Built-in Rust Resistance Means Better Beans

Although they work more than two thousand miles apart, close cooperation between two ARS scientists has brought forth an innovative new screening technique. As a result, snap and dry bean cultivars that resist two of the crops' worst leaf diseases could be in sight.

Leaf diseases caused either by the bean rust fungus, Uromyces appendiculatus, or by bean common mosaic virus (BCMV) often reduce crop yields up to 30 percent. In extreme cases, the diseases can wipe out an entire field, says plant pathologist Matt J. Silbernagel. He is in the ARS Vegetable and Forage Crops Production Research Unit in Prosser, Washington.

In addition to fungicide applications and "clean" seed certification programs, bean growers rely on varieties with limited resistance to the two diseases.

For currently, no single commercial bean cultivar offers combined resistance to all races of rust fungus and all strains of BCMV, says plant pathologist J. Rennie Stavely.

Stavely heads a cooperative bean improvement program at ARS' Molecular Plant Pathology Laboratory in Beltsville, Maryland. The program has resulted in the commercial release of 75 snap and dry bean germplasm lines with resistance to all 65 known strains of rust. Now, Stavely says, the goal is to provide breeders with new bean lines offering combined resistance to all strains of rust and all identified races of BCMV—something that hasn't been done before.

A direct benefit of breeding bean cultivars expressing such comprehensive resistance would be reduced fungicide use. Broader resistance could also mean: less insecticide to control insect vectors such as aphids, more profitable production, lower and more stable prices for consumers, and more competitive prices for U.S. beans in international markets.

Last year, the two researchers took steps in that direction by developing and then using a new inoculation technique to screen individual bean plants concurrently for resistance to both diseases.

Specifically, the technique pinpoints plants that carry gene combinations necessary for resistance to all races of rust and strains of BCMV—including a new strain occurring in Idaho, which is America's largest bean seed-exporting state. Left unchecked, the new BCMV strain could also spread to other leading bean-growing states such as North Dakota, Michigan, Nebraska, or Colorado.

Detected about 5 years ago on bean seed imported from Africa, the new strain—and possibly another, from Chile—threatens U.S. bean breeders' use of a dominant I gene that gives beans resistance to established BCMV strains. The new strain from Africa triggers a deadly response called black root in bean plants with the I gene—killing instead of protecting them.

The new inoculation technique identifies plants that carry genes that block the I gene's ability to cause black root. These genes, bc-2-2 and bc-3, complement the I gene's ability to confer resistance to BCMV.

"If one or both of the bc genes are added to plants expressing the I gene, you get resistance to all known BCMV strains," Stavely says. "Now our technique offers a way to screen bean populations for these gene combinations during early stages of plant development."

It involves inoculating plants first with eight key races of the rust fungus, followed by two key strains of BCMV. After 14 days, the researchers check the response, saving healthy plants for their seed. These undergo crossing with other varieties or bean lines in greenhouses, followed by additional testing to further confirm their resistance.

Eventually, resistant lines are test-planted in field plots by cooperating university scientists at various experiment station locations.

Anticipating upcoming tests, Stavely says, "We hope to have some resistant navy bean lines ready for release within the near future." — By Jan Suszkiw, ARS.

USDA-ARS Vegetable and Forage Crop Research Unit , 24106 N. Bunn Road, Prosser, WA 99350-8694; phone (509) 786-9928, fax: (509) 786-9277.

"Built-in Rust Resistance Means Better Beans" was published in the February 1995 issue of Agricultural Research magazine.