Rust appears as an orange or yellowish-orange powder (spores) on grass leaf blades, especially in late summer to early fall when the weather is dry. Rust typically develops on lawns growing slowly. Overall, the turf may assume a yellow, red, or brown appearance. Close examination will reveal the pustules, which easily rub off on your hand.
Low fertility (in particular nitrogen) and low water availability slow down turf growth, allowing rust to develop. Rust is fairly common on cool-season lawns late in the summer as the plants have slowed growth due to the heat. Often a return of cooler autumn temperatures and more rainfall, encourages vigorous grass growth. The rust is removed by the mower blade.
Cool nights with heavy dew and light, frequent rainfall add to the ideal conditions for rust to develop. Warm, cloudy, humid weather followed by hot, sunny weather also favors rust development on lawns. Occasionally, seasons with excess rain may have rust outbreaks due to depletion of available nitrogen.
Kentucky bluegrass, perennial ryegrass, and tall fescue are all affected, depending on cultivars. Rust spreads via air, water, shoes, equipment, and vegetative turf material (sod). Rust does not kill the grass plant but may weaken turfgrasses and make them more susceptible to other problems.
Control rust through sound turf management. Begin by choosing a quality turfgrass seed blend of several cultivars of the same species desired for the site. Resistance to rust can vary according to the race of the disease present. A diverse turf stand helps combat rust and numerous other turf problems.
Maintain lawns through sound watering, mowing, and fertilizing. Water early in the day so the grass dries quickly. Manage problem thatch. Increase vigor with an early fall nitrogen application, but don't overdo it. Check soil phosphorus and potassium levels through soil testing. Also assure good airflow over the site and light penetration by pruning trees and shrubs in the area near the lawn.
When rust occurs in late summer, improved growth conditions of early fall often get lawns growing more vigorously and the rust fades away. Early September is a key time for fertilization. If conditions are dry, irrigation is also needed to increase the growth rate of the lawn.
Fungicides are rarely suggested on home lawns for rust control.