in northern Michigan. Diseases and berry moth larvae have been found on clusters in southern Michigan.

Grape vineyard
Vineyards throughout the state look good. The warm, dry weather helped with fast growth and slow disease development, but we are getting into the season for heavy dew events. These can cause rapid disease development. Photo by Mike Reinke, MSU Extension.

Weather

The last week started hot with highs in the upper 80s in northern areas and lower 90s in southern Michigan. A cold front came through on Friday, July 10, bringing storms to much of Michigan. Most areas saw a half to 1 inch of rain. It also brought cooler temperatures for the weekend and start of this week, with highs in the 70s and lows in the upper 50s. The heat is returning with temperatures again near 90 by the weekend. A weather front on Wednesday into Thursday will the best chance of rain this coming week.

With the variable week, we still picked up a lot of growing degree days (GDD) last week: 150-180 GDD base 50. The southwest region is 345 GDD base 50 ahead of the northwest region.

Northwest Michigan GDD summary from March 1 - July 13, 2020
Southwest Michigan GDD summary from March 1 - July 13, 2020
Southeast Michigan GDD summary from March 1 - July 13, 2020

Northwest stations

GDD 50 F

GDD 47 F from wild bloom (grape berry moth model)

Petoskey (Petoskey)

912

661

Traverse City (NWMHRC)

1003

723

Old Mission (Old Mission)

947

682

Avg for NW region

965

 

Average last week

815

 

Southern stations

GDD 50 F

GDD 47 F from wild bloom (grape berry moth model)

Benton Harbor (SWMREC)

1344

1083

Lawton (Lawton)

1322

1007

Fennville (TNRC)

1189

917

Avg for SW region

1310

 

Avg last week

1136

 

Romeo (Romeo)

1232

943

Avg for SE region

1252

 

Avg last week

1073

 

Note: Wild grape bloom dates are estimates. See “Mid-season management of grape berry moth” for bloom dates.

Dew on grapes
Heavy dew events are common during mid-summer, especially during periods of high humidity like we have been experiencing. These are good conditions for downy mildew and other diseases. Photo by Mike Reinke, MSU Extension.

Vine growth

Grapes in southern Michigan are at buckshot berry and many are at berry touch. Tight cluster varieties such as Vignoles are at bunch closure. Growers have been doing leaf removal and continuing to do shoot positioning on most vinifera varieties. This allows sprays to reach the clusters more easily, increases air circulation and improves sunlight exposure. Now is the time to accurately estimate vine yields. With a good estimate you can determine if, when and how much fruit you should remove at thinning time. The Michigan State University Extension bulletin “Predicting Harvest Yield in Juice and Wine Grape Vineyards (E3186)” provides tools to reduce both annual yield and quality variability among years.

In northern vineyards, most vinifera are at or near buckshot berry, depending on the variety. Growers are starting to perform leaf removal while they are tucking shoots in the catch wires. See this chart for grape growth stages.

While the rains over the weekend provided a brief break, the hot, dry weather the last several weeks dried many soils around the state. Southwest and southeast Michigan are the most impacted at this point. Growers that have irrigation systems in their vineyards should inspect them to be ready if they start seeing drought stress. With the hot weather continuing for at least the next week, irrigation might be necessary soon, especially on young vines and vineyards on lighter soils.

Symptoms of drought stress will follow a general progression. Tendrils will first start to dry out, followed by shoot tips falling downward and leaves bending. Under severe drought stress conditions, shoots stop, tendrils will dry up and shoot basal leaves will turn yellow.

Diseases

At this time of year, the disease focus for most grape growers around Michigan is on downy mildew and powdery mildew management. Powdery and downy mildew leaf and cluster infections have been observed statewide.

Powdery mildew on cluster
Cluster infestations of powdery mildew are now being found. Both powdery and downy mildews produce a white infection on clusters, but powdery mildew sporulation is more diffuse. Downy mildew produces a denser white mat of spores. Photo by Tim Miles, MSU.

Downy mildew requires wet leaves to develop. Rain is the most common source for leaf wetness. As little as 0.5 inches of rain is enough, but warm nights with high humidity and heavy morning dews such as those most have experienced in the last few weeks are also good for development. During buckshot berry, downy mildew infections that occurred in previous weeks may become visible, especially with enough moisture. Fungicides applied more than one week post-bloom protect against defoliation but have limited control over fruit infections.

Unlike the other grape fungal diseases, powdery mildew doesn’t need leaf wetness to infect, just humid conditions. Even with low ambient humidity, the humidity inside the shaded canopy is higher. Opening up the grape canopy for air movement helps reduce powdery mildew and allows better spray penetration. Shoot positioning and cluster zone leaf removal can help. For more information on how weather conditions affect powdery mildew development, see “Why some seasons are worse for powdery mildew” from Good Fruit Grower.

For more information on both powdery and downy mildew, see “Protecting young clusters from powdery and downy mildew” from MSU Extension. For an overview on the general diseases and management options, see “Early season vineyard disease management” from MSU Extension.

