Peronospora destructor

Peronospora destructor is a plant pathogen. It causes downy mildew on leaves of cultivated and wild Allium. Allium cepa (onion and shallot) is most often affected, while Allium schoenoprasum (chives) and Allium porrum (leek) are only occasionally affected. Downy mildew is a major disease of onion. The pathogen persists as mycelium systemically infecting onion bulbs, but is not known to be transmitted in onion seed. The pathogen can persist in the soil for several years as oospores. Systemically infected plants are dwarfed and pale green. Under moist conditions, the pathogen sporulates on the affected tissues and spreads to other plants, on the leaves and stalks of which it forms greyish-violet local lesions. Infected leaves are often entirely killed. Critical periods for infection have been determined. Infected crops yield poorly, and produce distorted bulbs. Control is by crop rotation (at least 3 years between successive onion crops), use of healthy bulbs for planting (heat treatment has been used to eliminate the pathogen from bulbs), fungicide treatment of the bulbs for planting, and fungicide sprays of the foliage if downy mildew infection is nevertheless observed.

from Wikipedia