Downy mildew of Onions

Peronospora destructor

Onion downy mildew commonly starts in spots in the field and subsequently spreads to surrounding areas.

Elongated (3-30 cm long), pale-yellow lesions appear on the inner and outer side of leaves, covered by greyish-violet down, leaf tips shrivel. Drying up of leaves occurs, beginning with the older and outer leaves and progressing to young leaves, until the entire plant may be killed.

On seed stalks, similar lesions occur, especially on the upper part. These generally affect only one side of the stalk, often causing twisted or lopsided stalk growth. Lesions weaken the stalk and it breaks due to the weight of the seed umbel. Secondary infection by Alternaria porri or A. alternata and Stemphylium botryosum, frequently hastens death of leaf and stalk tissue.

Bulbs are systemically affected. After a period of storage, the outer fleshy scale becomes amber, wrinkled and watery