Control of Bean Diseases

by W. J. ZAUMEYER

GROWERS who plant two new varieties of pinto beans and use sulfur dust as a control measure have less need to fear the ravages of bean rust caused by the fungus Uromyces phaseoli typica Arth.

The rust, like the cereal rusts, can ruin a crop in a few weeks if the weather is right. It did tremendous damage in 1927 and 1928 in northeastern Colorado. It appeared again in 1938; in 1942, in epidemic proportions, it caused a loss of a million dollars in a county where 50,000 acres of pinto beans are grown and heavy losses also in other parts of Colorado. Severe and costly outbreaks occurred in 1944 and 1945 in fields of Great Northern beans in Wyoming and Montana.

The losses were particularly bad because they came when there was heavy demand for dry beans by the armed services and farmers were expanding their acreages in Colorado, from 421,000 acres to 595,000 acres in 1943; in Wyoming and Montana, from 49,000 and 21,000 acres, respectively, to 124,000 and 66,000 acres. Plant diseases are usually more severe where a concentration of acreage of any crop occurs, especially if environmental conditions favor the development and spread of the causal organisms. Because of the increase in acreage, farmers did not follow crop rotation as carefully as they did in previous years, and the rust was more destructive in fields that had grown beans the previous year than where another crop had been grown.

The two rust-resistant varieties became available to growers in 1946. They were developed by the Department of Agriculture in 8 years of breeding and selection at Beltsville and the Department's Field Station at Greeley, Colo. They are called Pinto No. 5 and Pinto No. 14.

As with the cereal rusts, races or strains of the bean rust occur. Before 1937 little was known about them, but since then scientists in the Department have identified 24 races. Because there are so many races and they show different degrees of infecting ability, the problem of breeding resistant plants is complicated. Before making crosses for the ultimate production of a rust-resistant pinto bean, a variety that resisted as many of these races as possible had to be found. After several years of testing, a white-seeded Kentucky Wonder type that could withstand most of the races of rust was found. In 1937 it was crossed with several pinto varieties. Pinto No. 5 and No. 14 were both derived from a cross between Idaho Pinto, an early, rust-susceptible type, and the resistant white-seeded Kentucky Wonder.

The early generations were tested in greenhouses at Beltsville by inoculation with practically all the rust races. Only the resistant plants were saved. These were grown in Colorado, where selections were made for an ideal pinto type. This process of elimination was continued until 1943, when we conducted a large field test and grew 110 different hybrid lines under commercial conditions. Rust was widespread, and all the commercial varieties of pinto in the test proved to be completely susceptible, but most of the hybrid lines were resistant. Sixteen lines were chosen for commercial tests at six locations in 1944. Rust was again widespread, but the two new sorts were immune.

Besides resisting rust, the two varieties are tolerant to common bean mosaic, a virus disease that causes considerable loss in the present commercial pinto beans. No. 5 and No. 14 are also highly tolerant to a bacterial blight of beans known as halo blight (Pseudomonas phaseolicola Dow.), which is rather widespread in many western areas.