MSU Extension Grapes

Announcement

This is our last statewide grape scouting report for the season. This year has been a good year for wine, juice and table grape production. We wish the processors and winemakers successful production. Stay tuned for our conference programs this winter. If you have not had the chance yet, stay connected with the Michigan State University Extension grape team by signing up at our newsletter signup page for weekly newsletters containing the most recent articles and announcements from Michigan State University Extension.

Weather

Last week was cool and rainy. A cold front moved through Michigan on Tuesday. A series of rain systems entered the state over the course of the week. Total rainfall amounts were 1-2 inches in western regions, near 4 inches in southeast Michigan. High temperatures fluctuated between the mid-50s and mid-70s in southwest Michigan, with warmer days occurring between rain systems. Warmer air pushed into the southern parts of the state over the weekend, bringing temperatures near 80 to the southern tier before being replaced with the cooler air yesterday. 

This week is expected to be cool and dry. High temperatures in most of Michigan should be near 70 and lows in the 40s, 5 degrees warmer in southwest portions of the state. The first chance of rain will be this coming weekend.

With the cool week, we picked up few growing degree days (GDD), 53-77 GDD base 50, half of the week before. The southwest region is 655 GDD base 50 ahead of the northwest region.

Vine growth

Statewide, sugar levels have been increasing at or ahead of the recent average dates. In southern Michigan, acid levels have been slow to come down for many grape varieties. This has resulted in some early ripening varieties measuring very high sugar levels before the acid levels indicate they are ready for harvest.

In southwest Michigan, wine grape harvest is continuing to expand. Picking of some red Bordeaux cultivars will begin soon. Niagara juice grape harvest ended over the weekend. Harvest of Concord for National Grape Cooperative will begin on Thursday. Concord harvest has been delayed a few days to give the grapes a chance to accumulate more sugar. The weather this week should be good for fruit maturation and sugar accumulation for most grape varieties. 

In northwest Michigan, ripening is moving along at a steady pace. Pinots are currently at 20-21 Brix, while Chardonnay and Pinot Noir are being harvested in some Leelanau Peninsula vineyards this week. The harvest of Riesling is expected to begin in mid-October.

Horticulture

During fruit maturation, bird damage can be 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.

This time of the season it is pivotal to properly monitor fruit chemical characteristics and maturity to make precise decisions on time of harvest and winemaking strategies to produce the best quality wine possible from this vintage. High quality wines are the natural convergence of fruit-derived flavor and aroma with ad hoc winemaking technologies. Therefore, in deciding the picking time for grapes, the priority needs are the quality and quantity of varietal aroma and flavor in the fruit.

Sampling fruit in the vineyard regularly will describe aromas, flavors and textures for each different cultivar. The next priority for red wines is the texture of the grape tannins in the skin and the seed because they determine the structure, body, astringency, bitterness and color intensity of the wine.

Brix or sugar content is a simple way to determine a ripeness scale but the levels can greatly vary from year to year. In some years the grapes will be ripe with perfect varietal character at 21 brix and another year they may not have a varietal character at 23 brix.

However, enologists have a target for grape ripeness for each cultivar they would like the fruit to achieve for the style of wine they produce. The target is different within the same grape variety, depending on the type or style of wine produced. For example, one winery may prefer to produce a wine (e.g., Pinot Noir) with prominent red fruit characteristics (e.g., cherry or strawberry) while another winery would prefer riper black fruit (e.g., black cherries) notes in the wine. In fact, grape ripening is a physiological process with a continuous production of aroma and flavor. Therefore, timing of harvest is determining that point of ripening of the grapes that fits the winemaker’s objective for the wine.

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 a 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), GoalTender (oxyfluorfen), Prowl H2O (pendimethalin), Chateau (flumioxazin) and Matrix (rimsulfuron) may be applied in fall.

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 main diseases to focus on are bunch rots including botrytis and sour rot. Botrytis is becoming more common around the state. Several strategies contribute to good botrytis bunch rot management including opening up the canopy, effective insect control, properly applying fungicides, and using resistant cultivars when possible. Good botrytis control depends on getting good coverage. A Michigan Grape Fact Sheet is available for managing botrytis bunch rot.

We have also been seeing sour rot in a number of vineyards in southwest Michigan. It is usually characterized by clusters that smell like vinegar, juice that drips over the fruit surfaces, a melting-type decay with skin slippage, and vinegar flies and fly larvae typically present. Recent research has shown a significant correlation between insect activity and sour rot movement. These insects have been primarily vinegar flies, but at MSU we have observed yellow jackets, honey bees and bald-faced hornets on sour rot clusters. Recent research into a well-timed insecticide at 14 Brix along with a sterilant (e.g., Fracture or Oxidate) prior to sour rot symptoms is an effective method of control. 

