Symptoms of Cercospora leaf spot first appear as individual, circular spots that are tan to light brown with reddish purple borders. As the disease progresses, individual spots coalesce. Heavily infected leaves first become yellow and eventually turn brown and necrotic. Blighted leaves soon collapse and fall to the ground. Warm nights combine with high humidity in irrigated fields to provide an ideal environment for disease development. The primary source of inoculum is residue from a previously infected crop, but the fungus can be carried on seed and is also hosted by numerous weeds. Spores produced by the fungus are dispersed by splashing rain and may also be carried by wind.