Rust is a common problem on ryegrass and Kentucky blugrass in the fall. The following is more information.
Rust on perennial ryegrass and other turfgrasses is favored by the cool, moist weather of fall. Look for yellowing of turf. Close examination of the individual grass blades will reveal the reddishorange pustules of the rust fungus. It can cause some thinning of turf but the best treatment at this time of year is fertilizing the lawn. Lawns will outgrow the fungus in most cases. Rust is usually a low fertility, low vigor problem on turf.
A variety of related fungi cause rust. Ccommon names include leaf rust, crown rust, and stem rust, and the disease occurs almost exclusively on Kentucky bluegrass and perennial ryegrass. Rust is largely cosmetic, but the orange spores that dislodge easily from leaf surfaces can be a greater nuisance, covering shoes, pets, and lawnmowers with a rusty residue. Rust can severely damage new spring-seeded lawns that lose vigor during heat and drought conditions.
From a distance, rust-infected turf appears chlorotic. Symptoms occur in a diffuse pattern around the initial site of infection as disease increases. Outbreaks often first occur in shaded or protected areas, such as around the bases of evergreens or next to a structure’s foundation. Close inspection of rusted leaves reveals numerous yellow-orange pustules on leaf blades. Walking through grass with significant amounts of infection will disturb and release the spores within these pustules and leave a distinct orange color on shoes and pets. These spores, carried by the wind, spread the disease to other areas during the growing season. Rust outbreaks are most common in late summer and early fall, although sometimes the disease is active in the early spring, especially on poorly nourished turf. Rust is a disease of slow growing turf, so factors such as summer heat and drought stress, low nitrogen fertility, compaction, and shade contribute to poor growth tend to favor rust development. Rust outbreaks require moderate temperatures (50°-60°F) and long evening dew periods (more than 10 hours). The pathogen survives as resilient spores over the winter or as inactive mycelium in dormant turf.
Maintaining a healthy and vigorous turf stand is the most effective and efficient method of rust control. Since slow growing turf in late summer is most vulnerable to outbreaks, small amounts of nitrogen fertilizer (0.25-0.5 pound of N per 1,000 square feet) in chronic trouble spots—shaded and possibly compacted areas—will help control the disease. The nitrogen will promote leaf growth and allow for regular mowing, which helps the turf outgrow rust’s relatively slow infection cycle. Avoiding irrigation during the early evening also will help limit disease spread by lessening the chance of extended dew periods.
DMI and QoI (strobilurin) fungicides are very effective against rust, but on well-established turf, should be considered only as a remedial treatment when cultural practices fail to prevent an outbreak. On newly seeded stands, fungicides should be appliedat the first sign of disease. In most cases, a single application of an effective fungicide, combined with efforts to encourage turf growth, will quell outbreaks.
Rust on perennial ryegrass. Photo from the August 10, 2009 edition of Turf Tips from Purdue University
Information from Bob Mulrooney, Extension Plant Pathologist, UD in the Current Ornamentals Hotline from UD Extension and Rick Latin, Turfgrass pathologist in the August 10, 2009 edition of Turf Tips from Purdue University