Red thread occurs during humid periods in spring and fall when daytime temperatures are between 60°F and 75°F. Fine-leaved fescues and some ryegrasses are most susceptible, but it also affects Kentucky bluegrass and other fescues. Slow-growing, nitrogen-deficient turfs are particularly vulnerable.
Look for water-soaked patches of grass that turn to a bleached tan as grass dies. Patches may be round or irregularly shaped and from about an inch to a foot in diameter.
In humid weather, the fungus produces coral-pink to blood-red hyphae up to an inch long on the tips of grass blades. These red threads can disperse the disease to healthy turf by mowing and foot traffic.
To help prevent the disease, maintain soil fertility and, where red thread has been a problem in the past, lime to maintain pH at 6.5 to 7.0. Avoid overwatering, provide good drainage, and keep grass blades as dry as possible by watering early in the morning.