Seed Disorders of Forage Plants

John R. Hardison

Seed diseases of forage plants are relatively few, if we do not count the smuts. The four seed disorders I discuss here are different from the seed-borne diseases, in which the causative agents primarily infect leaves, stems, or roots, but also may attack seeds.

Seed diseases are particularly important when they reduce the supply of seed needed to plant forage crops, lawns, or turf. In two diseases ergot and grass seed nematode disease the grass seeds are replaced by the sclerotic and galls, which are poisonous to animals.

BLIND SEED DISEASE of perennial rye-grass, Lolium perenne, apparently became established in the United States about 1940, although it has been a seed production problem in New Zealand since 1932. Poor germination of seed of domestic perennial ryegrass alarmed growers in Oregon in 1943 and led to positive identification of the disease. But three-fourths of the Oregon crop by then had become infested, and more than one-third of the 1943 seed could not be certified.

The causal fungus, Phialea temulenta, was identified in France on cereal rye in 1892, and was identified on perennial ryegrass in New Zealand in 1942. Since then the pathogen has been recognized on perennial ryegrass in England, Ireland, and Scotland; probably it can be found on the grass wherever climate permits infection.

Heavily diseased crops have many seeds that fail to germinate. Such dead seeds were referred to as blind seeds in New Zealand, and the malady was named blind seed disease.

It is hard to tell infected seeds from healthy ones unless the lemma and palea are removed. Then one can see the shriveled, soft, pasty appearance of diseased seeds. Healthy seeds have hard, plump, purple endosperms.

Diseased seeds reach the soil by pre-harvest shattering of the crop, planting diseased seed, feeding infested seed or screenings, harvesting operations, and, in unharvested areas, by the natural dispersal of seed.

The blind seeds remain dormant during the winter. In spring, when perennial ryegrass flowers, the small-stalked, cup-shaped spore-producing organs (apothecia) arise from the overwintered blind seeds and forcibly discharge the primary spores (ascospores). The ascospores are showered on the ryegrass flowers and infect the developing seeds. Asexual spores are produced in a slimy matrix surrounding infected seeds. These secondary spores can infect other developing seeds when rain and insects spread them from head to head. Badly infected seeds that overwinter at or near the surface of the soil produce spore cups the following spring and repeat the cycle. The entire life cycle is confined to the seed. Infected seeds are not toxic to livestock. The disease is important only when the grass is grown for seed.

To provide a basis for control, each sample of cleaned seed of perennial ryegrass entered for certification in Oregon is examined for disease. Growers then are advised through their county agents of the amount of disease in all fields. Plowing before May 1 is recommended for fields that appear too badly infested to produce a profitable crop of seed. The procedure makes it possible for each seed grower in Oregon to know how much disease is in his field and what to do each year. He can avoid unprofitable crops and use the land for spring grain or leave it fallow to reduce weeds. Thorough burning of the straw and stubble after harvest gives good control for a year.

Disease-free seed is selected after harvest and approved for planting in new seed-production fields. The blind seed fungus dies after 24 months in dry storage; aged seed therefore is also safe to plant. The planting of any seed more than one-half inch deep, with complete soil coverage, prevents emergence of the apothecia.

Much heavily infected seed is spread on the field with the straw during combine harvesting, as such seed is lighter in weight than healthy seed. Because the disease is perpetuated primarily by infected seed that is left on the soil, precautions should be taken during harvest so that light as well as heavy seed is removed from the field.

It is helpful also to destroy infested perennial ryegrass screenings, prevent heading in pastures until after July, plow clean to bury infected seeds deeply, have good soil drainage, and plow all ryegrass on a farm at the same time to prevent spread of disease from old fields to new plantings nearby.

ANTHER MOLD was first discovered in Ladino clover in 1947. A seed blight of white clover had been seen in Scotland before 1928, but the identity of the causal fungus was unclear until Mary Noble, of the University of Edinburgh, named it in 1948. By comparing data with Dr. Noble, we found that the seed blight of white clover in Scotland and the anther mold in Ladino clover in Oregon are different stages of the same disease.

Anther mold is caused by Sclerotinia spermophila, a fungus that can live inside the clover stems and leaves. Infected plants look the same as healthy plants. The disease is evident only in the flowers and seeds. Infected flowers bear gray, fuzzy anthers, on which the fungus produces spores that largely replace the pollen; hence the name anther mold. Bees carry the fungus spores to healthy flowers. Infection of young seeds in the flower is the only known way the fungus can enter the plant.

Infected seeds are shriveled and have a dull-brown or gray-pink color. Healthy seeds are plump and bright yellow or reddish brown. Most infected seeds will not germinate. The fungus therefore appears to be largely self-eliminating, and the disease has not been economically important. If a less lethal strain of the fungus would appear by mutation or other genetic change, however, more diseased seeds could grow and produce infected plants, which could furnish inoculum for infecting healthy seeds. The disease might then rapidly become a serious problem in the production of seed of white and Ladino clover.

The disease seems not to affect the vegetative growth of infected plants. Its dissemination has been retarded by the use of modern seed-cleaning machinery, which removes the lighter infected seed.

GRASS SEED NEMATODE DISEASE, caused by Anguina agrostis, is a main disorder of chewings fescue and Astoria and Seaside bents west of the Cascade Mountains in the Pacific Northwest, where those grasses are grown for seed. A similar nematode infests seeds of buffalograss in the Great Plains. Other related nematode species infest seeds sporadically in species of Calamagrostis, Danthonia, Elymus, Holcus, Sporobolus, and Stipa throughout the United States. I found a few heads of orchard-grass infested with a seed nematode in Oregon in 1947.

The life history of the grass seed nematodes is like that of Anguina tritici, which causes the eelworm disease of wheat and rye in Southeastern States. Only the seed is infested, so that utilization of a grass for lawn and turf purposes is not affected by the presence of the grass seed nematode.

Striking symptoms occur in bent-grass. The seeds are transformed into purple to black galls that may be much longer than the healthy seed. In orchardgrass the seed panicles are greatly malformed. In chewings fescue the symptoms are less easily seen.

Each gall contains many nematodes, which leave the galls after fall rains and migrate to grass leaves. Eventually they work their way between the folded leaves near the growing points. When the grass panicles develop, the microscopic roundworms penetrate the ovaries and stimulate the plant to produce the galls, within which many eggs are laid. Thus the cycle is repeated.

Satisfactory methods have been devised for control of the grass seed nematode in chewings fescue in the Pacific Northwest. Clean seed for planting seed-production fields can be had from dry-land districts or by disinfesting seed with a specific gravity separator, using air flotation. Chewings fescue seed weighing 21 pounds per bushel from such seed-cleaning machinery contained no nematode galls. Burning fescue fields after seed harvest is effective because it destroys many of the nematode galls in the grass stubble or in the straw. Vacuum machines, which were developed to pick up seeds on bare soil, could be helpful in removing the lightweight nematode galls from a grass field if they are adapted to work in the thick stubble. Crop rotations that use crops other than susceptible grasses effectively starve out the grass seed nematode.