MSU is still collecting grape powdery mildew samples. This post appeared in July 2019.

Powdery mildew in grapes (caused by the fungus Erysiphe necator) can reduce vine growth, yield, fruit quality, and winter hardiness. Symptoms of powdery mildew on leaves vary depending on cultivar, leaf age, and how long the mildew growth has been present. By mid to late summer, powdery mildew is noticeable as patchy white or gray growth on the upper surfaces of leaves. In some cases, the mildew growth covers the entire leaf surface (see photos).

Many types of fungicides are used to control grapevine powdery mildew, and an important group is the strobilurin fungicides (sometimes called QoI, FRAC code 11). You may recognize these fungicides by their trade names including Abound (azoxystrobin), Flint (trifloxystrobin) and Sovran 50WG (kresoxim-methyl), as well as products that are part strobilurin such as Pristine (pyraclostrobin and boscalid) and Quadris Top (difenoconazole and azoxystrobin). The strobilurins were first labeled for use on grapes in the early 2000s. The powdery mildew fungus is adept at becoming resistant to the strobilurins, which has prompted a study being done by researchers at Michigan State University.

As a part of the Fungicide Resistance Assessment, Mitigation and Extension Network (FRAME), MSU is conducting a survey throughout the Great Lakes region for strobilurin resistance. The MSU Small Fruit Pathology Laboratory along with collaborators at the USDA-ARS in Corvallis, Oregon, have developed a DNA-based test to quickly and rapidly identify powdery mildew resistant isolates. They want to include Wisconsin vineyards in this survey.

On this note, FRAME is looking for your help. If you are interested in having powdery mildew from your vineyard tested for resistance to strobilurin fungicides, please email Tim Miles at milesti2@msu.edu or Nancy Sharma at sharm115@msu.edu. The test will be paid for through the FRAME grant, and therefore will be free of charge to growers. The process is simple and requires only a few minutes to take a sample and place your sample in an overnight mailer (which they will provide) to be sent back to Michigan State University. The plan is to make these results available before the end of the year so you can make management decisions for next year’s growing season.