Spores are produced on the debris as warm, wet conditions occur in the spring. Wind and rain are responsible for dispersing spores to other plants and fields. Tops are more prone to infection as plants mature. Under favorable conditions, an unprotected crop can sustain a substantial amount of infection in a few weeks. Disease Control .
Crop rotation is important to prevent pathogen populations from building up to high levels.
Rotations out of onions for 2-3 years are recommended. Most commercial onion crops must be protected from purple blotch by using repeated applications of protective fungicides. Downy Mildew Introduction Downy mildew is a disease affecting onions and almost all related crops. Early-season epidemics have resulted in severe yield losses. Downy mildew is identified by a purple-brown mold occurring in irregular shaped patches on onion leaves.