MSU Extension Grapes

Weather

The past week was cool across Michigan. Much of the state saw highs in the mid-70s and lows in the 50s. The only rain of note came through the southern tier of the state on Tuesday, Sept. 8, resulting in about a half-inch. The temperatures are expected to be even lower over the next week with highs in the 60s for much of the state, rising into the low 70s over the weekend in southern grape growing areas. Many parts of northern Michigan are expecting lows in the 40s many nights in the coming week. Rain is forecast for Wednesday, Sept. 9, and again on Saturday for many parts of Michigan.

With the cool week across the state, we accumulated only 85-125 growing degree days (GDD) base 50 last week. The southwest region is 505 GDD base 50 ahead of the northwest region.

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

Vine growth

Harvest of table grape cultivars and some early hybrid wine grapes, such as Marechal Foch, Frontenac and Marquette, has begun in southern Michigan. Other varieties in southwest Michigan are getting near the harvest window, especially cold hardy grape cultivars and varieties used for sparkling production.

Crop adjustment work is finishing in the northwestern part of the state, both mechanically and by hand in wine grape vineyards. 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 by aiming for improved vine balance (fruit to vegetative growth). Growers are scouting the vineyards and using a green drop strategy to ensure the harvest of the highest quality grapes for the wines. Green drop reduces the variability in the ripening of the fruit in the vineyard removing “green” fruit that lags behind in maturation, making the grapes more uniform for better wine quality.

During fruit maturation, bird damage is a challenge in vineyards. Crop losses can reach up to 95% and 60% in red and white varieties, respectively. Several methods of protection can be implemented including the use of bird nets, streamers, scare-eye balloons, electronic bird distress calls and propane-fired bird-scaring cannons.

This is a good time to collect soil samples for nematode testing. Nematodes feed on vine roots, causing symptoms that can be misdiagnosed as disease or nutrient deficiency. See this announcement about nematodes and sampling for them. If you are interested in taking samples and testing, you can send them to MSU Plant & Pest Diagnostics.

Weeds

Weed control is always an important component in vineyards for keeping grape vines healthy and ensuring good quality grape yield. At this time of the year, weed control in vineyards is mainly important to reduce weed interference in machine or manual harvest and to reduce the soil weed seed bank for the following season.

The application timing for most of the herbicides during this time of season is mainly based on the preharvest interval (PHI). PHIs have been established for most herbicides except for paraquat (Gramoxone). Aim (three-day PHI), Rely (14-day PHI) and Gramoxone (Restricted Use Pesticide) provide quick burn-down of weeds. Rely and Gramoxone control both broadleaves and grasses, but Aim is only effective for broadleaves.

Poast and Fusilade are selective, post-emergence grass killers and can be used in vineyards based on 50-day PHI. Venue (zero-day PHI) can be added to improve burn-down and broaden the weed control spectrum. Some herbicides will damage grape green bark, new shoots, leaves or vines, so minimize contact with vines during application.

Glyphosate also has a short 14-day PHI, but it is not advisable to apply it after bloom. Significant injury may occur during the current season or the following year if glyphosate comes in contact with leaves, green shoots or is absorbed by bark.

Diseases

The disease focus for most grape growers in Michigan is still downy mildew and cluster rots. Pay close attention to preharvest intervals for specific products. Insect and bird damage and fruit splitting are major contributors to rot development. This week, some southwest vineyards with early ripening hybrids are showing botrytis symptoms. The time to manage sour rot is now approaching as berries begin to ripen.

At this time, we continue to be concerned about defoliation caused by downy mildew when will reduce vine winter hardiness. This article provides more information on late season downy mildew control.

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. Fungicide resistance management is also important. The most effective products for botrytis are site specific and prone to resistance development. A new Michigan Grape Fact Sheet is now available for managing botrytis bunch rot.

Sour rot can be particularly difficult to control on tight clustered varieties. Enhancing airflow through the canopy and clusters can help, so leaf pulling, thinning, shoot positioning and weed control can all provide some reduction in sour rot risk. Another important aspect of control is preventing berry damage (e.g., bird pecks, insect feeding and mechanical damage) to reduce the initiation of berry infections. We are finding more cracked berries, too, after the recent rainfall. Combining an insecticide with a contact fungicide can be effective in managing both the insect vector and the pathogen. In high risk cultivars (e.g., Vignoles) and in hot and humid conditions, these treatments should be applied before symptom development and clusters have reached 13-15 Brix. Under high disease scenarios, reapplication may be needed on seven- to 10-day intervals until harvest, with careful attention to pre-harvest intervals.

A complete list of grape fungicides can be found in the E154 Michigan Fruit Management Guide. Check the guide for potential phytotoxicity of certain sprays on Concord grapes especially. Phytotoxicity risk is higher with high temperatures and quickly growing vines. One important thing to remember at this time of year is to keep track of preharvest intervals for fungicides.

Insects

Grape berry moth damage continues to increase at the borders of many vineyards in southwest Michigan. Egglaying of the fourth generation of grape berry moth is predicted to begin at 2,430 GDD base 47 from the spring biofix. Some parts of Berrien County have reached this timing. Much of southwest Michigan will meet this threshold this week, with cooler areas in southwest and southeast Michigan a week later. This timing will result in the fourth generation overlapping with harvest season this year, potentially requiring control in some hot spots.

Preharvest intervals will be especially important for the earliest harvested wine grape cultivars and early Niagara blocks harvested before Sept. 26. For the later wine grapes, Concords and the last round of Niagara grapes, there is still time for the longer-lasting products to used. See “Berry moth control: Preparing for the 2020 grape harvest” for more details.

Yellowjacket populations are increasing in vineyards. They are attracted to the accumulating sugars in the berries. Yellowjackets can be very damaging to the berries directly, causing wounds that can lead to cluster rots and reducing harvestable crop. They can also be frustrating to sprayers and harvest crews, trying to avoid being stung while working with the ripening grapes. The colder and cloudier weather and rain will greatly reduce their activity over the next week, but expect it to pick up when the skies clear again next week.

As harvest approaches, it will be important to be looking out for spider mites, mealybugs and vinegar flies.

Related articles and resources

• Michigan Grape Scouting Report – September 2, 2020
• Southwest Michigan Fruit Update – September 8, 2020
• East Michigan Fruit Update – September 2, 2020
• A Mobile Guide for Grape IPM Scouting in North Central and Eastern U.S.
• Managing grapevine leafroll disease
• Michigan pesticide applicators receive temporary emergency license extension
• Agriculture support labs still open for business, with modifications
• Late season downy mildew control
• Do you suspect grapevine trunk disease in your vineyard?
• Berry moth control: Preparing for the 2020 grape harvest