Spore production by Helminthosporium avenae

Abstract

Spores of Helminthosporium avenae are liberated into the air in similar numbers per unit of leaf area during primary (seedling blight) and secondary phases of leaf-spot disease of oats. No spores could be found in the surrounding air during the intervening period of disease-free growth. When spores were present in the air, they were found by night as well as by day; most were found at ground level, and numbers diminished with increasing height. There was no correlation between numbers of spores in the air and temperature, relative humidity and wind velocity. It is concluded that some factor depresses or inhibits the activity of the fungus in relation to the host when growth is free from disease. The factor does not operate earlier during primary disease, nor later at the secondary leaf-spot phase. Since the factor cannot be climatic, it is suggested that an investigation of conditions within the host plant itself is most likely to provide an explanation.