Leaf Diseases of Grasses in the South

Howard W. Johnson.

The more important perennial grasses that provide summer grazing in the Southern States are Bermuda-grass (Cynodon dactylon), carpetgrass (Axonopus affinis), and Dallisgrass (Paspalum dilatatum).

Bermuda-grass is attacked by two species of Helminthosporium. H. giganteum causes a zonate eyespot with tan center and brown margin. H. cynodontis causes a bleaching and withering, particularly of the leaf tips.

The leaves of carpetgrass are sometimes spotted by a species of Helminthosporium, and one species of Curvularia has also been reported from leaf spots on this grass.

Dallisgrass is attacked in some areas by anthracnose (Colletotrichum graminicola). In spring the leaves sometimes are spotted by Stagonospora paspali. Neither has been severe enough to warrant intensive work by plant pathologists.

Johnsongrass (Sorghum halepense) is another perennial warm-season grass that is grazed and cut for hay in some parts of the South. It is quite susceptible to the numerous leaf diseases that attack Sudangrass and frequently suffers severe leaf injury.

Tall fescue (Festuca arundinacea) is Utilized commonly for cool-season grazing in the South. Three leaf diseases of some importance attack it. Leaf scald, caused by Rhizoctonia solani, has caused the most serious losses. It causes large, bleached, tan-colored areas on the leaves, especially noticeable in summer. A dark-brown, diffuse, netlike leaf discoloration (net blotch), caused by Helminthosporium dictyoides, sometimes becomes rather abundant on leaves of tall fescue during the cool months of fall, winter, and spring.

The leaf spot caused by Cercospora festucae has a gray center and a purplish border, which differentiate it from net blotch. Cercospora leaf spot is first evident in spring and becomes more severe as summer progresses. It was first observed on tall fescue in Kentucky in 1944. It became severe on tall fescue at College Station and Temple, Tex., in 1949, and appeared to be responsible for the death of both seedling and mature plants.

TEMPORARY WINTER GRAZING in the South is furnished largely by the early growth of winter cereals (oats, rye, barley, and wheat). The foliar diseases of these grasses are discussed in another section (page 344). Annual ryegrass (Lolium multiflorum) is also utilized throughout the South for winter grazing. Its chief foliar disease is crown rust, caused by Puccinia coronata. It is discussed on page 279.

Sudangrass (Sorghum vulgare var. sudanense) and pearlmillet (Pennisetum glaucum) are the grasses most widely grown for temporary summer grazing in the South.

Sudangrass is attacked by a number of bacterial and fungus diseases, which limit its usefulness. Pearlmillet is less widely grown and so far has been attacked by fewer parasites.

Bacterial stripe (Pseudomonas andropogoni) is probably the most common and destructive bacterial disease of Sudangrass. It occurs also on Johnson-grass and sorghums. It shows as stripes with blunt or jagged ends. The stripes are about one-fourth inch long when young and a foot or more when mature. The color, continuous throughout the lesion, ranges from purplish red to brown or tan, depending on the variety. Abundant bacterial exudate forms over the lesions and dries to form crusts or scales, especially on the lower leaf surface. The scales of exudate are the same color as the stripes.

Bacterial streak (Xanthomonas holcicola) is another widely distributed disease of Sudangrass, Johnsongrass, and sorghums. The young lesions are narrow, water-soaked streaks 1 to 6 inches long. They bear beadlike, light-yellow drops of exudate. Red or brown margins later develop, and irregular blotches of color appear in the streaks, breaking their continuity. The streaks may join to form irregular areas that cover much of the leaf blade. At that stage the exudate has dried to thin, white or cream-colored scales, which distinguish this disease from bacterial stripe, which has a darker exudate.

Bacterial spot (Pseudomonas syringae) is somewhat less common than the other two bacterial diseases, but some years it occurs abundantly on the leaves of Sudangrass, Johnsongrass, pearlmillet, foxtail millet, sorghums, and corn. The spots, circular to elliptical, vary from tiny dots to almost one-half inch circles in later stages. They appear water-soaked at first. Soon they become dry and light-colored in the center and develop a red or brown border. The spots may unite to form large diseased areas, but they do not elongate to form stripes or streaks. Exudate does not form on the lesions.

Leaf blight, caused by the fungus Helminthosporium turcicum, is probably the most destructive disease of Sudan-grass in the Southern States. The disease occurs also on Johnsongrass, sorghums, and corn. The causal fungus is seed-borne and lives also on dead plant material on or in the soil. It may cause seed rot and seedling blight, which make it difficult to obtain a satisfactory stand. When the leaves of older plants are attacked, the fungus causes long, elliptical lesions one-eighth to one-half inch wide and several inches long. The lesions may coalesce and kill large areas of the leaves, so that severely affected plants appear as though they had been blighted by an early frost.

The center of the individual lesions is usually gray to straw-colored. The border of the lesions varies from reddish purple to tan. In warm, humid weather, the lesions are covered with a dark, moldlike growth of fungus spores. Wind and rain scatter the spores and spread the disease.

Two other species of Helminthosporium attack Sudangrass, but the foliar diseases they cause are less damaging than leaf blight. Target spot, caused by the fungus Helminthosporium sorghicola, the more serious, occurs on Johnsongrass and sorghums, as well as Sudangrass. It forms small, round or oval lesions. They consist of alternate light-tan and brown bands of tissues. The tissues have a zonate or "target spot" appearance on varieties such as Tift Sudan, which contain a tan pigment. On common Sudan, which contains a darker pigment, the lesions are purplish black and are less zonate. The disease may become more serious in the South as the acreage planted to Tift Sudan increases, because while it resists leaf blight it appears to be quite susceptible to target spot.

The second species, Helminthosporium rostratum, is primarily a parasite of corn and pearlmillet but can attack Sudan-grass, Johnsongrass, and sorghums. The leaf spots it causes are small and light brown in color when young. Older lesions have straw-colored centers and may coalesce to form larger necrotic areas. On sorghums and Sudangrasses having dark pigment there is some purpling around the lesions.