Several phone calls have surfaced concerning the off colored turfgrasses around the state. Large areas turning yellow to orange in color are raising questions. What is it? And, why?
If homeowners take a closer look at individual leaf blades they will notice orange, powdery pustules. This is usually nothing to be alarmed about since weather patterns will change soon. The periods of cloudy, moist conditions followed by warmer sunny days present conditions favorable for this turfgrass disease.
Rust is most severe on susceptible cultivars of Kentucky bluegrass, tall fescue, perennial ryegrass and zoysiagrass. Rust symptoms usually appear in late August to early September and continue through the fall months. Rust can also develop at other times of year, depending on environmental conditions, turfgrass species and rust species. Disease severity can vary dramatically from one year to another.
The rust fungi (Puccinia spp.) overwinter as urediniospores in infected plants or are reintroduced each summer by windblown spores. Infection of leaf blades is favored by moderate temperatures (68 to 85 degrees F) and extended wet periods. The disease tends to be more severe in partially shaded areas, such as under trees or along fence rows. Once infection occurs, slightly higher temperatures favor symptom development. Turfgrasses under stress (nutrient deficiency, drought, shading, close mowing heights and high temperatures) are more likely to be seriously damaged by the disease.
Turfgrasses provided with optimal levels of fertilizer and water is less likely to be severely damaged by rust. Avoiding night watering decreases the length of time the leaf blades remain wet. However, we have not been irrigating here this fall. Mother-nature has been providing the extended leaf wetness periods. Regular mowing removes infected leaf tips from the plant and will help reduce inoculum levels. The best strategy is to mow frequently at a height not less than what is recommended for the turfgrass. Avoid close mowing or scalping of the turf. Many of us have been finished with mowing this season, therefore inoculum levels have remained high promoting highly infected areas.
For long-term disease management, choose grass cultivars with a high level of resistance to rust. The National Turfgrass Evaluation Web site at ntep.org will provide the most up-to-date information on relative resistance of selected turfgrass cultivars to rust.
The decision to use fungicides is often difficult because applications need to be made relatively early in rust development. Because the rust epidemic is dependent on weather, it is hard to determine if early fungicide sprays are warranted. In most years rust does not reach damaging levels before the turfgrass enters winter dormancy, so fungicides are not routinely used for rust control in home or commercial landscapes. As weather conditions change, the symptoms will disappear.
REVISED: September 30, 2015