Disease

Downy mildew

Peronosporaceae (Peronospora, Pseudoperonospora, Bremia)

A water mold, not a true fungus, that sporulates on the leaf underside in humid weather. Each crop has its own downy mildew, and it develops fast.

Signs

Yellow or pale patches on the upper surface, squared off by the veins.
Gray or purplish-gray fuzz on the underside below those patches, in humid weather.
Patches turn brown and dry; basil leaves develop black spots.

Cool, humid nights with dew or leaf wetness for several hours, and spores carried long distances on air currents.

Cucumber, squash & melon; Lettuce, spinach & herbs; Cabbage family (broccoli, kale, cabbage); Roses & ornamentals; Hemp & cannabis; Beans & peas.

Treatment by crop

Cultural, biological and chemical options in the order to use them, and notes written for each crop.

Identification is free. Treatment is on the Home and Pro plans.

Open in the key See plans

Look the plant over, leaf undersides included. Are insects, mites, slugs, eggs, webbing or droppings on it?
No, only damage or discoloration
Where is the problem?
Leaves
What does the leaf damage look like?
A coating or growth on the surface
What is the coating like?
Gray or purplish fuzz on the underside

Not a match? Start the key or describe what you see.

More from university Extension: search Extension pages on downy mildew, or find a plant diagnostic lab through your state's Extension.