Most of us think of rust of turf as a cool-season, spring or fall disease. Although there are a few cool-season rust diseases, most of the rusts of turfgrasses are favored by four to eight hours of overcast skies, temperatures between 70 degrees F and 80 degrees F, high humidity, and heavy dews or light rains followed by eight to sixteen hours of high light, high temperatures, and slow drying of leaf surfaces. This may seem a bit complex, but it means simply that the rusts are favored by warm, cloudy, humid conditions followed by hot, sunny conditions. Over much of Illinois, these conditions have been met in the last two weeks, so it should not be surprising that the disease is back.
All turfgrasses can be infected with rust fungi, but there are definitely differences in resistance within the turf species. Early symptoms of rust diseases include light yellow flecks on leaves and stems, giving the lawn a light green or yellow cast. The flecks enlarge until spores of the fungus are produced, causing the leaf tissue to rupture and expose powdery, spore-filled spots called rust pustules. The pustules may be yellow, orange, brown, or red. The spores rub off very easily on hands, shoes, clothing, and animals. The turf becomes weakened, unsightly, and more susceptible to injury from environmental stresses and attack by other pathogens.
Grasses growing slowly under stressful environmental conditions are most susceptible to rust, particularly when water, fertility, and soil conditions are inadequate for good growth. Logically, control measures should target stress areas. Rarely are fungicides recommended to control rust in a home lawn. Because leaf wetness is required for infection, it is important to water thoroughly early in the day so the turf can dry before night. Water turf infrequently but to a depth of six inches or more at each watering. Avoid frequent, light sprinklings. Fertilize to keep the grass growing about one inch per week in summer and early fall droughts. Use a balanced fertilizer; do not apply excessive nitrogen. As the grass grows, it pushes rust-infected leaves outward, making infected blades easy to mow and remove. Mow regularly to remove infected leaf tips, but avoid mowing below the recommended height for the particular turf species or cultivar. Prune surrounding trees and shrubs to improve light penetration and air circulation around densely shaded areas. Badly infected areas of turf may have to be renovated and reseeded. The best time to do this is mid- to late-August. Be certain to use a blend of turf cultivars with resistance to rust, as listed in Report on Plant Diseases No. 412.