The most critical time of infection and fungal activity usually occurs after mid-July in Colorado. Disease Symptoms Alternaria spores germinate on onion leaves and produce a small, water-soaked spot that turns brown. The margin may be reddish to purple and surrounded by a yellow zone. During moist weather, the surface of the lesion may be covered by brown to black masses of fungal spores. Lesions may merge or become so numerous that they kill the leaf.
Leaves become yellow then brown, and wilt downward two to four weeks after initial infection.
Lesions may form on seed stalks and floral parts of seed onions, and affect seed development. Spores may be blown or washed down to the neck region and infect the outer scales of bulbs through wounds or the neck tissue. Diseased tissue turns brown to black and dries out in the field or, more commonly, in storage. Downy mildew symptoms appear on older leaves as elongated patches that vary in size and are slightly paler than the rest of the foliage. With moisture, these areas become covered with a violet-gray mycelium, which contains spores that may be spread to surrounding healthy tissue.