Oblong to elongate, reddish-brown spots appear on seedling leaves soon after emergence. Spots on stems may be long and narrow or broad and irregular in shape. The outer edges of the spots often are poorly defined, brown color merging gradually into yellow or reddish shades, which frequently spread over the greater part of an infected leaf blade. Oat heads may also become infected. The fungus enters the hulls surrounding the kernels and may even penetrate the kernels slightly.
Diseased kernels turn brown at the basal stem.
Helminthosporium leaf spot fungus survives as spores or mycelium on crop debris and as spores on the seed, or as mycelium beneath the seed coat. When infected kernels germinate, the fungus renews growth or the spores on the seed germinate and infect the seedling leaves. Spores produced on these spots are dispersed by various means to other leaves of the same plant, as well as to other plants, and they produce new spots. Helminthosporium leaf spot, caused by Drechslera avenae . Crop rotation should help in reducing the amount of inoculum and lessen losses from this disease.