Black rot cluster damage is showing up in several research blocks and some commercial vineyards in southern Michigan. Black rot develops when rains trigger spore releases, typically in the spring. The timing for berry rot management is from immediate pre-bloom until three to four weeks after bloom. This window is closed for juice and hybrid grapes and rapidly closing for vinifera grapes in southern Michigan.

Black rot
Black rot damage is becoming more common. Take note of these symptoms and be prepared to manage this disease next spring. Photo by Rufus Isaacs, MSU.

Several grape varieties are at bunch closure and now is an excellent time to consider controlling botrytis bunch rot, particularly in vinifera and hybrid grapes where it can be an issue. Several strategies contribute to good botrytis bunch rot management. These include opening up the canopy, properly applying fungicides and using resistant cultivars when possible. Good botrytis control depends on getting good coverage. Effective systemic fungicides for botrytis are in FRAC codes 2, 7, 9, 11, 12 and 17. A new Michigan Grape Fact Sheet is now available for managing botrytis bunch rot for more information about the disease.

A complete list of fungicides for all these diseases with efficacy and other details can be found in the E154 Michigan Fruit Management Guide from Michigan State University Extension. 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.

Insects

Note: Thoroughly scout before assuming insects are an economic problem.

Grape berry moth populations are active this year with the usual variation among vineyards, highlighting the importance of scouting. Several vineyards scouted in southwest Michigan have had 10% of clusters infested during the first generation, some with multiple larvae per cluster. Moth catches are still low, but we are now finding young larvae in clusters, indicating that second generation has emerged and are laying eggs. First sprays for grape berry moth should have gone on last week or the start of this week for much of southern Michigan. Start of egglaying is expected within the next week for much of northern Michigan.

Wild grape bloom is used as biofix for the grape berry moth model predicting timing for insecticide sprays. This biofix is recorded as 50% of the clusters on wild grape are at 50% bloom. For For 2020, approximate wild grape bloom dates are June 2 in Berrien, June 4 in Van Buren, June 5 in Allegan, June 11 in Ottawa and June 12 in Grand Traverse counties. The date can be entered into the grape berry moth model in Enviroweather to predict when egglaying will start for the second generation of berry moth.

Using the approximate bloom dates above, the start of egglaying July 4 in Berrien, July 6 in Van Buren, July 9 in Allegan, July 9 in Macomb, July 13 in Ottawa, July 18 in Grand Traverse and July 20 in Emmet counties. This highlights the timing when growers should be starting to think about protecting clusters from the second generation of this pest.

Once egglaying has begun, there are a number of insecticides that can be used to prevent grape berry moth infestation, but their timing is critical to give the best performance. Insecticides that affect eggs should be applied at the beginning of egg laying at 810 GDD47 after wild grape bloom. Intrepid and Altacor are examples of these. Insecticides that are best against berry moth larvae should be applied when eggs start hatching. This is 100 GDD47 after the start of egglaying (910 GDD47). The first larvae are predicted to start hatching in the southwest around July 13 in Allegan County, and in the southeast around July 12 in Macomb County.

Keep a close eye on the grape berry moth model for your area to know when to make applications and to select products that will target the correct life stage. There are a number of chemical options that target larvae. For vineyards that saw significant damage last year or already have seen damage this year from grape berry moth, a second spray targeting larvae should be applied two weeks after your first spray. For more detailed information on choosing the right insecticide, see “Mid-season management of grape berry moth” from MSU Extension.

Since grape berry moth attacks berries in the clusters, good spray coverage is critical for control. Be sure to use enough water to penetrate the canopy and cover the clusters. A spreader/sticker can also help cover the confined spaces inside the canopy.

Japanese beetles and potato leafhopper have both been reported throughout the state. Potato leafhoppers feed on leaves. Some sensitive grape varieties can get yellowed and cupped leaves, and stunt growth in young vines. Most mature vines are tolerant of this damage and control isn’t often warranted. Japanese beetles feed between the leaf veins causing a lace-like skeleton on heavily damaged leaves. They seldom feed directly on clusters. Japanese beetles are generally only considered a pest of new vineyards where vines are more sensitive to leaf feeding.

Foliar infestations of grape phylloxera have been found in vineyards over the last month. Reducing root infesting phylloxera is the main reason rootstocks are used. Hybrid and juice grapes are significantly more tolerant of root infestations. Hybrid grapes, however, are more susceptible to significant foliar infestations of this insect. Vines can withstand a lot of leaf damage. If there is a high infestation, Movento is an effective insecticide for phylloxera. This should be applied to the canopy at the first sign of infestation. Movento needs to penetrate the waxy leaf surface, so application with a penetrating adjuvant is essential to get control.

Treatment recommendations for these insects can be found in the MSU Fruit Management Guide (E154).

Upcoming meetings

Our grower meetings have moved online. We are using Zoom for our meetings. Here is more information on Zoom.

In place of Viticulture Field Day, we are having Viticulture Virtual Field Days - Webinar Series. This will be a week of webinars from 12-1 p.m. during July 27-31. Registration is now open!

See also

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