Onion Downy Mildew is a water mold or oomycete disease of alliums caused by Peronospora destructor. It causes irregular foliar lesions that begin as pale-green, then progress to yellow or brown necrotic tissue. Eventually, lesions coalesce and lead to the collapse of the leaf. During periods of high moisture, fuzzy, gray-to-violet sporangia appear on leaf surfaces. These symptoms can also be seen on seed stalks and flowers. Bulbs can be stunted and sponge-like during systemic plant infection and, when the disease infects the bulb itself, can become watery. P. destructor primarily overwinters in volunteer onions and cull piles, as well as in infected soil, plant debris, and perennial hosts. The airborne spores spread via wind currents and splashing water, infecting through water on the leaf surface.
Favorable conditions include high moisture and cool-to-moderate temperatures. Cultural management involves planting disease-free bulbs and seed, rotating away from susceptible allium crops in diseased fields for 3 years, maintaining proper soil and leaf moisture, avoiding overhead irrigation and planting in poorly-draining soils, improving airflow between plants, and properly managing volunteer onions, cull piles, and infected plant debris. Several preventative fungicides can be applied when conditions for onion downy mildew become favorable, as well as those applied at the first signs of disease.