Printer Friendly

Rust-no-more beans; new varieties fend off all 55 known rust-causing fungi.

New varieties fend off all 55 known rust-causing fungi.

Every summer afternoon, tens of thousands of commuters drive past J. Rennie Stavely's bean garden on their way home for supper.

For many of the commuters, supper will feature a dish of this high-protein, high-fiber food--perhaps refried beans in a Tex-Mex medley...snap beans zapped in the microwave...hearty navy bean soup...a tangy cold salad of green and yellow wax beans and kidney beans...or maybe Aunt Molly's secret six-bean chili.

Soon, Aunt Molly's recipe may use beans from new, up-and-coming varieties being tested in Stavely's 2-acre research plot growing near Interstate 495, the freeway girdling the Nation's Capital.

Those new beans, says Stavely, a plant pathologist with the Agricultural Research Service, will stand up to all 55 identified U.S. strains, or races, of the fungus that causes bean rust.

"Rust is among the worst diseases of bean plants," says Stavely, who is with ARS' Microbiology and Plant Pathology Laboratory in Beltsville, Maryland. "In a bad year, it can cost $250 million in losses nationwide. An epidemic can cut yields by 50 to 80 percent or more."

The rust fungus is Uromyces appendiculatus. It robs water and nutrients from leaves and stems, and the plant struggles just to survive.

Rusts, among the most highly evolved fungi in nature, are obligate parasites, meaning they reproduce only on their specific host plant, in this case the bean plant.

"Rust rarely kills a bean plant--that would destroy the fungus' only source of reproductive energy," says Stavely.

The fungus produces five kinds of spores at different stages in its annual cycle. Plants become infected with wind-carried white spores in spring. These produce more white spores as well as rust-colored spores that give the disease its name.

Both spore types germinate and enter a leaf through stomates--openings through which the plant draws water and carbon dioxide. Germ tubes from spores grow between leaf cells and produce suckers that enter the cells to rob nutrients. Rounded brown pustules form on the leaves, which may die. The plant becomes debilitated and produces fewer pods.

Stavely says the risk of rust is worst in humid climates such as the Southeast, mid-Atlantic, and parts of the Midwest. It's usually not a problem in the arid West.

From 1971 to 1991, severe rust epidemics occurred in 10 states and for at least 5 years in Florida, Maryland, New Jersey, Tennessee, and Virginia. In 1991, epidemics struck dry beans in Colorado and snap beans in Florida.

But Stavely and other scientists around the country are rustproofing Phaseolus vulgaris, the common bean plant. That will safeguard this species' uncommon diversity of foods: green snap beans, yellow wax beans, and an assortment of dry beans--navies, pintos, great northerns, and others.

Rustproof beans get their start in thousands of mini-plots--like the 392 at Beltsville, Maryland, in 1991--in a cooperative program of ARS, seven universities, and commercial breeders. Stavely coordinates the program.

There's no real cure for rust, he says. Fungicides can reduce infection, but the most widely used one was recently withdrawn because of environmental concerns.

Now, however, farmers and gardeners can look forward to rust-resistant plants--without resorting to chemicals. That's because nature gave a few P. vulgaris plants--and related species--the genetic know-how for stopping rust from taking hold, or bowing so slightly to it that there's little or no loss in bean yield or quality.

Stavely and colleagues are putting those genes into breeding lines that commercial breeders are turning into marketing varieties. Since 1984, the scientists have released 53 lines of beans resistant to all 55 rust races.

Like Jack's magic beanstalk, the work has sprouted the new beans at a quickening pace: 17 lines released from 1984 to 1988, 36 lines since then.

Bush-type green snap beans for the fresh market make up a dozen of the lines. Another 30 are green snap or yellow wax beans for processing into frozen or canned products. Seven are navy beans, and there are two lines apiece of pinto and great northern beans. This year, at least two more green processing lines and three more of pintos are due out.

Many of the new lines boast more than rust resistance. Seven can resist or tolerate root rot that threatens beans in Florida, according to Robert T. McMillan of the University of Florida. And Matt J. Silbernagel of the ARS Vegetable and Forage Crops Production Research Laboratory, Prosser, Washington, verified that most of the lines resist most North American strains of bean common mosaic virus, the crop's worst virus disease.

Many rust-resistant lines began as primitive beans that USDA plant explorers have collected since the 1940's in Guatemala and other Latin American countries. Collecting and preserving germplasm--the genetic instructions in seeds and plant tissue--now pays big dividends.

"Several years ago," says Stavely, "we found that many strains of wild and cultivated beans resist one to as many as a dozen races. A few resist most of the 55 races. So at first we bred plants that combined resistance borrowed from several of these strains. The final product has resisted all 55."

