MSU Extension

Harvest is again upon us, and soon we will all be increasingly busy with picking and processing, so we are closing down our vineyard scouting reports. We hope these reports have provided timely information to remind you when to scout, what to look for in your vineyards and offered options to help with your pest management needs. We will continue to scout some representative vineyards through much of the harvest period and will report any significant increases of insect pests and diseases should they occur.

Michigan State University Extension does recommend that growers, scouts and vineyard managers continue checking vineyards until they are harvested to help identify any increases in cluster rots and late-season insect pests such as common fruit flies, multi-colored Asian lady beetle, brown marmorated stink bug, wasps, spotted wing Drosophila (SWD) and mealybugs. Although we do not expect a significant fourth generation of grape berry moth this year and the window has passed for using fungicides to control many diseases, a walk through the vineyard will let you know if there are problem areas where special care should be taken during picking. It will also provide information on the efficacy of your spray program during the growing season.

Recently, we have seen mature grape berry moth larvae in clusters on the borders of vineyards near woodlots, and sour rot and botrytis have also increased in some vineyards. Earlier varieties with heavy grape berry moth populations and substantial disease pressure have higher risk for these problems at harvest. For later harvested cultivars, we expect damaged fruit to drop from the clusters so that these issues will be less of a problem.

Preharvest is a good time to reflect on the past season, and we recommend growers make note of areas of high insect pressure or high incidence of diseases to help with management plans for the future.

Although we experienced more heat late this season compared to last year, 2015 was still a cool season with above average rainfall when compared to the last five years. This made management of diseases and insect pests very challenging for many growers. Not only did the abundant rain that occurred during the critical period around bloom favor disease development, but pesticide wash-off led to the need for reapplication of some sprays or resulted in less effective control of insect pests and diseases. Many growers had a particularly difficult time controlling powdery mildew on leaves and clusters and downy mildew on leaves in both wine and juice grape vineyards. Phomopsis and black rot were problematic in some vineyards, berries are now dropping from infected clusters and bunch rots were especially prevalent in varieties with compact clusters.

In general, grape berry moth was also out in force this year, and we observed some of the highest infestation levels of this insect in the vineyards that we monitored for this newsletter. Although recent hot weather has added growing days and speeded insect development, we do not expect a significant fourth generation of grape berry moth before harvest in most varieties. Even though the table below shows we have reached the 2,430 growing degree day (GDD) target for a fourth generation, this heat has occurred too late in the season to bring on a harvest generation. This is because when grape berry moth eggs and young larvae are exposed to day lengths that are less than 14 hours, the vast majority won’t develop into adults, but will instead enter a resting stage that prepares them for overwintering. When egglaying began in early August, the day length was only about 13.5 hours per day, therefore most of these eggs should not develop into adults, but rather will enter their overwintering phase, drop to the leaf litter and emerge next spring. Many other insect pests including flea beetle, rose chafer, grape leafhopper, potato leafhopper and Japanese beetles did not reach levels that required treatment in most vineyards this year.

Recently, we have seen an increase in the number of split berries in many vineyards and with that we saw an increase in the occurrence of common fruit flies, wasps and ants. Other insect pests that can appear at harvest time, such as multi-colored Asian lady beetle, brown marmorated stink bug, wasps, and mealybugs, were not seen during our scouting. We are still catching SWD in traps on vineyard borders, but because we generally have to deal with common fruit flies at this time, we do not expect this pest to pose many additional problems. Insecticides with short preharvest intervals (PHI) such as Mustang Max (one day PHI), Scorpion (one day PHI) or Belay (zero day PHI) can be valuable tools for controlling pests during this time of year. Be sure to consult MSU Extension Bulletin E0154, “Fruit Management Guide,” or a more comprehensive list of late-season insect and disease control options.

We will have some dry, pleasant weather this week just in time for the start of Niagara harvest in southwest Michigan. Highs are predicted in the 70s with lows in the 50s for the next week to 10 days, and there is little rain in the forecast for southwest Michigan for the coming week. Across the region, we are currently about 200 GDD₅₀, or about 15 days, ahead of the same time last year, but compared to the past five years, we are about 150 GDD₅₀, or about 10 days, behind the average accumulation for this date. As the weather is cooperating and the grapes are getting ready, we wish you good luck with harvest, and we will see you at meetings this fall and winter.

Weekly scouting report

Berrien County farms visited Monday, Sept. 14

Concord is in late veraison and the largest berries are about 2 centimeters across. Concord harvest is estimated to begin around Sept. 25-28. Vignoles are ripening steadily with harvest also expected to begin later this week.

The third flight of grape berry moth males is continuing, but, over the past two weeks, the number of moths caught in traps has been steadily decreasing. On the last scouting visits to these farms, captures ranged from zero to 12 moths per trap and this indicates the third flight is ending.

