Downy mildew of Onion

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.

Leaves: When looking at individual leaves, necrotic spots begin as yellowing spots that eventually turn brown or black as the leaf tissue dies. Older and outer leaves often show symptoms earlier than younger leaves. Leaf tips shrivel as the pathogen moves inward toward the stalk of the plant itself. The symptoms begin as elongated, pale yellow lesions which progress into small patches of fungal colonies that are gray in color. As the disease continues to progress, secondary infection by other pathogens may occur, leading to purple or brown colored spores in the lesions on the leaves, which characterizes the downy mildew disease.

Stalks: The stalks of onion plants can also be infected by Peronospora destructor, with symptoms appearing as yellow or brown necrotic areas along the stalk itself. Although P. destructor does not usually kill the entire onion plant, the pathogen reduces the growth of the onion.

Bulb: The bulb tissue typically becomes soft and watery, lacking the firm quality that typical healthy onions have. The outer portion of the bulb also appears wrinkled and may take on an amber hue.