Department of Crop and Soil Sciences - Cooperative Extension
Turfgrass Diseases
Rust diseases, Puccinia spp.
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 during other times of the year.
Signs of the causal fungus of rust disease on Kentucky bluegrass.
(photo courtesy of Dr. Noel Jackson).
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 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.
Cultural control
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.
Chemical control
Fungicides are used only as a last resort; one or two sprays applied after the onset of symptoms usually will suffice.
Slime molds, Mucilago, Physarum, and Fuligo spp.
Slime molds are different from most other turfgrass pathogens in that they do not directly infect the plant. Instead, this group of organisms feeds on microorganisms and decaying organic debris. During periods of warm, wet weather, slime molds migrate onto the surfaces of turfgrass leaves where they produce massive amounts of their reproductive structures, thus producing a white, blue, or gray patch.
Symptoms and signs
Grass blades and the surface of the soil may be covered with a translucent slimy, creamy-white growth. In a few days, this slimy growth changes to pinhead-sized masses of various colors (usually gray or blue). The affected areas may be from a few inches to many feet, with shapes ranging from rings to streaks or patches.
Disease cycle
Slime molds survive adverse conditions in the spore stage. In cool, humid weather, the spores absorb water. The spore wall cracks open, and a motile spore emerges. The motile spores feed on other microorganisms and decaying organic matter, but do not infect living turfgrasses. Eventually, pairs of spores unite and increase in size. This form of the organism is called a plasmodium, and it is this stage that produces the slimy overgrowth on turfgrasses. Slime mold plasmodia and reproductive structures may shade the grass leaves to the extent that leaves are yellowed, but damage seldom is severe.
Spore masses of slime mold fungus on Kentucky bluegrass lawn.
Cultural control
Slime molds do not damage turf (apart from shading) and can be removed by sprinkling the leaves with water after the onset of dry weather. Removal of the dry spore masses by mowing, raking, brushing, or sweeping with a pole will aid in returning the grass to normal appearance. Chemical control is not necessary.