Gray Spot Leaf Fungus

Causal Agent

• Gray leaf spot is the result of an infection from the Pyricularia grisea fungus, also called Magnaporthe grisea. Though a number of cool-season grasses, such as perennial ryegrass, annual ryegrass and tall fescue, are prone to infection, only the St. Augustine grass is affected by the fungus from the warm-season grass group. The damage to St. Augustine grass and perennial ryegrass is severe enough to deem the disease deadly when the infection is widespread.

Symptoms

• The disease starts with the appearance of an oblong shaped spot on grass blade with a gray, felt like center caused by spore growth. Leaf spots may also be green to brown colored with darker borders in the absence of spores. Inspecting grass closely reveals several smaller spots that merge to create larger patches. These characteristic gray spots are barely visible on cool season grasses. Early signs on these grasses are very small lesions that are frequently missed.

Favorable Conditions

• Fungus commonly infects grasses during the warm rainy periods between May and September where the grass stays wet for prolonged periods of time. Humidity of over 95 percent favors fungal growth. Most favorable weather temperatures range between 80 to 90 degrees F. Grasses that are growing under less than adequate cultural conditions and are consequently stressed are more prone to infection than vigorously growing grasses.

Control

• Improving cultural conditions is a long-term remedy to reduce stress and chances of infection in turfgrasses. Common reasons for stress include compacted soil, applying the herbicide atrazine to the grass in summer, a low nutrient level in soil, poor irrigation or an already existing pest infestation. Fungicides for the control of infection include strobilurin, chlorothalonil and propiconazole.