This disease is caused by a group of fungi (Bipolaris, Drechslera, and Exserohilum spp. (previously classified as Helminthosporium fungi) that is active over a wide range of temperatures. At any given time of the year, at least one species within this fungal group can be isolated. Thus, diseases caused by these fungi can occur at any time of year. However, as a general rule, the leaf spot disease occurs during mild, wet periods during fall and winter. Leaf spot symptoms tend to vary with each pathogen/host pair from very small (pinhead size), solid brown to purple lesions or spots to expanded lesions with bleached centers that girdle the leaf blade.