Helminthosporium, genus of fungi in the order Pleosporales (phylum Ascomycota, kingdom Fungi) that exists as asexual anamorphs and causes leaf blight, especially of grasses (e.g., bluegrass, corn, oats), in humid areas. Symptoms include grayish green, tan, or brown elliptical spots that appear on lower leaves and spread later to upper leaves. Control is possible through spraying of fungicide and use of resistant plants.