There are more than 30 species in the genus Oidium of the Erysiphaceae family of fungi. These fungi are erysiphales that cause powdery mildew on numerous plants, including the tomato. Powdery mildew on tomato plants has been increasing internationally in recent decades; two species of fungi cause most of the mildew on tomato plants in the United States. The recommended treatments are similar for both.
Irregular, bright yellow blotches appear on leaves infected with Leveillula taurica, sometimes called Oidiopsis sicula. Dead spots eventually form in the blotches and are sometimes surrounded by a yellow halo. A light gray mycelium, the branching hyphae by which the fungus obtains nutrients, sometimes appears on the bottoms of older leaves. Oidiopsis sicula is most often found in the dry, semi-arid climates of California and the southwestern U.S.
A powdery mildew appears on the leaves of tomato plants infected with Odium neolycopersici, also known as Oidium lycopersicum. White spots of spores and mycelium form on the tops of the leaves and on the stem. The leaves eventually shrivel, turn brown and die. This mildew often appears along the Atlantic coast and in the midwestern U.S.
The wind carries powdery mildew spores to tomato plants. The fungi appear when temperatures are below 86° Fahrenheit, often on cool nights in areas that have high temperatures during the day. Once symptoms appear, hot weather accelerates them.
To treat Leveillula taurica, horticulturalists at the University of California, Davis, recommend two applications of fungicides containing the active ingredient azoxystrobin seven to 14 days apart, with no more than three subsequent applications before switching to a different fungicide. Other recommended fungicides include those with the active ingredients pyraclostrobin, myclobutanil or trifloxystrobin.
To treat Odium neolycopersici, horticulturalists at the University of Connecticut recommend the application of wettable or dry sulfur early in the season, and fungicides containing azoxystrobin on more mature plants. Sulfur should not be applied on young plants when the temperature is above 90° F.
Early applications of sulfur will prevent infections. Powdery mildew often appears late in the growing season. Removing older leaves from the bottoms of plants may control the diseases.
Oil sprays and applications of potassium bicarbonate will kill mycelium and spores on the surfaces of the leaves; these are best applied when the disease first appears.
Researchers at Oregon State University say organic growers should apply two to four quarts of Sonata per acre every seven to 14 days. Sonata is a biological control agent containing the bacterium Bacillus pumilus that kills fungi.
Cures may vary for introductions of new fungi that cause mildew with slightly different symptoms. Washington State University maintains a comprehensive international list of all species of fungi reported to have caused powdery mildew, arranged by country, state or province.