SOME DISEASES OF FORAGE LEGUMES

Howard W. Johnson

ALL LEGUMES that we grow for forage purposes are attacked by diseases, some of them serious and widespread.

Because of the nature of forage legumes, direct control measures, such as spraying and dusting, have only limited application. Control must be sought in most cases through the development of disease-resistant varieties by selection or hybridization and the use of cultural practices designed to prevent infection from contaminated seed and soil crop rotation, use of disease-free seed, time of planting, time of harvesting, and plowing under of crop residues.

Of the many diseases, a few can illustrate some of the types and demonstrate the progress that is being made in developing ways to fight them. The types of diseases considered here include bacterial wilts, anthracnoses, fungus and bacterial leaf spots, powdery mildews, rusts, mosaics and other virus diseases, and fungus stem and root rots.

Besides these are the disease-like injury caused on numerous forage legumes by the potato leafhopper (Empoasca fabae) ; the root knot disease caused by a nematode (Heterodera marioni) ; and the diseases caused by mineral deficiencies in the soil. In the latter category boron and potash deficiency are probably of most common occurrence on forage legumes.

A serious threat to alfalfa growing in the United States is the bacterial Wilt disease that kills out stands of susceptible alfalfas in 2 to 4 years. When it was first discovered in 1925, bacterial wilt was most severe in the river valleys of Nebraska and Kansas, but it is now found in considerable abundance eastward to the Atlantic seaboard. It is prevalent and destructive also in the irrigated valleys of the Western States to the Pacific coast. The losses due to the disease include the crops that are destroyed, the costs of seeding, and loss of production from the land until a new crop is established. If growers could maintain stands of alfalfa for even 2 years longer than at present by growing disease-resistant varieties, they would be able to save millions of dollars.

The first evidence of bacterial wilt in an alfalfa field is the presence of dwarfed plants with small, abnormal-shaped leaves of a yellow or pale-green color. Such top growth may wilt during summer droughts and the infected plants gradually weaken and die because water-conducting vessels in the alfalfa roots become plugged with bacterial growth. The wood of such roots is yellow; the color of a healthy root, uniform creamy white. In almost all sections, dwarfed alfalfa plants, with yellow or pale-green foliage and discolored wood in the roots, indicate the presence of bacterial wilt.

Efforts to control the disease by cultural practices have proved largely unsuccessful, and efforts have been directed toward testing alfalfa varieties for resistance and breeding new varieties that contain a large proportion of resistant plants and possess other desirable characters. Tests for varietal resistance have been made by inoculating large populations of seedlings and determining by examination those that remain free from disease at the end of a suitable period of growth in the field or greenhouse.

These comparatively rapid but controlled tests have not measured exactly the percentage of plants in a variety that will withstand wilt over a long period under all conditions, but they have made possible provisional estimates of relative resistance, and have been helpful in forecasting the outcome of long-time field trials.

F. R. Jones, of the Department, reported in 1940 that controlled tests show that Peruvian, Grimm, Hardigan, and the common varieties of alfalfa contain few resistant plants, usually fewer than 1 percent, and that Cossack alfalfa usually has fewer than 10 percent. In Ladak, on the other hand, about a third of the plants are highly resistant; in Hardistan, Orestan, and other more resistant strains from Turkestan importations, about one-half the plants are highly resistant. As a result of these tests and field trials of varieties, Cossack, Ladak, Hardistan, and Orestan have been recommended for use in wilt-infested areas until seed of new, disease-resistant varieties become available.

The introductions from Turkestan have provided the foundation material for a plant-breeding program for control of this destructive disease. Seed of these wilt-resistant plants, which were to prove so valuable to American farmers, had been collected in Turkestan and neighboring countries by plant explorers of the Department and had been grown in plant nurseries in various parts of the United States. The aim of the breeding program has been to combine the bacterial wilt resistance of the Turkestan alfalfa with the good qualities of commercial alfalfas, such as Grimm and Common.

Resistant plants for breeding purposes have been obtained by the same method of inoculation and subsequent examination that was used in the tests for varietal resistance. These selected plants have then been increased by vegetative cuttings and the resulting clonal lines have been inoculated artificially and tested for resistance in field nurseries.

By using that general method of getting wilt-resistant lines, workers in the Department and the Nebraska and Kansas Agricultural Experiment Stations have developed two new varieties of alfalfa, Ranger and Buffalo, which are highly resistant to bacterial wilt and possess good characteristics for growth and seed production. Seed of these varieties will soon be available to alfalfa growers in substantial quantities.

A bacterial wilt disease somewhat similar to the one that attacks alfalfa was reported on annual lespedeza by T. T. Ayers, C. L. Lefebvre, and H. W. Johnson, of the Department, in 1939. The disease is widely distributed in the lespedeza-growing areas of the United States.

