MSU Extension

This is the first southwest Michigan vineyard IPM report for 2015. It is part of the continuing Michigan State University Vineyard IPM Project supported by the Michigan Grape and Wine Industry Council and National Grape Cooperative. These biweekly scouting reports will provide summaries of insect and disease incidence at commercial juice and wine vineyards in southwest Michigan. These reports are written to inform growers, vineyard managers, crop consultants and other grape industry members of current pest and disease occurrence to be used as a resource for vineyard pest management. Additional information on the biology and management of insect pests and diseases can be found by following the links in these articles. These summaries should not be used as a substitute for scouting your own vineyards, but should be used as a guide for when particular pests and diseases are likely to occur.

Weekly scouting report

To guide our use of the growing degree day (GDD) model for timing grape berry moth sprays, we have been checking for bloom on wild grape vines (Vitis riparia, “riverbank grape”) in woodlots and tree lines adjacent to vineyards in Berrien and Van Buren counties. As of June 1, all sites we monitor as part of this project are at or past full bloom (50 percent of the flowers are open on 50 percent of the clusters). Wild grapes in areas to the north are probably not at full bloom yet and should still be assessed. When checking for bloom in wild grapes, be sure you are checking the most common wild grape species in the area, the riverbank grape (V. riparia). The undersides of leaves of riverbank grape are reflective green and smooth. Another common species in woodlots, “summer grape” (Vitis aestivalis), has leaves that appear white or gray underneath because they are covered with tiny hairs (Photo 1). These two species bloom at least two weeks apart, so it is critical to use riverbank grape (V. riparia) as the indicator species. Black locust, a very common tree species with very noticeable white flowers, blooms at about the same time as the riverbank grape. See the Michigan Grape and Wine Newsletter from May 15, 2012 for more about this tree.

The wild grape bloom date is used in the grape berry moth model on the Michigan State University Enviro-weather website to determine the ideal timing of insecticide sprays against this pest. The wild grape bloom date in your area is used in the model to predict the timing of the beginning of second and third generation egglaying later in the summer, which is when insecticides should be applied. It is best to use the wild grape bloom date on your own farm to make the best predictions. That said, our recent observations on farms part of the MSU Vineyard IPM Project put full bloom in riverbank grapes between May 26 and 28 in Berrien County, and May 28 and June 1 in Van Buren County. Cultivated grapes bloom about a week after wild grape bloom, so wild grape bloom indicates timing for final pre-bloom fungicides and insecticides.

Bloom is expected to occur in the next week in most vineyards in southwest Michigan. Grape berries are very susceptible to diseases such as powdery mildew, downy mildew, black rot, phomopsis and anthracnose from bloom through early fruit development. Most diseases are active during this period and are favored by moderate temperatures and frequent rains. For that reason, between now and four weeks after bloom ends, Michigan State University Extension advises growers to maintain cover with fungicide or biological protectant to prevent infection by the principal grape diseases: powdery mildew, downy mildew, phomopsis and black rot.

In susceptible wine grapes, initial Botrytis infections can also begin at this time if conditions are sufficiently cool and wet during bloom. These remain inactive for most of the summer and then develop in clusters after veraison, so at least one spray of a Botrytis-active material is recommended during bloom. Currently, the weather prediction is for low rainfall. If this occurs, the risk of fungal infection in clusters will be lower than average, though stress from winter injury in many varieties may make them more vulnerable to infection.

Considering the winter low temperatures, shoot growth in vinifera grapes in the southwest has been much better in many vineyards than it was last year. However, we are watching for shoot collapse. Survival of grape buds and good shoot growth in the early season does not guarantee vines have enough functional trunk veins to supply water and nutrients for heavy foliage or a crop. The variety, site and sources of vine stress such as the size of last year’s crop will all play a role in determining whether and when shoot collapse occurs in a vine. The worse the trunk damage from the last two winters, the more likely it will be. Some shoot collapse in tender varieties has already been observed in the field (Photo 2).

 
Photo 2. Shoot collapse beginning to occur in tender V. vinifera on a newly-trained cordon.

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

Current GDD accumulations and estimated accumulations for the week ahead are given in the table below. We are about 50 GDD₅₀ ahead of the accumulations on this date last year, and that puts us about three days ahead of 2014. Nevertheless, it is still a cool year and we remain 50 GDD₅₀ behind the five-year average.

Berrien County farms visited Monday, June 1

Concord shoots are 18-24 inches long and bloom is expected to occur in the coming week. Most wine grapes are now at 8- to 16-inch shoots.

The number of first generation grape berry moth males caught in pheromone traps has remained steady and low at all sites over the past two weeks and most recently, captures ranged from zero to 14 moths per trap. We expect females to become active in vineyards in the next week to 10 days. Very little damage results from this first generation. In vineyards where there is low to moderate infestation at harvest, growers should not use an insecticide for this generation. Instead, we recommend control for the second and third grape berry moth generations; ideal timing for these is based on the grape berry moth model.

A very low incidence of early Phomopsis leaf lesions was seen in commercial Concord and Vignoles vineyards during scouting on June 1. Powdery mildew and downy mildew symptoms were not observed in these vineyards. In unsprayed vineyards, leaf infections of black rot are highly developed, and leaf infections of downy mildew are beginning. Growers are preparing to put on their final pre-bloom fungicide cover sprays targeting Phomopsis, powdery mildew, downy mildew and black rot.

Eastern grape leafhoppers are starting to appear. In addition, low numbers of young galls formed by grape tumid gallmakers were found in some vineyards and in wild grapevines in adjacent woodlots. In vineyards with a history of infestation, be on the lookout for symptoms of this pest as well as the leaf feeding stage of phylloxera.

Van Buren County farms visited Monday, June 1

We began to see trace bloom in Niagara with 18- to 24-inch shoots, and hybrids were at 16- to 18-inch shoots. Grape berry moth males are being captured county-wide, and the number caught in traps remains high, ranging from 32 to 136 moths per trap. This suggests we are at or approaching the peak of male flight. No action for control of this pest is necessary at this time, except in cases of a history of severe pressure (see above). As in Berrien County, other insects and disease symptoms were hard to find in these vineyards during scouting on June 1. Only a few leaves with Phomopsis symptoms were seen. Growers are preparing for the final fungicide application before bloom.

Reminder: Southwest Grape 2015 Bloom Meeting

The Southwest Grape 2015 Bloom Meeting is June 3 from 1-4 p.m. at Lemon Creek Winery, 533 E. Lemon Creek Rd., Berrien Springs, MI 49103. For more information, contact Brad Baughman at baughman30@anr.msu.edu or Jamie Styburski at 269-944-4126.

From the meeting location, the group will travel by van to one or two additional farm sites in the area where some innovative grape growers will be showing off novel horticultural practices for wine grapes and discussing 2014-15 winter injury in detail. There will be presentations on pest insect life cycles and effective disease control during bloom.

Three RUP recertification credits will be available for this meeting. There is no cost for registration. We hope to see you there!

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