Zoysia and other turf grasses are susceptible to a variety of fungal diseases in hot, humid weather and when they are wet for prolonged periods. Some ways exist to prevent some of these diseases, although many are not treatable by fungicides. For close-up images of these diseases and recommended treatment, visit the Web site of the Plant Disease Diagnostic Clinic at Cornell University (see Resources).
The fungal disease that poses the worst danger to Zoysia grass is brown patch (Rhizoctonia solani), which forms roughly circular, light brown patches from 2 to 3 feet wide. When the turf is wet, a grayish-black ring of blighted and wilted leaves sometimes marks the advancing patch. Brown patch kills the leaf blades, which are often then infected by algae that forms a hard crust.
On closely cut turf, dollar spot (Sclerotinia homeocarpa), appears as sunken, round, straw-colored or tan spots the size of a silver dollar. Uncontrolled spots can merge to produce large sunken areas of turf. The fungus often produces white cobweb-like growth in humid air.
Fusarium patch (Microchium nivale), also called pink snow mold appears as water-soaked, round spots 1 to 3 inches wide that evolve into orange-brown, reddish-brown, or yellow patches. They may form light gray or light tan rings 1 to 2 feet wide with reddish-brown or orange-brown borders. Under wet conditions or beneath snow, the fungus grows on matted grass. When the snow melts, the white growth at the edges turns pink.
Several related fungi called Helminthosporium attack the roots, crown and leaves of Zoysia. In early spring to autumn, small, dark-brown or purplish spots form on the leaves. As they grow, they develop light-colored centers with purplish-black borders.
Helminthosporium causes rotting roots and crowns, turning them reddish-brown to black. The weak, thin turf, appearing to suffer from drought, develops growing, irregular to round patches in the summer. The turf can turn yellow, wither and die.
Zoysia grass infected with powdery mildew (Blumeria graminis), develops powdery patches of white mildew. Powdery mildew can kill new plants in the shade.
Pythium blight (Pythium spp.), also called cottony blight, grease spot, and spot blight, causes slimy, water-soaked sunken patches of matted grass from 6 to 12 inches wide. These develop in wet weather.
Long streaks of pythium blight often appear after the grass is wet. The reddish-brown diseased areas turn light brown when the grass dies. If the air is saturated with moisture, a fluffy, white to purplish gray cobwebby mold appears on the blighted grass.
Red thread (Laetisaria fuciformis) forms ragged, light tan to pinkish patches 2 to 24 inches wide. These irregular to circular patches, which can be confused with dollar spot, may merge to form tan to reddish-brown "scorched" areas. It develops in hot, humid weather in soils that lack nitrogen and weakens the turf, often leading to other diseases.
Powdery spores of rust (Puccinia spp. and Uromyces spp,) cause infected Zoysia to turn reddish-brown to yellow-orange. These spore can rub off. Severe infections can cause the grass to turn yellow and die. The weakened grass is susceptible to drought, winterkill and other diseases.
Slime molds are caused by Mucilago, Didymium, Fuligo, Physarium and other species of fungi. Slimy, watery, white, cream-to-yellow, gray or black masses grow over the turf in irregular to round patches. These patches dry to form black, bluish-gray, grayish-white or yellow powdery growths.