Gray Leaf Spot

Text adapted from Turfgrass Pest Management Manual: A Guide to Major Turfgrass Pests and Turfgrasses
North Carolina Cooperative Extension Service

CAUSE: Pyricularia grisea

HOSTS: St. Augustinegrass

SYMPTOMS: Gray leaf spot begins as small lesions on leaves and stems during warm-humid weather. In time, the lesions enlarge to circular or oval shaped spots that are tan in the center and bordered with a purple to brown margin. A gray growth may cover the lesions during warm-humid weather. Severely affected leaves may wither and die, and St. Augustinegrass may appear brown.

FACTORS AFFECTING DISEASE DEVELOPMENT: The fungus survives as mycelium and spores on infected plant tissue. Spores are produced during warm-humid weather and are spread by wind, water and mowing. The disease is more severe on immature plants and in turf that has been fertilized with high rates of nitrogen.

CONTROL: Select slow release fertilizers or limit the amount of quick-release nitrogen applied to St. Augustinegrass when warm-humid weather is expected. In general, do not exceed one pound of nitrogen per 1,000 square feet per month during the months of July and August especially on newly planted St. Augustinegrass.
Avoid late afternoon and evening waterings to reduce the duration of leaf wetness. Prune trees and remove undergrowth to aid in air movement and allow for light penetration.
Fungicides are available that will effectively control gray leaf spot. Repeat treatments may be necessary if conditions favoring the disease persist.