MSU Extension

Weekly scouting report

Warm weather and rain have fueled plant growth as well as disease and insect development. Juice grape vineyards in Berrien and Van Buren counties are past bloom with berries on primary clusters ranging from “bb” to buckshot size range. Hybrid varieties are in the middle of bloom, and bloom in vinifera wine grapes has begun.

Rose chafers are out, but primarily in vineyards with sandy soil and a history of this pest. Grape leafhopper adults have also been detected in low numbers. Growers and scouts should keep an eye out for these insects on the undersides of leaves. Also, be on the lookout for other species of leafhoppers that infests grapes, including potato leafhoppers, on younger leaves. The first flight of grape berry moth is nearing its end, and low numbers of young larvae have been found in clusters. Sprays can still reach larvae in webbing between berries, but insecticide applications for this generation of berry moth are not cost-effective in most cases. There is very low frequency of damage in the first generation, and grape clusters will compensate for some berry loss by increasing berry size. Consult the Michigan Fruit Management Guide (E0154) from Michigan State University Extension for information on pest management options for insects, diseases and weeds.

Rain over the weekend and long periods of moisture and shade have increased the risk of fungal infections of flowers and developing fruit from downy mildew, powdery mildew, black rot, fruit Phomopsis, and latent infections of Botrytis. Current weather predictions call for alternating rain and clear weather in the coming week – with grapevines in bloom or immediate post-bloom, this is a critical time for applying sprays with a broad range of efficacy in all varieties. Tank-mixes of two materials, or pre-mixes of multiple chemical classes, are called for at this time. In unsprayed vineyards, Phomopsis shoot lesions and leaf infection symptoms of black rot are widespread.

The key time for control of Phomopsis on non-fruiting plant parts has passed, and any Phomopsis infections that have occurred up to this point will produce inoculum next spring, if they are not pruned out before then. Fruit are still vulnerable to infection from Phomopsis at this time and spores are expected to be available for the next two to three weeks. High pressure is expected from powdery mildew this year in vineyards where control of leaf infections was lost last year in mid-September or earlier. Conditions conducive for the establishment of powdery mildew infections are cloudy days, humid weather without rain and moderate temperatures. Excessive rain is detrimental to powdery mildew. For downy mildew, rain splash off of saturated soil and leaf debris where the pathogen overwinters allows infections to form. For black rot, even in vineyards that have gotten good control of leaf infections, fruit infection may still occur from the last bits of overwintered inoculum, so make sure to protect the fruit. Concord and Niagara fruit will become naturally resistant to black rot 4-5 weeks after bloom.

Enviro-weather growing degree day summary for 2015 (GDD₅₀ from March 1)

Growing degree day (GDD) totals are about 65 GDD₅₀ ahead of the accumulations on this date last year, and that puts us about three or four days ahead of 2014. However, we are still about 40 GDD₅₀ behind the five-year average for this date.

Based on our observations at farms in the Vineyard IPM Project, we are using May 26 as the full bloom date for wild grape for the vineyards in Berrien County and May 28 for those in Van Buren County. It is best to use the wild grape bloom date on your own farm to make the most accurate predictions, but the dates given below can be used for MSU Enviro-weather’s grape berry moth model if there is no date recorded for your vineyard. The date of wild grape bloom in your area is used in the model to predict the beginning of second and third generation egglaying later in the summer, which is the ideal time to spray insecticides for grape berry moth. Second generation egglaying is currently projected to begin sometime during the first week of July.

Berrien County farms visited Monday, June 15

Concord is past bloom and berries are almost at the buckshot stage. Vignoles is at full bloom.

The first flight of grape berry moth males is still lingering and the numbers caught in pheromone traps was low at all sites this week, with catches that ranged from zero to 52 moths per trap. Females are active in vineyards, and there are a few larvae in clusters in vineyard hot spots. Infestation currently ranges from 2 to 5 percent of clusters with feeding damage on MSU IPM project farms. We expect to see very little damage from this first generation of grape berry moth.

The incidence of disease symptoms is still low. A few Phomopsis and black rot leaf lesions have been detected in commercial Concord and Vignoles (less than 5 percent of leaves infected). Powdery mildew and downy mildew symptoms have not yet been observed in these vineyards. However, downy mildew has been found in wild grapes close to the ground, so the conditions have been favorable for activity of the pathogen.

Eastern grape leafhoppers are starting to appear, but potato leafhoppers were not seen on new growth. Rose chafers are still few in number.

Concord berry development in north Berrien County.

Van Buren County farms visited Monday, June 15

Niagara and Concord berries are approaching buck shot, and Chancellor and other hybrids are in full bloom. The number of grape berry moth males caught in traps has decreased over the past two weeks as the first flight winds down; the number of males caught ranged from 12 to 80 moths per trap. Berry moth damage in these vineyards was low and ranged from 7 to 11 percent of clusters with larvae. No action for control of this pest is necessary at this time except in cases of a history of very high pressure (see above).

We did see low numbers of rose chafers during scouting last week, but none were observed this week. Low numbers of grape leafhoppers were seen during scouting, but numbers were well below the need for action. Early downy mildew leaf lesions were seen in Niagara (3 percent of leaves with symptoms). Leaves with symptoms from Phomopsis and black rot were seen in commercial Chancellor and Niagara, but these infections are infrequent (less than 5 percent of leaves infected) and appear to be held in check by bloom-time fungicide applications.

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