Turfgrass Diseases: Rust Diseases (Causal fungi: Puccinia spp.)

Rust diseases are caused by fungi in the genus Puccinia. A few of the common rusts that occur on cool-season turfgrasses include stem, stripe, crown, and leaf rust. Although there are differences among these diseases with respect to the symptoms and portion of the plant affected, they generally occur under the same environmental and cultural conditions.

Symptoms and Signs

Initial symptoms of rust diseases appear as pale-yellow speckling of leaves or stems. As the disease progresses, the yellow areas become larger and tend to align in rows parallel with the veins of the leaves. Eventually, the leaf surface ruptures and the yellow spots develop into reddish-brown spore masses called uredinia (more commonly referred to as pustules). Severely diseased plants have an appearance similar to rusty iron, hence the name rust. When infected leaves are rubbed between fingers or walked upon, an orange powder collects on fingers or shoes. This powder is composed of millions of tiny spores (urediniospores) of the causal fungus. Rust is normally a late summer or early fall problem and does not occur at other times of the year in Pennsylvania.

Disease Cycle

The cycle of development for rust diseases is quite complex. Of the dozen or so species of rust fungi that affect turfgrasses, all but three go through five distinct spore production stages. Some of these stages, which are necessary for the completion of the entire life cycle, must occur on plants that are unrelated to turfgrasses. For a specific rust species, completion of the rust life cycle may require grasses and woody shrubs or grasses and herbaceous ornamental plants. In general, rust diseases do not kill turfgrasses but may weaken them to the point that they become more susceptible to stress-related problems, such as drought and heat.

Cultural Control

Adequate nitrogen and irrigation to maintain growth through late summer will minimize rust infections. In most years, the disease does not become severe enough to thin stands, although infected turf may take on a yellow-orange color. Although most cool-season turfgrass species are susceptible to infection by at least one species of Puccinia, perennial ryegrass and Kentucky bluegrass tend to be more commonly and severely affected than other species. Turfgrass cultivars vary in susceptibility to rust diseases; thus, turfgrass managers should select rust-resistant cultivars of perennial ryegrass and Kentucky bluegrass whenever possible.

Chemical Control

Fungicides are rarely used to control rust diseases in lawns or sports turf. However, when newly seeded turfgrasses begin to show symptoms in high visibility settings, one or two fungicide applications usually will control rust infestations.

*FRAC is an abbreviation for Fungicide Resistance Action Committee. The FRAC code/resistance group system consists of numbers indicating classes or groups of fungicides based on the mode of action, and letters that refer to broad classifications of fungicides (P = host plant defense inducers; M = multi-site fungicides; and U = unknown mode of action and unknown resistance risk). Due to the risk of fungicide resistance, turf managers should avoid excessive use of fungicides within the same FRAC code/resistance group and alternate products among different FRAC codes/resistance groups.

**Plant mobility classification refers to a fungicide’s ability to penetrate plant surfaces or remain on plant leaf or stem surfaces without penetration. Fungicides that penetrate plant surfaces and are translocated mostly upwards through plant xylem tissues are called acropetal penetrants (acropetal = toward the apex). Fungicides that enter plant cuticles or move limited distances in internal plant spaces, but do not translocate through vascular tissues (xylem and/or phloem) are called local penetrants. Contact fungicides do not penetrate plant surfaces and only inhibit fungal pathogens residing on leaf and stem surfaces.

***Follow label precautionary statements, restrictions, and directions regarding tolerant turfgrass species, rates, and timing of applications.

References

Buhler, W. Fungicide spraying by the numbers.

Clarke, B.B., P. Koch, and G. Munshaw. Chemical control of turfgrass diseases 2020. University of Kentucky, Rutgers University, and University of Wisconsin.

Latin, R. 2011. A practical guide to turfgrass fungicides. American Phytopathological Society Press, St. Paul, MN.

Smiley, R.W., P.H. Dernoeden, and B.B. Clarke. 2005. Compendium of turfgrass diseases, 3rd Edition. American Phytopathological Society Press, St. Paul, MN.