Symptoms begin as star-shaped, white lesions on the upper surface of the foliage. Lesions gradually coalesce, and leaf surfaces appear dusted with white powder. Infection on cabbage or cauliflower can reduce head or curd size. On Brussels sprouts, the disease moves onto the stems where sporulation is accompanied by a purplish discoloration of host tissues. Sprout buds may be heavily infected, resulting in an unmarketable product.
Conditions for Disease Development
This obligate pathogen overwinters as cleistothecia on dead host tissue or as mycelium in living tissue. Weed species serve as alternate hosts in the off-season and commonly serves as an inoculum source for subsequent disease cycles. Disease development is generally favored by dew and moderate temperatures [15-20°C (59-68°F)] and conidia are easily spread by wind or by harvesting the crop. Water stress within the host also favors infection.
Apply preventive fungicidal sprays and eradicate cruciferous weeds and volunteers to control this disease.