There are a number of rust diseases that affect turfgrasses. A few of the more common rusts that occur on cool-season grasses 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 conditions.
Symptoms and signs
Early infection appears as a light yellow flecking of the leaves. As these flecks enlarge, they may become somewhat longer than broad and when numerous they are arranged in rows parallel with the veins of the leaves. Soon, the epidermis ruptures and the spots develop into reddish-brown pustules. Severely infected plants have an appearance similar to rusty-iron, hence the name rust. When infected leaves are rubbed between your fingers or walked upon, a red powder collects on fingers or shoes. This powder is composed of millions of tiny spores of the fungus. Rust is normally a late summer or early fall problem and does not occur at other times of the year.
Signs of the causal fungus of rust disease on Kentucky bluegrass. (photo courtesy of Dr. Noel Jackson).
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 the grasses. 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.
Adequate nitrogen and irrigation to maintain growth through late summer will minimize rust infections. In most years, the disease will not become severe, although infected turf may become yellowish-orange.
Fungicides are used only as a last resort; one or two sprays applied after the onset of symptoms usually will suffice.