MSU Extension

In our regularly scouted vineyards in southwest Michigan, we are seeing an increase in the incidence of grape diseases including powdery mildew, downy mildew, Phomopsis, black rot and Botrytis. In light of recent rainy weather, additional fungicide applications may be needed to protect vines from these fungal diseases. Cool, wet weather is likely to continue over the next week with little heat accumulation, and these conditions are good for development of most grape diseases. We are currently about 120 growing degree days (GDD) base 50 behind where we were on this date last year, and about 260 GDD50 behind the average heat accumulation for the past five years. This now puts us roughly one week behind last year and two weeks behind the five-year average.

MSU Enviro-weather GDD summary for 2014 (GDD50 from March 1)

There also continues to be a steady increase in grape berry moth damage in clusters on vines that are adjacent to wooded areas. We are still seeing a few freshly laid grape berry moth eggs and new feeding activity from second generation larvae. Captures of male grape berry moth in traps have generally declined over the past two weeks, but the catch is up at some high pressure sites where we normally see grape berry moth appear first. We can expect grape berry moth egglaying to continue from now until harvest, although cooler weather will reduce the rate of development of eggs and larvae in clusters, and prolong activity of the current second generation.

Growers and vineyard managers should check vineyard hotspots for damaged clusters to determine the need to apply insecticides to limit damage to clusters at this time. As we are well into the egglaying period for the second generation, contact insecticides such as Imidan (14 day re-entry), Baythroid, Bifenthrin, Delegate and others will be the best options to apply now to target eggs and larvae. Consult the “2014 Fruit Management Guide” (E-154) from Michigan State University Extension for a more comprehensive list of insect and disease control options.

Looking forward, egglaying by the third generation of grape berry moth is not expected to start in the next week. Given current and projected temperatures, this generation will likely start laying eggs 10 to 14 days from now in Berrien County vineyards, and it could be two to three weeks before we see the beginning of egglaying in vineyards in Van Buren County. You can check the GDD model at MSU Enviro-weather to track development of these predictions.

MSU Enviro-weather grape berry moth model summary (GDD47 from wild grape bloom)

 Other pest insects remain at very low levels in the vineyards we scout in Berrien and Van Buren counties. Eastern grape leafhoppers and potato leafhoppers are still well below economic thresholds. Japanese beetles have increased in numbers at some sites, and some light feeding is visible on leaves. Control of this pest is not needed at this time as the number of beetles observed is still low and the vines have abundant leaf area to ripen the potential crop. Although wine grape varieties are more adversely affected by Japanese beetle feeding than are juice grape varieties, the beetles often feed at the top of the canopy. On a vertical trellis system, their feeding typically does little to reduce the capacity to ripen the crop. This will be particularly true this season with the lighter crop load in many vineyards. Growers of susceptible vinifera and hybrid varieties should still scout regularly to detect possible build up of Japanese beetle populations and plan to treat if canopy protection is needed.

Berrien County farms scouted Monday, July 28

Concord berries are sizing nicely and many are 12-14 millimeters, or about 0.5 inches, in diameter. Vignoles clusters continue to tighten and berries are 8-10 millimeters, or 0.33 inches, in diameter. Eastern grape leafhoppers and potato leafhoppers were not seen in the vineyards we scouted in this county. Very few Japanese beetles were seen during scouting and light feeding damage on leaves can be seen, but numbers are low and there is no need to treat for this pest at this time. Low numbers of spotted wing Drosophila (SWD) continue to be trapped in vineyards, but as there is no ripe fruit yet, we do not expect any infestation in vineyards at this time.

The second flight of grape berry moth males has remained low over the last week, and the number of males captured ranged from zero to four per trap. Second generation larvae can be found in clusters in vineyard hot spots, and in some border areas, as many as 60 percent of the clusters have some evidence of active berry moth feeding (webbing and frass). In vineyard interiors, grape berry moth damage ranged from 6 to 16 percent of the clusters with grape berry moth feeding damage. Treatments with contact insecticides can be considered at this time to prevent further increase in infestation, and these can be focused at the vineyard borders if the rows run perpendicular to the edge.

Disease incidence has remained steady or increased over the past two weeks. In a Vignoles vineyard, Phomopsis leaf lesions were fairly widespread with about 11 percent of the leaves showing symptoms during scouting on July 28. Some Botrytis was seen developing on berries with grape berry moth infestation, but no other leaf, shoot or cluster disease symptoms were seen. Low levels of Phomopsis leaf lesions were seen in Concords, 2 percent leaves with symptoms. Phomopsis lesions were also seen on 5 percent of the shoots, and on the rachis in 1 percent of the clusters.

Van Buren County farms scouted Monday, July 28

On July 28, Niagara and Chancellor clusters were tightening and berries were sizing rapidly. Over the past week, captures of grape berry moth males have increased at the vineyards we monitored, one to 36 per trap, and the level of feeding by larvae in clusters is similar to that seen in Berrien County, ranging from 34 to 80 percent of clusters infested on borders and 2 to 8 percent of clusters infested in vineyard interiors. As was seen in Berrien County, very low levels of grape leafhoppers and potato leafhoppers were present in the vineyards we scouted. Japanese beetle numbers are increasing, but populations are not large enough to warrant treatment.

We did detect the presence of Phomopsis on 1 percent of leaves, black rot  on 2 percent of clusters and powdery mildew on 2 percent of leaves and 13 percent of clusters this week in a Chancellor vineyard. In the Niagara vineyards we monitor as part of this project, we recorded Phomopsis leaf lesions on 3 percent of the leaves, black rot symptoms on 3 percent of the clusters and 1 percent of the leaves, and we observed early downy mildew lesions (oil spots) on 3 percent of the leaves that we checked. Botrytis associated with grape berry moth infested berries was recorded on 2 percent of the sampled clusters.

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

Drs. Schilder and Isaacs’ work is funded in part by MSU‘s AgBioResearch.