Gray leaf spot, caused by the fungus Pyricularia grisea, is a disease that affects mainly annual and perennial rye grasses and tall fescue. It has been shown to cause minimal damage in fescues, bentgrasses and Kentucky bluegrass. The disease is of particular significance on the warm season St. Augustine grass and has been common in the Southern states since first being reported in 1971. Recently infections have been found as far north as Long Island, the lower Hudson Valley of New York State, and Pennsylvania.
Infections and subsequent symptomatic tissue can appear quite quickly. Damage is usually noticed during the warmer months of August and September. Conditions favoring infection include hot days over 80°F, nights with prolonged cloud cover when humidity is high, and prolonged leaf wetness. Infected leaves may have water soaked lesions and appear chlorotic. The youngest leaves often take on a characteristic fishhook shape. The disease is most severe on young seedlings. Gray to brown lesions range in size from 2-5 cm. At times, a yellow margin may surround the lesion and the leaf blades may have dark brown borders. The large amount of spores produced by the fungus creates a "felted" look to the leaf blades.
Pathogen: Pyricularia grisea
Primary Hosts: Perennial ryegrass, tall fescue and St. Augustinegrass
Environmental Conditions Conducive to Disease Development: In
perennial ryegrass and tall fescue, this disease is one of the last to be seen in lawns
(mid-July through early October). It's more severe following long periods of drought stress.
In St. Augustinegrass, the disease is most active in late spring through mid summer following high humidity and rainfall. Newly sodded or sprigged areas are most susceptible.
Brown or gray leaf spots with a yellow halo around them signal the early stages of the disease. This is especially true of tall fescue and St. Augustinegrass. In perennial ryegrass stands, leaves may have a twisted, water-soaked or velvety appearance in the morning. In perennial ryegrass, large areas of turf will appear to be wilting even under good soil moisture conditions. Under the right environmental conditions turf will die in a matter of days. This fungus can produce large quantities of airborne inoculum in the forms of spores, which is why lawn devastation can occur so quickly.
Mid to late summer into the early fall provide conditions most conducive for the disease in ryegrass and tall fescue. Gray leaf spot is most active in St. Augustine in late spring through mid summer.
It's believed that the cycling of wetting and drying the turfgrass canopy (making conditions favorable for spore production and dispersal) can enhance the severity of the disease. Irrigate deeply and infrequently. Avoid significant amounts of water-soluble nitrogen during these times; the fungus thrives under lush, high nitrogen fertility.
Fungicides may be necessary during the first year for the establishment of St. Augustinegrass lawns from sprigs or sod since nitrogen and water requirements will be higher, creating conditions conducive for disease development. In perennial ryegrass and tall fescue, brown patch may be present at the same time as gray leaf spot. Consider a preventive fungicide program for managing gray leaf spot as well as brown patch. The QoI and benzimidazole fungicide classes are the best on gray leaf spot.
The fungus that causes gray leaf spot, Pyricularia grisea, has developed resistance to the QoI fungicide class in some locations. When using the QoI fungicides in a preventive spray program rotate with thiophanate methyl, propiconazole, triadimefon, fludioxonil or mancozeb.