Rust diseases are favored by warm and humid conditions and develop most frequently on grasses subject to stress-droughty conditions, low nitrogen fertility and shade. Low mowing heights, particularly on Kentucky bluegrass, also increase the susceptibility of grasses to rust. The disease first appears on grass leaves as small orange to reddish-brown flecks that enlarge to form raised pustules on leaves and stems. Individual pustules are usually oval or elongated and contain a powdery mass of orange to reddish-brown spores. As the pustules mature they turn brown to black.
Infected leaves turn yellow, wither and die.
In southern states, ryegrasses are highly susceptible to rust in the spring, particularly where nitrogen fertility is low. Zoysiagrasses are most often affected by rust in the fall as the growth rate of grass slows and environmental conditions favor disease development. Cultural practices which improve the vigor of the turf help prevent rust. To reduce the incidence of rust, keep nitrogen levels adequate for turf growth, avoid moisture stress or overwatering and adjust mowing heights according to the grass needs. In the case of Kentucky bluegrass and perennial ryegrass, use varieties that have good resistance to rust where the disease is a problem.