MSU Extension Grapes

Weather

The past week was warm. Statewide, daytime highs were in the mid- to upper 70s for much of the week. A cold front came through Michigan on Monday and Tuesday, Sept. 28 and 29, dropping temperatures by 15-20 degrees. The front also brought around a half-inch of rain for much of the state. The cool weather will continue through the next week, with highs in the 50s and lows in the 40s. The coldest morning will be Saturday, Oct. 3, when parts of northern grape growing areas are predicted to see below freezing temperatures. Chances of rain are decent almost every day over the coming week, but the best chances will be on Wednesday, Thursday and Sunday.

The long lead forecast for Michigan is for a warmer than average October and November with average precipitation. A La Niña weather pattern is developing over the Pacific Ocean. This is expected to bring average winter temperatures for Michigan, but above average precipitation, especially in January and February.

With the above average week across the state, we accumulated a lot of degree days for this time of year: 90-100 growing degree days (GDD) base 50 last week. The southwest region is 550 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

Niagara juice grape harvest was paused on Friday to switch to Concord harvest. So far, the early Niagara harvest was looking positive with lower than average insect damage and good volumes. Concord harvest is looking similar. Harvest of wine grapes are continuing in southern Michigan. Varieties being harvested include Vignoles, Chardonnay, Pinot Noir, Gewurztraminer, Traminette and Gruner Veltliner, among others. In northwest Michigan, Pinots are at 20-21 degrees Brix and in southwest Michigan, Bordeaux and Rhone varieties are at about 20-22 degrees Brix. They will be picked starting this week, depending on location and wine style.

In the northern part of the state, Chardonnay and Pinot Noir are being harvested for sparkling production. Most of the grape cultivars are reaching good sugar levels and the long-term forecasts remain positive for good harvesting weather through most of the season. Riesling is one of the last cultivars to be picked and currently is hanging at about 17-19 degrees Brix. The threat of a frost next week in northern parts of the state could impact fruit maturation.

Except for last week, there have been very few growing degree days (GDD) accumulating in the northwest region recently and a frost next week could induce partial or complete leaf fall. If this defoliation occurs, fruit maturation will stop and any increase in sugar accumulation (Brix) would likely be due to berry dehydration and sugar concentration from water loss. If fruit remains on the vine after the first significant fall frost and partial/total defoliation happens, there will be no post-harvest foliated period. This period, where the plant can focus its energy towards preparing for winter and the subsequent season instead of ripening grapes is pivotal for grapevine cold acclimation. During this time, grapevines accumulate nitrogen and carbon, which are important for early shoot growth the following season.

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. For more information on these methods, see “Bird damage reduction strategies in viticultural practices.”

Weeds

Weed control is important at this time of the year for both harvested vineyards and blocks yet to be harvested. Weed control in non-harvested 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 before harvest is mainly based on the pre-harvest interval (PHI). 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. 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.

After grape harvest, weed control might not be the top priority for many vineyards. But fall is the best time to control perennial weeds and apply residual herbicides to vineyard floors. After harvesting, the most important step is to perform thorough scouting of your vineyard and prepare the list of weeds that are problematic in your vineyard.

The next step is to select the herbicides in combination with non-chemical management tools that are effective against these weed species. Fall applications of preemergence herbicides should be made before the soil freezes. Preemergence herbicides such as Alion (indazaflam), Princep (simazine), Solicam (norflurazon), Casoron (dichlobenil), Kerb (pronamide), Goal Tender (oxyfluorfen), Prowl H2O (pendimethalin), Chateau (flumioxazin) and Matrix (rimsulfuron) may be applied in fall. “Weed management in vineyards starts with fall herbicide application” provides more information on fall weed management in vineyards.

Detailed information related to the herbicide rates and efficacy on weeds can be obtained in the Herbicide section of E154 Michigan Fruit Management Guide, which contains lists of all currently labeled herbicides along with specific remarks for their use in vineyards.

Diseases

The disease focus for most grape growers in Michigan is on cluster rots. Botrytis symptoms are becoming more widespread as brix levels increase. Sour rot is also starting to show up in vineyards throughout Michigan. See “Critical practices to control late season grape diseases and the potential effects of fungicides on fermentation” on how to balance your control strategies and minimize fungicide residue at harvest.

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 events.

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.

This is a good time to collect soil samples for nematode testing. Nematodes feed on vine roots, causing symptoms that can be misdiagnosed as abiotic stress or nutrient deficiency. Several of the most important nematodes vector plant viruses such as Tobacco and Tomato ringspot virus. 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.

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.

Postharvest growers should consider providing some type of protection from trunk diseases like crown gall during the winter. Crown gall is the soil-inhabiting bacterium, Agrobacterium tumefaciens. Cultural practices that protect the trunk like hilling protect the vine from freezing injury during the winter and can reduce gall formation on the trunk and graft union. Please see this recently published fact sheet on managing grapevine crown gall for additional information.

Insects

The recent weather has slowed down development of damage from grape berry moth and other insect pests. Cold night temperatures have limited moth activity and reduced the risk of damage from the fourth generation.

Yellowjacket activity has also followed the weather patterns with wasps active especially during warm sunny days and feeding on the accumulating sugars in the berries. Yellowjackets can be very damaging to the berries, reducing harvestable crop and disrupting hand harvest. Picking earlier in the day can help, and if there are extreme amounts of wasp activity some effective insecticides with short PHIs are registered. If considering this, check with winemakers and processors to make sure treatments are approved.

Vinegar flies have been less active this season, but with the presence of the invasive spotted wing Drosophila in Michigan it is more important to keep a regular check on clusters for vinegar flies and their larvae. The presence of vinegar flies in berries can be determined by putting fruit into a gallon bag with half a pint of strong salt solution for an hour, then looking in the solution to determine whether there are larvae present. If treatment is needed, there are several short PHI insecticides available. Be aware, resistance has been documented for pyrethroids in vinegar flies in other states and is being evaluated in Michigan. To do this, we need to find sites where vinegar flies are active. If you know of a location, please contact isaacsr@msu.edu.

Related articles and resources

• Michigan grape scouting report – Sept. 23, 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