New varieties offer disease resistance

Whether it’s in the Great Lakes country of Michigan or a USDA lab in Maryland, as well as numerous other research sites, scientists are diligently working toward developing better, more disease-resistant bean seeds. Dry beans are among the world’s leading food crops providing nutrients and are popular for both their nutritional value and just plain good taste. Traditional plant breeding plays a major role in the development of disease-resistant varieties, and work is underway at numerous sites across the nation. An exciting new aspect has entered the arena of plant breeding with gene identification.

Photos courtesy of Dr. Marcial Pastor-Corrales.
1. Pinto 114 bean that does not have a gene for rust resistance. 2. Pinto that has been inoculated in the greenhouse with rust pathogen race 67, making it very resistant.
3. Early Gallatin snap bean has a rust resistance gene named Ur-4 and is resistant to rust pathogen race 108 but susceptible to race 67. 4. Early Gallatin snap bean is susceptible to race 67 rust.
5. BelFla-RR01 snap bean has two genes resistant to race 67 rust.

This avenue complements traditional plant selection in breeding plants that carry broad resistance to various disease pathogens. The process is producing results that will help not only individual growers, but also consumers and the national economy. Food supplies will not only be improved through higher yields, but opportunities for growers to export their products can be expanded as new, disease-resistant varieties make their way to grower seed suppliers.

Dry beans include legumes such as pinto beans, black beans, navy beans and red kidney beans as well as the ever-popular great northern beans. About 20 percent of U.S.-produced beans are exported, including otebo beans exported to Asia.

The U.S. is among the top six bean-producing countries, and close to 1.5 million acres of dry beans are harvested annually. While North Dakota, Michigan and Nebraska lead the nation in dry bean production, dry and snap beans are grown almost everywhere. Despite being so widely produced and consumed, beans are extremely vulnerable to a number of diseases. Anthracnose, common mosaic, common blight and common rust continue to be major issues for dry bean growers, and white mold is a significant threat to snap beans. Some of these diseases are caused by pathogens with many different strains, so bean varieties that are resistant in one year or location may be susceptible the next year or in a different location.

A new disease, Asian soybean rust (ASR), threatens both dry and snap beans in the U.S. While freezing temperatures found in Midwestern and Northern states have prevented its spread to those states, Southern states have experienced the spread of ASR since it entered the U.S. in 2004.

Pyramid building

Dr. Marcial Pastor-Corrales, plant pathologist with USDA’s Agricultural Research Service (ARS), has collaborated with other USDA scientists and plant breeders at various state university research programs in developing disease resistant dry and snapbean cultivars. “In our work, we use gene pyramiding,” Pastor-Corrales said. “Gene pyramiding is a long process of combining several disease resistance genes, resulting in new cultivars with broad disease resistance. These bean cultivars are resistant to all known strains of the hypervariable pathogens that cause these diseases.”

Dr. Pastor-Corrales examines a bean plant in the laboratory.

Understanding the role of genes and finding ways to pyramid those genes into new varieties is a way of taking the traditional selection process in plant breeding to a new level. The work of Pastor-Corrales at the Beltsville, Md., ARS Soybean Genomics and Improvement Laboratory and that of other researchers has led to the release of a number of new cultivars in recent years with broad resistance to diseases. The cultivars developed are then crossed with other cultivars and released as varieties by state university or other researchers.

Pastor-Corrales noted, “Gene pyramiding makes it possible to breed varieties with extremely broad resistance to all strains of certain highly variable pathogens.” He noted that while the world gene banks hold about 40,000 bean varieties, very few are produced for consumption. He cited the limited genetic diversity as partly responsible for the vulnerability of beans to pathogens. However, Pastor-Corrales, along with researchers at several universities and other entities, is working to change that with the release of a number of disease-resistant varieties in the past few years.

The process used in developing these disease-resistant varieties involves selecting varieties that contain genes with resistance to specific diseases. As these varieties are crossed with other plants containing resistance to other specific diseases, the resulting cultivars carry complete resistance to the various diseases.

A 2004 release was made of six genetically resistant great northern bean germplasm lines numbered 8 to 13. These lines were designated BelMiNeb-RMR-8 to BelMiNeb-RMR-13.

Pastor-Corrales said, “BelMiNeb-RMR-9 is one of the six great northern beans that we released that are the first great northern beans in the world that combine four genes for rust resistance and two genes for resistance to two viruses known as bean common mosaic virus and bean common mosaic necrosis virus.” The four genes for rust resistance are Ur-3, Ur-4, Ur-6, and Ur-11, and the two for the mosaic viruses are 1 and bc-3.”

