Leaf Diseases of Range Grasses

John R. Hardison.

Of the troubles that beset forage grasses, the leaf diseases are the most numerous and often the most conspicuous. Although one or only a few of them may be serious on a specific grass and their individual effect on grasses may be relatively mild, their total damage can be serious.

Leaves killed by disease become weathered, low in food value, and less palatable for livestock. They cut the quantity and quality of hay and pasture. The weakened plants are less likely to withstand drought and severe winters. Even worse is the result in areas of low rainfall. There grasses may have only one chance to make their season's growth. Disease, by robbing plants of normal leaf growth, can impair the carrying capacity of a range or pasture and reduce the yields of seed to a point where self-propagation or artificial reseeding is retarded.

The more prevalent or striking types of leaf diseases are considered here. We discuss successively those of western grasses, southern grasses, and northern grasses.

WHEATGRASSES (species of Agropyron) are prey to at least 70 diseases. More than half are leaf troubles, notably rusts, leaf smuts, leaf blotches and spots, bacterial chocolate spot, scolecotrichum brown stripe, rhynchosporium scald, and powdery mildew.

Powdery mildew is caused by the fungus Erysiphe graminis. It occurs throughout the northern Great Plains and the Pacific Northwest. It usually is more severe in cool, humid, cloudy climates, although it needs little moisture for spore germination. Therefore the disease also survives in drier places and attacks most of the wheat-grasses there, too.

Powdery mildew occurs as white patches on the leaf blades, sheaths, and inflorescences. The fungus is conspicuous, because most of the mycelium and spores are on the surface. Small rootlike organs, haustoria, penetrate the leaf tissue and absorb nutrients at the expense of the plant. Infected leaves turn yellow and brown. The fungus may kill all the leaves in very susceptible grasses. The result is premature dormancy or unthrifty growth of the plant.

New infections start from wind-borne spores. Within a week new infections produce abundant spores, which spread to other plants. Thus a little powdery mildew rapidly can become an epidemic. The amount of loss it causes in yield of forage has not been determined, but comparable infection in barley has caused a reduction of 30 percent in yield of grain.

Chemical dusts and sprays control powdery mildew, but they are impractical except when grass is intensively grown for seed. The development of resistant varieties of many grasses is possible.

Strains of powdery mildew that infect wheatgrasses have been found on barley, wheat, wheatgrass, and wild-rye grasses. Other mildew strains that attack barley occur on quack-grass (Agropyron repens) and a wild-rye grass (Elymus dahuricus). Grasses, therefore, can serve as sources of infection of wheat and barley. Other strains of mildew attack only wheatgrasses and wild-rye grasses. The existence of several different mildew races complicates the problem of breeding mildew-resistant wheatgrasses.

Powdery mildew is also prevalent and sometimes destructive on blue-grasses in the Pacific Northwest and northern Great Plains. A similar problem in breeding for resistance exists there, because a number of strains infect different species of bluegrass and strains within the species. Resistant plants in many species of bluegrass are known to exist, however.

Wheatgrasses in dry regions are often free from serious leaf diseases. Lack of moisture and coarse leaf texture may explain why few leaf diseases are important on those grasses.

MEADOW FOXTAIL (Alopecurusfiratensis) is attacked by at least 15 diseases. The most serious leaf troubles are scolecotrichum brown leaf stripe and rhynchosporium scald.

Meadow foxtail grows throughout the northern Great Plains and Pacific Northwest where enough moisture is available. Growth begins in early spring and is best in cool, moist weather which also favors maximum development of scald.

Scald, caused by Rhynchosporium orthosporum, makes blotches on the leaf blades and sheaths, which at first are water-soaked, ovate to irregular, and scaldlike. The color of the blotches changes from a solid, bluish green to zonated, scalded, and brown zones. Finally the centers become pale. West of the Cascade Mountains in Oregon and Washington the disease develops throughout winter and spring and usually causes much leaf killing. A related fungus, Rhynchosporium secolis, damages barley, wheatgrasses, wild-rye grasses, western brome grasses, and sometimes reed canarygrass in the Northwest and the North Central States.

Upper left: Spots on leaves of smooth brome attacked by the fungus Helminthosporium bromi. Upper right: Orchardgrass infected by the leaf spot fungus Stagonospora maculata. Lower left: Blotching due to a bacterial disease on leaves of smooth brome. Lower right: Leaf infection on smooth brome caused by the scald fungus.

No variety of meadow foxtail is resistant to scald. Crop rotation and attention to sanitation are recommended. Careful spring burning of residues will reduce infection in some areas but would be difficult in regions where the grass grows during the wet winter and spring months.

BLUESTEM, or beardgrass (species of Andropogon) is attacked by a great variety of leaf disorders, including leaf rusts' black choke, anthracnose, cat-tail, leaf spots, and tar spot.

Tar spot, incited by Phyllachora luteornamlata, is of striking appearance. The fungus produces black, sunken, glossy spots on the leaves. Spores produced in organs immersed in these black masses start new infections. Many grasses are attacked by the tar spot diseases caused by similar species of this fungus. Control of tar spot has not been reported. The disease does not kill the grass, and the effect on forage and yields of seed has not been measured.

Septoria leaf spots occur from North Dakota to New Mexico and are occasionally serious. Bluestem grasses generally are subject to more injury from disease in the southern, humid part of their range.

Research to produce disease-resistant varieties of bluestem has been started in Kansas. The Kaw strain of big bluestem, Andropogon gerardi (A. furcatus), released by the Kansas State College in 1950, is relatively free from disease.

GRAMA GRASSES (species of Bouteloua) are subject to 30-odd diseases. In some areas phyllachora tar spot and selenophoma eyespot are important leaf diseases. Leaf rust is often serious. Other leaf disorders include leaf spots, choke, scald, and black ring.

Black ring disease gets its name from the peculiar black organs of a fungus, Balansia sirangulans, which surround grass culms with a tight collar that strangles the stem and leaves above the fungus body. The seed head is often blighted or is unable to form.

Burning could possibly help control it by destroying the fungus bodies outside the plant and thus eliminating spores that otherwise would be disserninated. New spore organs will arise from the mycelium inside the plant, however.