Rust, caused by Puccinia spp., is not seen every year, but sometimes becomes severe on susceptible varieties during hot periods of the summer when grass growth has slowed. When rust is severe, the lawn may have a yellowish to reddish-orange appearance. A red-orange dust fills the air when the grass is mowed and also collects on shoes and clothing. Individual blades of grass will have slightly elongated yellow-orange to red-orange spots or pustules (filled with a rusty colored powder -- the spores of the rust fungus) that break through the leaf surface. When rust is severe, the grass blades turn yellow, wither and die. Rust may also weaken a lawn, making it more susceptible to winter kill the following winter.
Rust is favored by humid weather with night temperatures of 70 - 75 F, day temperatures of 85 - 95 F, wetness from dew lasting many hours after sunrise, and frequent light rain (or watering). Rust may be especially severe on Merion and Touchdown varieties of bluegrass, which are highly susceptible. When weather favors rust, the disease is more likely to be severe on low maintenance lawns -- lawns with low soil fertility and some degree of drought stress. It is also apt to be a problem in shady areas, on closely cut grass, and on newly laid sod.
Rust is easily controlled by maintaining good lawn growth with adequate fertilization and adequate watering. Once normal growth is obtained, mow the grass frequently at recommended mowing heights and remove the clippings, an important source of the rust fungus. Fungicides are not usually needed or economical for homeowners, but may be required to help protect new growth when rust is severe and weather promotes rust development.