MSU Extension

The incidence of grape diseases in our regularly scouted vineyards in southwest Michigan are still increasing due to the cool, wet conditions and we are seeing more powdery mildew on leaves and clusters as well as more downy mildew lesions on leaves. Phomopsis symptoms on leaves and clusters have remained steady while black rot lesions on leaves and Botrytis in clusters with grape berry moth feeding are also more common. Additional fungicide applications may be needed to protect vines from foliar fungal diseases that are increasing because of recent rain. The berries have now become generally resistant to infection by black rot, powdery mildew and downy mildew pathogens, although already-infected berries may continue to rot or sporulate. Bunch closure, veraison and pre-harvest are important spray timings for Botrytis bunch rot control.

The recent trend of cool, wet weather is likely to continue over the next week. Across southwest Michigan, we are currently 80-100 growing degree days (GDD) base 50 behind where we were on this date last year, and about 300 GDD50 behind the average heat accumulation for the past five years. This now puts this region 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)

 Grape berry moth damage is generally easy to see in clusters on vines that are adjacent to wooded areas, but few larvae are still feeding in those clusters, and we are also seeing an increase in the number of grape berry moth males captured in pheromone traps. This tells us that the second generation of this pest is ending and the third generation will be starting soon. The GDD model for grape berry moths is available at the MSU Enviro-weather website and for sites in Berrien County, this model predicts egglaying will begin today, Tuesday, Aug. 12. In Van Buren County, grape berry moth egglaying is predicted to begin over the coming weekend around Aug. 17.

Insecticides that are active against eggs and young larvae, such as Intrepid, Altacor or Belt, should be applied at the start of egglaying for maximum effectiveness. Coverage is crucial with these compounds, so it is recommended to use a higher spray volume of 50 gallons per acre in juice grape canopies and focus on the clusters to help get control of the third generation of berry moth. Consult the “2014 Fruit Management Guide” (E-154) from Michigan State University Extension for a more comprehensive list of insect and disease control options.

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

 Similar to our observations over the last month, 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 present, but still well below economic thresholds. Japanese beetles have increased in numbers at some sites, and some light feeding is visible on leaves. Vineyards with a healthy canopy and a light to moderate crop load do not need treatment at this time. 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, Aug. 11

Concord berries continue to increase in size, many are 15-18 millimeters or about 0.75 inches in diameter, but no coloring has been observed during our scouting. Vignoles clusters continue to ripen slowly and berries are 10 millimeters, or about 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 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 third flight of grape berry moth males is just beginning, and the number of males captured ranged from zero to 28 per trap. Feeding damage from second generation larvae can be found in clusters in vineyard hot spots, but larvae have left the berries to pupate. In some border areas, as many as 64 percent of the clusters have evidence of second generation feeding. In vineyard interiors, grape berry moth damage ranged from 0 to 16 percent of the clusters with grape berry moth feeding damage.

Disease incidence has generally increased over the past two weeks. In a Vignoles vineyard, Phomopsis leaf lesions were fairly widespread with about 30 percent of the leaves showing symptoms, but no symptoms were seen on clusters during scouting on Aug. 11. Black rot leaf lesions were found at low levels – 14 percent of leaves. Some Botrytis was seen developing on berries with grape berry moth infestation. Phomopsis leaf lesions were seen in Concords – 16 percent leaves with symptoms. Similar levels of black rot symptoms were seen on leaves, and downy mildew was found on 6 percent of the leaves during scouting.

Van Buren County farms scouted Monday, Aug. 11

In the Niagara vineyards we scouted, clusters were tightening and berries sizing rapidly, and Chancellor has reached veraison. Over the past week, captures of grape berry moth males have increased at the vineyards we monitored. The damage from second generation larvae in clusters is generally higher than what we have seen recently in Berrien County, ranging from 48 to 90 percent of clusters infested on borders and 12 to 24 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.

This week in the Chancellor vineyards we scout in Van Buren County, the incidence of Phomopsis leaf symptoms – 7 percent of leaves – and black rot leaf lesions – 6 percent of leaves – have increased. Powdery mildew incidence on leaves has also increased – 9 percent of leaves – but cluster infections – 12 percent – have remained steady. In the Niagara vineyards we monitor as part of this project, we recorded black rot symptoms on 4 percent of the clusters and 4 percent of the leaves. Downy mildew lesions (oil spots) have also increased and were found on 11 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.