Downy mildew and powdery mildew are fungal infections that affect plants. To determine which particular infection is afflicting a plant, the symptoms are examined. Both attack the leaves, but damage them differently.
Downy mildew belongs to the Peronosporaceae family and powdery mildew belongs to the Erysiphaceae family.
Powdery mildew spores appear on both sides of the leaf, where as downy mildew only produces spores on the underside of leaves. Powdery mildew produces spores in a tree-like formation, while downy mildew spores are produced in chains.
Powdery mildew requires a nighttime temperature of at least 60 degrees F and a daytime temperature of 80 degrees F. It also requires a nighttime humidity of 90 to 99 percent and a daytime humidity of 40 to 80 percent, according to R. K. Horst, professor of plant pathology at Cornell University. Downy mildew can't survive below 40 degrees or above 80 degrees F. Sixty-five degrees is ideal for downy mildew and humidity needs to be above 85 percent.
Powdery mildew spores are spread by wind. Downy mildew spores spread as a result of splashing water. Spores are usually launched from dead leaves on the ground upward to the underside of plant leaves during rainfall.
Powdery mildew produces a thick web of white spores that cover the leaves, stems and shoots. The leaves turn yellow and fall off. Downy mildew produces a gray or white fuzz on the underside of leaves. The top of the leaf has yellow blotches.