MSU Extension

Weekly scouting report

A recent warmup and more summer storms have dominated our weather over the past two weeks. In general, we are having a very similar growing season to 2013 and 2014. That is to say that frequent rain has made it challenging for growers and vineyard managers to keep up with disease management. In addition, insect pests such as leafhoppers and Japanese beetles have remained at low levels in most vineyards as in the previous two years. Michigan State University Extension recommends growers and scouts still keep an eye out for Japanese beetles on the canopy, grape leafhoppers on the underside of leaves and potato leafhopper on younger leaves.

Grape berry moth male second flight is in full swing. Similar to what we have observed in most years, the number of males caught during second flight is much lower than during first flight. Juice grape berries are gaining natural resistance to black rot, Phomopsis, powdery mildew and downy mildew; however, young leaves remain susceptible to downy mildew and powdery mildew infection. European varieties are still susceptible to most fungal diseases and need to be actively protected. Cluster infections of fruit have been relatively few so far in vineyards that are part of the integrated pest management (IPM) project.

We expect to see high pressure from grape berry moths this year as relatively cool and wet conditions are forecasted, and this is ideal for grape berry moth survival. Recent warm nights have increased second generation female activity, and eggs and young larvae have been found in some vineyards. Low numbers of older grape berry moth larvae have been found webbing berries together, indicating there are still some late first generation larvae in vineyards we scout. Growers using contact insecticides for berry moth management will get better control if they apply another insecticide spray before second generation flight is over; pyrethroids have a short residual and organophosphates and carbamates will have washed off in areas that received rain. Longer-residual materials, like insect growth regulators and diamides, are more likely to remain on clusters in an inch or less of rain, but also may require a second border application if there is cool weather and a long second generation flight. Focus on vineyard borders for the second insecticide spray where berry moth pressure is likely to be higher. Looking ahead, third generation egglaying begins at 1620 growing degree days (GDD ) base 47 after wild grape bloom in early August sometime.

Enviro-weather GDD summary for 2015 (GDD₅₀ from March 1)

GDD totals are about 30 GDD less than totals on this date last year, and that puts us about one to two days behind 2013 as well. We are about 185 GDD₅₀ or close to a week behind the five-year average for this date.

Berrien County farms visited Monday, July 13

Concord is at berry touch and berry diameters range from 1 to 1.5 centimeters. Vignoles clusters are also at berry touch with berries ranging from 0.5 to 0.75 centimeters.

Grape berry moth male second flight of is occurring, and the number of captures was low at all sites, ranging from zero to five moths per trap. A few new stings are visible in clusters at some sites, and there are still a few first generation larvae in berries in some vineyard hot spots. In the vineyards we scouted July 13, infestation ranged from 4 to 24 percent of clusters with grape berry moth feeding damage.

Potato leafhoppers were not seen during scouting, but Eastern grape leafhoppers are out in low numbers in some juice grape vineyards. Growers and scouts should continue to look for these pests on foliage. Japanese beetles are still present at the farms we scouted this week. Numbers of this pest are low, and no treatment is required at this time.

The incidence of disease symptoms is still low with a few Phomopsis and black rot leaf lesions in commercial Concord and Vignoles vineyards seen during scouting July 13 (5-10 percent of leaves infected). Very low incidence of black rot-infected berries was observed, and it looks as though these infections will not spread before the berries become naturally resistant. Powdery mildew and downy mildew symptoms were not observed in these vineyards.

Van Buren County farms visited Monday, July 13

Niagara clusters are at berry touch with berry diameters 1 to 1.5 centimeters. Chancellor berries are sizing well after bloom and the largest berries are 0.75 centimeters. The number of grape berry moth males caught in traps ranged from zero to 13 moths per trap, and we are well into second flight in this region. Damage in these vineyards was low and ranged from zero to 28 percent of clusters with damage. Low numbers of grape leafhoppers were seen during scouting July 13, but numbers were well below any need for action. No Japanese beetles were observed during scouting, but heavy rains earlier in the day may have kept the beetles off the vines.

Downy mildew and powdery mildew symptoms were not seen on clusters in Niagara or Chancellor, even though these varieties are susceptible to this pathogen. Leaves with Phomopsis or black rot symptoms were seen in Chancellor and Niagara, but these infections were low (5-10 percent of leaves infected) and appear to be held in check by fungicide applications. We did see a few Niagara leaves with downy mildew lesions (4 percent of leaves infected), but it appears fruit was protected from infection.

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

Disease development

Concord and Niagara berries are now resistant to infections of powdery mildew, downy mildew and black rot. In vineyards with black rot leaf lesions, if good coverage of black rot-active materials did not occur near the end of the susceptible period for fruit, fruit symptoms may appear several weeks from now from currently developing infections in the berry. Fungicide applications should now be focused on protecting foliage from powdery mildew and downy mildew infection, especially where there is a heavy crop, as the maximum leaf area will be needed to ripen it.

Wine grape clusters should become resistant to powdery mildew and downy mildew in another week or two, but are still vulnerable to both of these fungi, so growers should continue to protect clusters. If late clusters developed due to spring frost injury, these are particularly susceptible to powdery mildew infection. While rainy weather and warm, humid nights promote downy mildew development, dry, cloudy weather is more conducive to powdery mildew. So far, 2015 has been more favorable for downy mildew development than powdery mildew. However, in canopies with heavy foliage, leaves and clusters are sufficiently protected from rain that powdery mildew can still develop.

In vineyards with black rot leaf lesions, these can continue to produce secondary spores, and fruit are still vulnerable to infection. Expect fruit to begin developing resistance to black rot in a week or so, and then become fully resistant two weeks afterwards. With frequent rains, Botrytis may be especially a problem this year, so fungicides active on Botrytis are good to get on before bunch closure, especially in tight-clustered varieties that are susceptible to bunch rot.

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