Enemies of Grass

SOME DISEASES OF FORAGE GRASSES

J. Lewis Allison

GRASSES, like all the other economic plants, are hosts to many diseases. To illustrate: In the United States, more than 45 diseases attack Kentucky bluegrass, 35 attack timothy, and 30 attack orchard-grass.

The bacteria, fungi, viruses, nematodes, and a few parasitic higher plants are among the pathogens that cause the diseases. The fungi are most to blame, but the others also are responsible for several economically important diseases. Nonparasitic disorders that frequently are mistaken for parasitic diseases are common to some grasses; actually they are caused by hereditary or physiological factors.

The diseases can be classified into three major groups according to the parts of the plant they attack : Root disorders, foliage disorders, and flower and seed disorders.

Root disorders are caused by the soil-inhabiting fungi and nematodes. Foliage disorders (blight, mildews, leaf Spots, rusts, and smuts) are caused by bacteria, fungi, and viruses. Fungi and nematodes cause flower and seed disorders (ergot, head and kernel smuts, and seed galls).

Of these, root disorders are the least conspicuous. but they include several of the economically important pathogens and generally are responsible for the inability of grasses to establish initial seedings or the rapid depletion of already-established stands.

Foliage disorders, the most conspicuous group, include only a relatively small number of pathogens that are economically important. They destroy only the foliage, the part of the plant that is readily regenerated.

Flower and seed disorders have economic importance only when crops are grown for seed. Some of this group are highly destructive and seriously hamper seed production. One ergot, a fungus disease can poison livestock if infected grasses are heavily grazed.

The diseases generally are limited to certain regions. The pathogens are usually specific, attacking only a single grass species. Accordingly, the diseases of economic importance in a given region are those that attack the dominant grass or grasses in the region, although a few pathogens are destructive to several different grass species.

Environmental conditions, especially temperature and moisture, limit the range and destructiveness of the diseases. The two factors limit certain diseases to geographic regions; they may be the reason why a disease becomes economically destructive one season and diminishes to one of minor consequence the following season. Environmental factors are largely responsible for the common incidence of root disorders in the Great Plains region, the especial prevalence of leaf spots in the North Central and Southern States, and for the sporadic occurrence of rusts from year to year.

A few of the diseases occur in even more limited areas; for example, the blind seed disease, especially destructive to certain grasses when grown for seed, is localized in a small area in the Pacific Northwest. Most of them are not readily controlled by methods effective for other plant pathogens.

Sanitary measures, important in checking many diseases, are impractical in checking grass diseases under field conditions. For one thing, waste areas into which grasses spread by natural means are inaccessible for control practices. Crop rotation, important in combating many soil-borne diseases, is not readily applied to grasses. Many economic grasses are perennial and are used in permanent pastures where diseases can run an uninterrupted course. Chemical control of foliage diseases with sprays and dusts is impractical because of the possible poisoning hazard to livestock. Seed treatment for seed-borne diseases is not commonly practiced; many grass seeds are difficult to treat because they are so small or have awns and other parts that interfere with treatment. Sanitary handling and processing of grass seed can control diseases carried with the seed ergot, blind seed, and nematode gall.

The development by means of selection or hybridization of strains and varieties of grasses resistant to the diseases that attack them is the only practical control for most grass diseases.

Such resistance has been demonstrated for many species. Breeding and improvement programs are under way in an effort to incorporate the factors for disease resistance into the germ plasm of improved varieties.

The knowledge that many pathogens causing grass diseases are themselves composed of physiologic races, each distinct from another, complicates improvement programs, but complete knowledge of any plant pathogen aids in assuring more complete success in breeding for disease resistance with any crop plant.

We cannot here consider all the diseases of grasses; rather, I have selected representative pathogens from each of the major groups root disorders (damping-off, seedling blight, root rot, stem canker, sheath spot, brown patch) ; foliage disorders (bacterial blight, powdery mildew, brown spot, leaf blight, anthracnose, rusts, stripe smut) ; and flower and seed disorders (ergot, blind seed, heat smut, grass seed nematode).