The first visible symptom of infection is the appearance of dark, water-soaked spots on the leaflets. The infected leaflets soon become grayish brown, desiccated, and curled. Systemic infection follows rather rapidly in the case of susceptible strains of annual lespedeza, and within a few weeks entire plants wilt and die. Bacteria are abundant in the leaves and stems of infected plants, and it appears that the plugging of the water-conducting tubes is responsible for the general wilting and death.

Because the bacterium causing wilt of annual lespedeza may occur either in or on the seed, prevention of the disease requires primarily the use of disease-free seed. There seem to be no practical control measures that will check its spread once it appears in a field of annual lespedeza.

Greenhouse inoculations have shown that many strains of annual lespedeza are susceptible to bacterial wilt. In the field, the disease has appeared to be most severe on strains of Early Korean lespedeza in Illinois, Iowa, and Missouri. The disease has not yet been observed to affect perennial species of lespedeza in the field, although greenhouse inoculations have shown some species of perennial lespedeza to be susceptible.

Anthracnose of Clover

The two common anthracnose diseases of red clover are caused by the fungi Colletotrichum trifolli and Kabatiella caulivora. The symptoms they produce are similar, and most clover growers would be unable to distinguish them. In early summer, anthracnose occurs as small dark spots on the stems, petioles, and flower stalks. The girdling of these structures causes wilting and browning of the tissues above the girdles and results in breaking or bending over of the petioles and flower stalks to produce characteristic "shepherds' crooks."

Considerable defoliation may occur in the first cutting, but when anthracnose is severe during late summer the tops are sometimes killed completely and the crown and taproot of infected plants may be invaded and rotted. This type of general infection frequently destroys a stand of red clover after it has produced a satisfactory crop of hay and results in loss of the seed crop and a greatly reduced value for green-manure purposes.

Various methods of controlling red clover anthracnose have been suggested, such as crop rotation, seed disinfection, fall planting, and mixture with grasses. Such methods have decidedly limited application, however, and control by means of resistant varieties offers the most promise.

Varieties of red clover resistant to southern anthracnose (Colletotrichum) are known, and the development of strains of red clover resistant to northern anthracnose (Kabatiella) appears possible.

The Tennessee strain of red clover, which was developed at the Tennessee Agricultural Experiment Station during the first decade of the present century by selection of the surviving plants from badly diseased fields, is still highly resistant to southern anthracnose. Other strains of red clover, grown for many years in regions such as Kentucky and Virginia where they are subjected regularly to severe outbreaks of the disease, also possess considerable resistance to Colletotrichum. It appeared that mixing together the seed Of several of these strains might yield an anthracnose-resistant red clover of wider adaptation than any one strain alone would possess.

Using this method of procedure, the variety Cumberland, which resists southern anthracnose and is adapted to the southern part of the red clover belt, has been developed and released to farmers. Similarly, the variety Midland, which has some resistance to northern anthracnose and is adapted to the northern part of the red clover belt, was developed from old Corn Belt strains of red clover.

While the anthracnose caused by Colletotrichum trifolii is most severe on red clover, the disease is common also on alfalfa in the Eastern States. It is found also on sweetclover, crimson clover, sub clover, and bur-clover.

Leaf Spots

The common leaf spot caused by the fungus Pseudopeziza medicaginis is probably the most destructive of the foliage diseases of alfalfa. The disease is present in almost every alfalfa field; it does small damage to foliage under dry conditions and great damage under more humid conditions.

The disease has been regarded in the past as one of the unavoidable evils to which the alfalfa plant is subject, but recently resistant lines have been found in alfalfa breeding nurseries. It seems probable, therefore, that strains of alfalfa resistant to both bacterial wilt and leaf spot will eventually be made available for the humid areas of the United States. This fungus attacks other species of Medicago as well as alfalfa; a closely related but distinct fungus causes a similar leaf spot on clovers, as reported by F. R. Jones in 1919.

Leaf spotting and stem blackening of alfalfa are caused by the fungus Ascochyta imperfecta. The leaf spots are irregular and dark brown; when infection is severe the spots coalesce and the leaves turn yellow and fall. Infection by this fungus is favored by cool, wet weather and considerable defoliation may occur in the spring before the first cutting is made. Blackening of the stems occurs progressively from the base upward and in wet springs in some localities may result in the death of the less vigorous young shoots. Late in the season, in stands left for seed, the blackening may progress up to the racemes and even to the seed pods, according to a report by M. W. Cormack, of the Canadian Department of Agriculture in 1945. Dr. Cormack showed also that the pathogene is seed-borne; he suggests that seed infection occurs directly through the pods of severely diseased plants.