Symptoms first appear as small dark purple to black colored spots on the leaf blade. As the spots enlarge, the centers often turn light tan. In warmer temperatures (> 85° F), the entire blade often appears dry and straw colored. The disease is mostly confined to the leaf blades during the cool weather, but can infect leaf sheaths, crowns and roots during hot, humid weather.
Leaf spot is a warm-weather disease, but the pathogen overwinters as dormant mycelium in infected plants and dead grass debris. Leaf spot can become evident when temperatures reach 70° F (21.1° C). The disease is most severe when temperatures are above 90° F and humidity is high. Conditions of drought stress followed by rewetting intensify the disease.