Damping-off and Blight

Damping-off, Pythium debaryanum, is caused by a soil-inhabiting fungus that has a wide host range among economic and noneconomic plants. Disease symptoms are a seed rot and decay that occur during seed germination before the seedlings can emerge from the soil.

This fungus causes extensive seed rotting of many spring-seeded grasses in the Northern Great Plains. The disease is favored by wet, cool weather following seeding, which retards seed germination and allows the fungus a longer period to cause rotting. Many strains of the fungus are known to occur and these vary greatly in pathogenicity. Some strains isolated from infected grasses will attack many species of grasses but are mildly parasitic on nongrass crops, while in other cases the reverse is true.

No adequate control measures for this disease are known.

Seedling blight, Pythium arrhenomanes, is caused by a soil-inhabiting fungus and is destructive to many grasses. It has a wide host range among the grasses and cereals. It is not known to attack other crop plants. The fungus is widespread through the Northern Great Plains; in some years it causes almost complete destruction to spring seedings of crested wheatgrass, slender wheatgrass, and smooth brome.

Symptoms are a general blighting and dying of young plants a few weeks after they emerge from the soil. The fungus is favored by wet, cool weather.

Crop rotation, using crops other than grasses and cereals, is an effective control method. Fall planting of grasses is also effective because in fall environmental conditions are unfavorable for the development of the fungus, and after stands are established the fungus is no longer able to attack them.

Rhizoctonia solani

Rhizoctonia solani is a fungus present in most soils, especially acid ones. It has a wide host range among economic and noneconomic plants.

Disease symptoms on the plants it attacks vary greatly. On grasses alone its symptoms are recognized from the common names root rot, stem canker, sheath spot, and brown patch. The disease attacks most grasses after stands are well established; it is most severe in the spring during wet periods.

Brown patch symptoms occur on well-established turf such as golf courses, cemeteries, and lawns where Kentucky bluegrass is dominant.

Crop rotation is ineffective as a control method because the disease attacks so many different plants in one form or another and can thus persist in the soil indefinitely. With one exception, no other control methods are known. Brown patch can be controlled by treating diseased areas of turf with certain fungicides, among them tetra-methyl thiuram disulphide (Thiosan), a mixture containing two parts of mercurous chloride with one part of mercuric chloride (Caloclor), and hydroxymercurichlorophenol plus hydroxymercuricresol ( Special Semesan).

Parasitic races of the fungus are known to occur.

Bacterial Blight

Bacterial blight, Pseudomonas coronafaciens var. atropurpurea, is a destructive disease on smooth bromegrass throughout the North Central States. Initial symptoms appear as circular to oblong water-soaked areas of uniform size on the leaf blades, which turn purplish black and frequently coalesce to form typically blighted areas involving the entire blade and sheath. Blighted leaves wither and die.

The disease is favored by periods of warm, humid weather and is most severe about mid-June.

There are no known control measures for blight.

The second regrowth of smooth brome is rarely infected, because the disease does not develop during hot, dry, midsummer. Other bacterial diseases attack smooth brome but are not so destructive as blight.

Powdery Mildew; Brown Spot

Mildew, Erysiphe graminis, is a fungus disease that attacks many grasses. Among the economic grasses, Kentucky bluegrass is highly susceptible. Mildew produces conspicuous symptoms that appear to be more damaging than they really are. The white powdery growth visible on the surface of leaf blades is the vegetative and sporulating parts of the fungus. Dried blotched areas develop later at the points where the fungus has penetrated into the leaves. Severely infected plants become weakened and retarded in growth.

Mildew is widespread in its occurrence, but it is especially prevalent in the North Central States where bluegrass is a dominant grass.

The disease is seasonal in development, first appearing in the spring, diminishing during the summer, and reappearing again during the fall.

Resistance to mildew is known to occur in plant lines of bluegrass. Several physiologic races of the fungus have been determined.

Brown spot, Pyrenophora (Helminthosporium) bromi, is one of the many fungus leaf spots attacking grasses. It is selected as a representative example because both the asexual and sexual stages of the fungus are known. For most of the leaf-spotting fungi, only the asexual stage is known.