To breed such beans, the scientists start by crossing a resistant strain with a commercial variety. In a greenhouse, they challenge each of the progeny of these crosses with at least eight races of rust. Then--at least three times--they backcross fully resistant plants to the commercial variety and retest the progeny. Next, in outdoor plots in bean production areas, the program's cooperating scientists test lines whose resistance is uniformly and reliably inherited.

The cooperators release the best lines to commercial and public breeders. These breeders work further with these lines to ensure that they have desirable yield, tenderness, flavor, seed and pod color and size, and other traits important to quality.

To complete the test cycle, many breeders then send these advanced lines back to the bean rust nurseries, where Stavely or other cooperators evaluate their rust resistance and other traits. This final evaluation helps the commercial and public breeders decide whether to release the beans as a new, rust-resistant commercial cultivar.

Since 1980, project technician Gene Frazier has made nearly 3,000 different bean crosses for the program in Beltsville. And the results have made Frazier and Stavely justifiably proud of the rust-free lines in their Beltsville bean plots--even though many other plants look, well, rusty to a casual visitor by late summer. By this time, one or more of five races of the disease take their toll of plants that didn't inherit the special genes.

"We only test five races here," Stavely says, "because they already occur naturally in our area. That way we don't risk introducing a new race. We test the other 50 races in the greenhouse in the winter and at other locations where they're already present."

By September, in several rows surrounding the nursery's hundreds of 3- or 6-foot-long mini-plots, thousands of bean plants lie mostly dead, brown, and leafless. These plants--of strains highly susceptible to rust--serve as hosts for rust spores that summer winds spread over the test plants in the nursery.

Many of the test plants succumb. But others stay green and lush, like a rust-resistant Slenderette--a popular green snap bean--that debuted in supermarket frozen-food cases this winter.

Stavely and Joseph Steinke of Rutgers University released these beans to breeders in 1986. Lynn Kerr of Idaho--and other commercial breeders--liked them.

Kerr, after further work to strengthen the beans' commercial qualities, sent them to Stavely for evaluation in the rust nurseries.

A bean producer--Seabrook Brothers and Sons of Seabrook, New Jersey--decided to try the superrust-resistant Slenderettes on a small scale. Each year, Seabrook processes about 30 million pounds of green snap beans for the frozen-food market. Last fall's Seabrook harvest included about 200 acres of rust-resistant Slenderette grown from seed obtained from Kerr.

Seabrook contracts with about 60 farmers who grow the beans the company processes. Hank Wakai, a contract field man for Seabrook, says the rust-resistant Slenderette acreage was scattered among several New Jersey farms.

Rust epidemics struck the state in 1970-74, 1978-79, and 1983-84, and "that's why we need rust resistance," says Wakai. "Fortunately, it wasn't a problem last summer."

The rust-resistant Slenderette yielded as well as regular Slenderette, he notes. "We'll probably grow a larger acreage of the rust-resistant beans next year, and we may gradually switch over completely," Wakai says.

Stavely and Steinke made Slenderette rust resistant by first crossing it with B-190, a line bred and released in the early 1980's by ARS plant geneticist George Freytag in Puerto Rico.

"B-190 was one of the grand-daddies of rust resistance. It's susceptible to only 8 of the 55 races," says Stavely, "and the beans we're working with now are even better sources of resistance."

If new rust races appear, he's confident that these beans--including ancient wild strains from Latin America--can again be called on to rescue modern-day growers.

PHOTO : Plant pathologist Rennie Stavely (center) and technician Vansie Blount (right) examine a rust-resistant bean variety, while technician Eugene Frazier records their findings. (K-4392-6)

PHOTO : Alternating rows of susceptible and rust-resistant beans. (K-4394-20)

PHOTO : Bean leaf at right is from a plant that can fight off most rust fungi. (K-4392-18)

J. Rennie Stavely is at the USDA-ARS Microbiology and Plant Pathology Laboratory, Room 318, Bldg. 011A, 10300 Baltimore Ave., Beltsville, MD 20705-2350. Phone (301) 504-6600.

To contact other scientists mentioned in this article, write or telephone Jim De Quattro, Bldg. 419, BARC-East, 10300 Baltimore Ave., Beltsville, MD 20705-2350. Phone (301) 504-8648.
COPYRIGHT 1992 U.S. Government Printing Office
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:includes related article on beans
Author:De Quattro, Jim
Publication:Agricultural Research
Date:Feb 1, 1992
Previous Article:Victims no one mourns.
Next Article:Rain, runoff, and underground water.

Terms of use | Copyright © 2017 Farlex, Inc. | Feedback | For webmasters