Grape berry moth control appears to be good in the vineyards that we visited this week. Some clusters on the outermost vines on vineyard borders have grape berry moth damage, but much of this damage involves only a small entry wound on a single berry with no further increase in the severity of damage. In vineyards we scouted Sept. 14, infestation ranged from 8 to 84 percent of clusters with some grape berry moth feeding damage on vines that are adjacent to woods containing wild grape. As we have seen all season, very little damage was recorded in vineyard interiors. As we are already starting harvest, this is likely the last chance for growers and scouts to check vineyard hotspots to determine if a clean-up treatment for grape berry moth is required. Insecticides with a broad range of activity and a short PHI should be used for clean-up sprays where necessary.

Third generation egglaying and feeding by young larvae is still occurring, but this activity appears to be declining. The observed decline is likely due to damaged and diseased berries getting consumed by fruit flies, ants and wasps, and then drying and dropping from the clusters.

Japanese beetles were not seen during scouting this week, but some old feeding damage is visible on some leaves. Overall, beetle numbers are still low, but growers with susceptible vinifera and hybrid varieties should still scout regularly to check for increasing Japanese beetle populations and plan to treat and protect the canopy until harvest. Potato leafhoppers and Eastern grape leafhoppers were not seen during scouting at these farms, but growers and scouts should continue to look for these pests on the foliage, particularly for late harvested varieties.

Other than common fruit flies, ants and wasps, harvest time insect pests such as multi-colored Asian lady beetles, brown marmorated stink bugs and mealybugs were not seen during our scouting. We are still catching SWD in traps on vineyard borders, but because common fruit flies are already an issue at this time, we do not expect SWD to pose an additional problem.

These vineyards were last scouted for diseases on Sept. 14. At this time of year, we are scouting leaves for active infections of downy mildew and powdery mildew, and examining clusters for evidence of downy mildew and powdery mildew, as well as botrytis and sour rot infections. The incidence of disease symptoms at the Berrien County scouting sites remains low. In Concord, no black rot infections were seen on Concord clusters, but downy mildew lesions were seen on 8 percent of leaves. In Vignoles, old infections of phomopsis were seen on 10 percent of clusters, and this is causing some berries to drop. Single black rot infected berries were seen on 17 percent of clusters, but no further infection is expected. Downy mildew and powdery mildew were not observed on leaves or clusters in Vignoles, but Botrytis (12 percent of clusters infected) and sour rot symptoms (70 percent of clusters infected) are increasing.

Growers should continue to monitor leaves for downy and powdery mildew and, especially in tight-clustered varieties, growers should check clusters for developing symptoms of botrytis and sour rot infections. Treatment may be necessary to control these late-season leaf diseases to ensure that the canopy remains healthy. Early picking may be necessary in instances where sour rot control has been lost.

Van Buren County farms visited Monday, Sept. 14

Early Niagara harvest is underway and about half of the crop is expected to be picked early this week before Concord harvest begins Sept. 24 or 25. The remaining Niagara will be harvested after Concord is finished. Chancellor is ripening well, and harvest should begin around Oct. 1.

The number of grape berry moth males caught in traps has decreased over the past two weeks and ranged from zero to 22 moths per trap. This trend is similar to that in Berrien County and shows the third flight is ending in this region. Grape berry moth damage in these vineyards has remained steady or decreased over the past two weeks and feeding damage by third generation larvae ranged from 60 to 96 percent of clusters with damage. The incidence of damage in these high-pressure vineyards is highest at vineyard borders, but the severity of feeding remains low, and many of the damaged clusters contain only one or two damaged berries. At this point in the year, we are not expecting a fourth generation of grape berry moth; however, depending on the length of the third generation and the level of grape berry moth pressure, an additional insecticide application may be required in some vineyards before harvest.

Potato leafhopper and grape leafhopper were not seen during scouting on Sept. 14, and no Japanese beetles were observed. However, growers with late-harvested varieties should continue scouting for these pests, as well as the harvest-time pests mentioned above, from now until the grapes are picked.

Diseases have been well-managed in the vineyards we scout in Van Buren County. Downy and powdery mildew symptoms were absent from clusters in Niagara or Chancellor, despite the susceptibility of these varieties to these pathogens. Downy mildew leaf lesions, or “oil spots,” were seen in the Niagara vineyards, and the incidence of infected leaves has not increased over the past two weeks (14 percent of leaves with symptoms. Black rot lesions were seen on clusters in Niagara (33 percent of clusters with symptoms). In Chancellor vineyards, symptoms of black rot (5 percent infected clusters) and Botrytis (7 percent of clusters with symptoms) were seen on fruit.

For Chancellor and other late-harvested varieties, growers and scouts should still be checking vineyard canopies for active infections of downy mildew and powdery mildew on leaves, and botrytis and sour rot infections on clusters. It will be important to ensure that these pathogens do not increase between now and harvest, so additional fungicide applications may be needed before season’s end. If a fungicide application is warranted, check the PHI to make sure there is sufficient time before harvest. Also, check with your winemaker to ensure late fungicide applications don’t interfere with the wine-making process.

Please consult the Michigan Fruit Management Guide (E0154) for management options for insects, diseases and weed.

This scouting report is supported by the Michigan Grape and Wine Industry Council and the National Grape Cooperative.