Purple blotch occurs primarily on onions as oval-shaped tan and deep purple lesions on leaf blades. Yellow streaks, which turn brown, extend along the blade in both directions from the lesion. In advanced stages lesions may girdle and kill leaves and seed stems. Concentric zones may develop within the lesions. These diseases are favored by heavy dew and by foggy and rainy weather. The spores are airborne. These diseases are often associated with downy mildew lesions on onions; they occur less commonly on garlic. Infection of seed stalks can reduce seed yield and quality.