Early symptoms are usually noticed as light green or yellow angular patches on the upper leaf surfaces. Under high humidity, downy mildews form fluffy or fuzzy (downy) areas of spores only on the lower leaf surfaces. If viewed through a hand lens, these downy areas look like very small bunches of branched hairs. Older leaves near the bottom of the plant are usually infected first and wither and turn brown as the disease proceeds up the stem. Some species can be seed borne and others may cause systemic infections. Some weed species serve as reservoirs of inoculum.
Under high humidity, down mildews for fluffy or fuzzy (downy) areas of spores only o the lower leaf surfaces.
Most foliar diseases can be lessened with proper watering and humidity regulation. Water plants as early in the day as practical to allow foliage to dry before nightfall. Alleviate poor air circulation or crowded conditions with proper plant spacing. Inspect new growth and older foliage regularly for signs of infection to catch infections early. Sometimes simply removing the infected spotted leaves or plants will solve the problem. Selection of resistant varieties will also help to eliminate the application of costly controls.
Impatiens downy mildew, a strain of the disease specific to impatiens, has been a concern in Maryland since 2012.