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Gray Leaf Spot — Pyricularia grisea

© 2017 MSU Plant Pathology
© 2017 MSU Plant Pathology
© 2017 MSU Plant Pathology
AKA

Gray leaf spot is considered a problem on primarily perennial ryegrass and St. Augustinegrass. The disease first appears as tiny brown spots that enlarge and become oval or elongated. The mature spots usually have depressed gray centers with irregular brown margins, and a ring of chlorotic tissue surrounding. Severe infection shows burned or scorched blades of turfgrass. Lesions are also found on the leaf sheath, spike and stems. Stem lesions tend to be brown to black.

The disease is most severe during warm, humid weather. The pathogen overwinters as spores and dormant mycelium in the lower leaves of infected plants and thatch. When temperatures rise, the spores are spread by wind and water to healthy plants. Under conditions of high humidity and high temperature (80° to 90° F [26° to 32° C]), the disease may develop rapidly. Newly established stands of St. Augustinegrass are more susceptible to gray leaf spot than mature turfs.