This plant in a field setting shows it is very susceptible to rust.
Great northern bean seed BelMiNeb-RMR-9 is one of the six great northern beans that are the first great northern beans in the world that combine four genes for rust resistance and two genes for resistance to bean common mosaic virus and bean common mosaic necrosis virus.

“BelMiNeb-RMR-8 to BelMiNeb-RMR-13 are resistant to all known strains of the bean rust pathogen, which numbers in the hundreds, and all known strains of the bean common mosaic virus and bean common mosaic necrosis virus,” Pastor-Corrales added. “This is how gene pyramiding works. When we inoculate these beans with any strain of the rust pathogen, they are resistant.” A release of five resistant pinto bean lines was designated BelDakMi-RMR and numbered 19 to 23. The letters represent the collaborating laboratory locations such as Beltsville, North Dakota State University, Michigan State University and the University of Nebraska, and RMR denotes rust and mosaic resistance.

Although gene pyramiding is a long process, the same pyramid building techniques that have been used in the development of varieties that are resistant to traditional diseases will be employed to develop varieties resistant to the more recently identified ASR. Pastor-Corrales recently worked with ARS molecular biologist Dr. Reid Frederick and University of Illinois plant pathologists Dr. Monte Miles and Dr. Glen Hartman to identify five dry bean cultivars that are resistant to various strains of ASR. That identification represents a first step in developing ASR-resistant cultivars.

Michigan bean releases benefit growers

With a 100-year history behind it, Michigan State University Bean Breeding Program has contributed a number of bean variety releases to the bean growing community. The program’s first release was the navy bean Robust in 1915, which was crossed with Early Prolific to produce Michelite in the 1930s with higher yields, better quality and resistance to bean common mosaic virus, a significant problem at that time.

Dr. Jim Kelly, plant breeder and program director, said, “We start off with a widely grown variety, and we look at characteristics.” Plants with the desired characteristics are crossed in recurring trials until an optimal variety is achieved. A highly competitive production environment has evolved during the last century. “Yield is critical,” Kelly noted. Because diseases can change with time, durable resistance to diseases is an important consideration.

Kelly cited the Michigan Crop Improvement Association and the Production Research Advisory Board as major contributors to the success of MSU Bean Breeding Program. He emphasized that gathering data from growers’ fields is an essential part of the plant breeding program. A number of varieties have been developed over recent years, providing benefits to the growing community in several locations. Recent developments include the release of black bean, pinto bean and otebo bean cultivars that will be available as certified seed in 2010.

Dr. Jim Kelly inspects dry bean research plots, with new Zorro black beans in foreground.

A black bean named Zorro exhibits both improved yield and quality as well as disease resistance and suitability for direct harvest. It is among the highest-yielding black beans and carries partial resistance to bacterial blight and specific races of rust. The development of Zorro should allow Michigan to again become a seed producer, after most of the seed production has been in Idaho and Washington following a Midwestern epidemic of common bacterial blight in the 1980s. Contributing significantly to growers’ opportunities to become a part of the export community, Kelly has developed a new variety of the otebo bean, named Fuji. “It’s a very popular bean in Japan,” Kelly noted. The otebo bean is a small, white-seeded bean from Japan. It is exported to Japan and to other Asian countries for use primarily in a confectionary paste. The market for this bean began in the early 2000s and appears to be stable, with broader markets for growers. Fuji was developed from a backcross line from the commercial cultivar Hime but carries resistance to common mosaic virus.

Although much of the bean crop is grown in Northern states where cooler temperatures prevail, various types of beans are heavily consumed in the Southern states. Pinto beans have long been a popular item in the South and across the Southwest. The new pinto bean Santa Fe is based on a combination of favorable characteristics for upright architecture, high yield, midseason maturity and dry down. It exhibits suitability for direct harvest, along with improved levels of resistance to white mold and rust, and has acceptable canning quality.

Looking ahead

Pastor-Corrales noted, “In the final analysis, the dry bean is an important food crop. It offers complete protein like meat, includes fiber, iron and folic acid, and has a low glycemic index.”

With worldwide importance as food, dry bean production in the U.S. is a significant economic contributor, and developing new disease-resistant cultivars remains an essential area of research.

Rust is a major disease, and newer strains of diseases are continually surfacing. Controlling disease is essential. Pastor-Corrales reiterated that extensive cooperation is occurring and will continue to occur among researchers at various entities. These entities include not only USDA and state university research programs but also private seed and processing companies.

Nancy Riggs is a freelance writer and frequent contributor. She resides in Mount Zion, Ill.