Breeding Better Alfalfa

by H. M. TYSDAL

THE BACTERIAL wilt disease of alfalfa forcefully brought to the attention of alfalfa growers the need for improved, disease-resistant varieties of alfalfa. This serious disease, first identified in 1925, is caused by the bacterium Corynebacterium insidiosum. It can be classed with winterkilling and drought as among the most serious enemies of alfalfa. Although it was first noticed in the river valleys of Nebraska and Kansas and in some of the Corn Belt States, it is now found in every major alfalfa-producing State. It kills out stands in 2 to 3 years, and even before killing greatly reduces yields.

The Department of Agriculture and several State agricultural experiment stations undertook the problem of combating bacterial wilt by whatever method seemed feasible. Various control methods were tried, including rotation of crops, soil amendments, and breeding for disease resistance. The trials indicated that the selection of resistant varieties was the only practical method of solving the problem. Because domestic varieties were not resistant, the Department sent explorers over much of the world searching for wilt-resistant alfalfas. Through the efforts of H. L. Westover, resistant strains were found in Turkistan.

By intercrossing, plant breeders combined the resistance in the Turkistan varieties with desired characters of domestic strains. Within the past few years two new varieties have been produced and increased that are resistant to bacterial wilt. They have been named Ranger and Buffalo. In areas where bacterial wilt is present, they maintain a good stand after 3 or 4 or more years, while the old varieties, such as Grimm or Common, have been completely killed.

Ranger, produced by Department plant breeders in cooperation with the Nebraska Agricultural Experiment Station, originates from selection within Turkistan, Cossack, and Ladak. In regions where bacterial wilt is not a factor, Ranger is about equal to Grimm in productivity of forage and seed and cold resistance. One disadvantage, particularly in the Eastern States, is that Ranger is slightly more susceptible to leaf-spot diseases and leafhopper yellowing than Grimm or Hardigan. Ranger, however, is recommended for the northern regions of the United States wherever bacterial wilt is serious. It was released for commercial production in 1940, and now several thousand acres are producing seed.

Buffalo, produced by plant breeders of the Department and the Kansas Agricultural Experiment Station, originates from selections made from an old Kansas Common field. Buffalo compares favorably with Kansas Common in yield and adaptation. It surpasses Kansas Common in resistance to the bacterial wilt disease. Buffalo, like Kansas Common, is best adapted in the general latitude of Kansas, and south and east from that State, but its range of adaptation probably will be farther north than that of Kansas Common because of its greater cold resistance. It is recommended for use anywhere within this range where bacterial wilt is a problem.

Ranger and Buffalo have been tested by many experiment stations. At the Iowa station, for example, Buffalo produced 3.60 and Ranger 3.55 tons to the acre in their third year of production. In the same replicated test, the better standard varieties, such as Grimm and Baltic, produced only 2.5 and 1.5 tons an acre, respectively. Similar results were had in Idaho, Ohio, Minnesota, and elsewhere—in fact, wherever the bacterial wilt disease is present. In areas where the disease is unimportant, Ranger and Buffalo yield about the same as an ordinary variety.

Strains resistant to bacterial wilt are not the only new alfalfas introduced to alfalfa growers during the war years. Two other newcomers were released for commercial production: Atlantic, produced by plant breeders of the New Jersey Agricultural Experiment Station; and Nemastan, an introduction from Turkistan.

Atlantic is a vigorous-growing, high-yielding variety and is adapted to the Eastern States. It originates from selections within many varieties, including Hardigan, Grimm, and Baltic. Although it does not withstand the bacterial wilt disease, it is somewhat more tolerant of that disease than standard varieties such as Hardigan or Grimm.

Nemastan, one of the many introductions brought in by Mr. Westover, has been found by research men in the Utah and Nevada Agricultural Experiment Stations and the Department to resist stem nematode. Efforts were started to increase its seed stocks for use in localities in Nevada and Utah and neighboring States where the stem nematode is a serious limiting factor. It is also resistant to the bacterial wilt disease. Nemastan is not recommended for use anywhere in the Eastern States, however, because of its susceptibility to leaf spots. Progress is being made in improving its seed and forage yield and its resistance to other diseases. It is now serving an extremely useful purpose in areas where alfalfa cannot be grown because of the stem nematode.

Plant breeders can use ordinary selection methods to produce good strains of improved alfalfa, but the best strains with maximum improvement can be produced only by making use of hybrid vigor.