Text adapted from Turfgrass Pest Management: A Guide to Major Turfgrass Pests in Oklahoma. Oklahoma Cooperative Extension Service.

CAUSE: Leaf spot diseases are caused by species of the fungi Bipolris, Curvularia, Drechslera and Exserohilum (all formerly known as Helminthosporium species).

HOSTS: Bentgrass, bluegrass, fescue, ryegrass and bermudagrass.

SYMPTOMS: These fungi can cause leaf, crown and root diseases. The leaf spot diseases are usually characterized by dark circular or oval shaped lesions in the early stages of development (Figure 1). The lesions may enlarge and girdle the leaves resulting in a light tan or brown turfgrass (Figure 2). Many of the fungi infect the roots and crowns, and cause a gradual thinning or "melting-out" during stress periods. The melting-out phase is characterized by dark lesions that occur on the sheaths of stunted, spindly shoots. Areas may at first appear chlorotic (yellow) and, then, may turn brown and die if the disease is severe. These fungi can also cause seedling blights on recently planted turfgrass.

DISEASE CYCLE: These diseases can develop anytime during the year. The fungi survive in thatch during unfavorable conditions. Many of these fungi become most active during periods of cool, moist weather and are spread to leaves by wind, rain, irrigation, equipment and foot traffic. The fungi may spread to the crowns and roots during periods of drought stress.

FACTORS FAVORING DISEASE DEVELOPMENT: Lush, succulent turfgrass due to excessive nitrogen, wet conditions in the spring and autumn, drought stress and continual mowing at close heights are factors that encourage the development of these leaf spot diseases. Shaded areas with little or no air movement result in weak turfgrass and extended periods of leaf wetness that favor disease development.

CONTROL: Fertilize to meet the nutritional needs of the turfgrass, but avoid over-stimulation and the development of lush, succulent growth. Avoid excess nitrogen fertilizer on coolseason turfgrasses in late spring and summer.
Avoid continual close mowing of the turfgrass by raising the cutting height whenever possible. Keep the mower blades sharp to reduce the area of open wounds through which the fungi can enter.
Reduce extended periods of leaf wetness by watering deeply but infrequently to a depth of 6 inches to 8 inches. Avoid late afternoon and evening watering, and ensure good surface and soil drainage. Remove unwanted vegetation, and prune trees and shrubs to allow good air circulation. Prevent excess thatch accumulation by power raking or core cultivating and topdressing golf greens on a regular basis.
Fungicides are available that will control these diseases. They should be applied when leaf spot fungi are active and in the early stages of development for best control.

Figure 2. -- "Turfgrass 'browning out' as a result of leaf spot fungi", Courtesy of C.S.S.A.