If you are planning on making a fungicide application, the most important thing to remember is the pre-harvest interval. See this article for more information about late season disease management.

Another disease to not forget about is downy mildew. This disease is causing particular trouble for growers this year. The recent waves of rain and high humidity have created good conditions for repeated infection events. If they are permitted, fungicides that are broad-spectrum/contact like captan are effective products for resistance management. Viticultural practices that reduce canopy wetness such as good irrigation timing, leaf removal and good weed management can reduce these diseases in a vineyard.

The main concern with downy mildew at this time is late season defoliation. So far this season we have observed downy mildew more frequently than normal due to the high relative humidity and longer dew periods. Defoliation will impact winter hardiness and survival, as well as health of the vine next year. Effective fungicides for downy mildew include products in FRAC codes 4, 11, 21, 40 and 45 as well as phosphorus acid salts, captan and some biologically-based products.

After harvest, protecting vines from trunk diseases and crown gall should be the major focus. Other items like clearing the vineyard floor may also reduce black rot and Phomopsis by removing plant debris. Crown gall (caused by the bacterial pathogen Agrobacterium vitis) is a widespread and devastating disease, particularly in cool-climate regions in the world. Unfortunately, there are no synthetic chemical treatments for controlling crown gall of grape. A protective practice that can be used to reduce winter injury is “hilling-up”, which is mounding soil over the graft union in the fall to insulate it from exposure to low temperatures.

Soil needs to be carefully removed (so not to cause mechanical damage) in spring once all pruning-related activities have ended to avoid scion rooting. Plowing snow from the rows can offer similar protective benefits. However, unlike soil, snow is unpredictable in its occurrence and duration. For more information about managing crown gall in Michigan, please see this Michigan Grape Facts sheet on Managing Grapevine Crown Gall

Insects

There is significant grape berry moth infestation in some vineyards as harvest approaches, with rows at vineyard border especially badly affected. This reflects a strong late-season increase in berry moth after what was generally a successful early season of insect control with relatively little damage through July. Catches of this pest have declined in pheromone traps recently, indicating that the fourth generation of moth activity is wrapping up, but post-veraison infestation from this generation has been very high in some regions. Affected vineyards may also have rots and vinegar flies associated with these rots, and depending on the level of infestation some control of these insects is worth considering. With Concord harvest starting this week, there is limited time until harvest for some blocks but vinegar fly control with short PHI products can reduce their activity in the fruit.

Wasps, bees and ants were common in vineyards on sunny days when the temperatures are in the 60s or warmer. They are attracted to the accumulating sugars in the berries. Yellow jackets 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. Picking in cooler weather can help avoid wasp stings, but it is challenging to reduce their abundance with sprays at this time of the year due to the difficulty in controlling all wasps in a nest. Interception of wasps using traps deployed at the vineyard border can help reduce their abundance if the traps are placed at a high enough density.

Vinegar flies are becoming common as more berries ripen and some fall from the vine or are opened by other insects or birds. These typical vinegar flies can develop on grapes only when the skin is broken, but with the presence of the invasive spotted wing Drosophila (SWD) in Michigan, it is more important to keep a regular check on clusters for development of vinegar flies in berries. These are associated with cluster rots and move the pathogens around.

The presence of vinegar flies in the canopy can be easily determined by disturbing the clusters, as the small flies will take off and be seen around the clusters. Managing berry quality after veraison and treatments for sour rot can help reduce the impacts of vinegar flies. If these are a challenge during this harvest, consider how you’ll change your management in 2022.

If treatment is needed this season, there are several shorter PHI insecticides available that can provide temporary protection from SWD and other vinegar flies, including Mustang Maxx, Leverage and Verdepryn. Check PHIs and talk to your processor before deciding on any treatments at this time of the year.

Related articles

• Post-harvest vineyard management: Preparing vines for winter
• Southwest Michigan Fruit Update – September 7, 2021
• A Mobile Guide for Grape IPM Scouting in North Central and Eastern U.S.
• Mid-season management of grape berry moth
• Michigan grape scouting report – September 22, 2021
• Pest management approaches in a winter or freeze damaged grape vineyard
• Bird damage reduction strategies in viticultural practices
• What can be done for late season downy mildew in grapes?