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Downy mildews can become active with cool to warm (not hot) temperatures, wet conditions and high humidity. Six continuous hours of wet leaves will provide an excellent environment for downy mildew to become established in a crop. Downy mildews are in the same pathogen class as Pythium and Phytophthora (water molds). Symptoms are not always easy to recognize and may appear as other plant problems. There may be microscopic sporangia (spore sacs) on the bottom of the leaf with a darker blotch on the upper surface. Some downy mildews are more aggressive than others. The fungus grows within the plant tissue and not on the surface and symptoms may not appear until conditions are perfect for spore formation, when they emerge on the underside of the leaf. On some plants, symptoms include distortion, stunting and curling of new leaves that appears much like damage from aphids.

To control downy mildews it is important to irrigate early in the day and have good air circulation to avoid wet leaves for long periods, especially overnight. Even this might not be enough to avoid conditions favoring a downy mildew outbreak.

Control is difficult and most fungicides only offer protection, not eradication. When conditions are favorable for downy mildew it is very important to use a fungicide rotation program to manage these diseases as part of a fungicide resistance management program.

The OHP Fungicides registered for downy mildew control include:

Aliette® 50WDG
Apply 1.25–5 pounds per 100 gallons as a foliar spray.

Compass™ O 50 WDG
Apply 1–2 ounces per 100 gallons as a foliar spray.

FenStop™
Apply 7-14 fluid ounces per 100 gallons as a foliar spray.

Triact® 70 – Clarified Hydrophobic Extract of Neem Oil
Apply 0.5 to 1 gallon per 100 gallons as a foliar spray.

Using OHP and other fungicides in a Downy Mildew management program*

The following rotation programs should provide good to excellent management of downy mildews. The application intervals will vary with disease pressure and restrictions on product labels.