Keep monitoring for powdery mildew in grapes
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EmailEditor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Powdery mildew is becoming more apparent in vineyards, on the fruit as well as the leaves. Although we had suitable conditions (0.1 inch of rain at temperatures over 50°C) for primary infections numerous times in spring and early summer, the cool weather appears to have kept the disease at bay. In northwest Michigan, even unsprayed vines are not showing much disease. However, warm weather and moderate to high humidity in upcoming weeks may still allow some development of powdery mildew on grapevines. This would be mainly on the leaves, since berries are resistant to infection by now. If berries are already infected, the colonies may become more visible due to growth and sporulation. Continued monitoring for colonies on leaves and clusters is advised. Symptoms and signs may be inconspicuous.
Once the fungus gets established, it does not need water or rain for infection. In fact, rain is detrimental because it washes the spores from the leaves and causes them to burst. Under optimal conditions, the disease can spread rapidly, as the time from infection to production of the airborne conidia can be as short as seven days. Although infections can occur at temperatures from 59 to 90ºF, temperatures between 68 and 77ºF are optimal for disease development. Temperatures above 95ºF inhibit spore germination, and the fungus may be killed at temperatures above 104ºF.
Sulfur remains an effective and inexpensive protectant fungicide for powdery mildew control in non-sulfur-sensitive grape varieties. However, sulfur residues remaining on grapes at harvest have the potential for forming hydrogen sulfide and other sulfur compounds during fermentation, depending on the type of yeast. It would be best not to apply sulfur within 30 days of harvest to avoid excessive residues on grapes. The most effective systemic fungicides for powdery mildew control are the sterol inhibitors: Rally (formerly called Nova), Elite, Vintage, Procure, etc.; and the strobilurin fungicides: Pristine, Sovran, Abound and Flint. Since Abound is now much more expensive, some growers are switching to Sovran or Flint. New fungicide options that provide good to excellent control of powdery mildew are Quintec, Endura, and Adament (mixture of Flint + Elite). Do not apply Pristine, Flint, or Adament to ‘Concord’ grapes, as crop injury may result. JMS Stylet Oil and Sulforix are good “eradicant” materials to knock down existing colonies after infection has started. Applying these materials in sufficient water is important to get excellent coverage because these compounds have to contact the fungal colonies to be effective. Do not apply Sulforix to sulfur-sensitive grapes, and make sure to read all label warnings with respect to tank-mix incompatibility with other products.
Dr. Schilder's work is funded in part by MSU's AgBioResearch.
