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Photo: Snap bean pods showing symptoms of chocolate pod. Link to photo information
New, chocolate pod virus-resistant snap beans could soon be on tap, thanks to genetic sleuthing by Agricultural Research Service scientists. Click the image for more information about it.

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Chocolate Pod: Not So Sweet for Bean Growers

By Jan Suszkiw
August 11, 2009

New, virus-resistant snap beans could soon be on tap, thanks to genetic sleuthing by Agricultural Research Service (ARS) scientists in Prosser, Wash.

The target of their investigation, a strain of the clover yellow vein virus, is the culprit behind chocolate pod, a disease that causes unsightly defects on snap bean pods, ruining their marketability.

Soybean aphids transmit the virus while feeding on bean plants, but spraying insecticide to prevent such feeding isn't always effective or economically feasible. Incorporating genes for resistance into the crop offers a better approach, according ARS plant pathologist Richard Larsen.

Toward that end, he and ARS geneticist Phil Miklas developed a polymerase chain reaction (PCR)-based test for detecting the chocolate pod virus and distinguishing it from other bean pathogens.

They were able to do so by identifying the sequence of amino acids that make up the virus' coat protein, explains Larsen, who, along with Miklas, works in the ARS Vegetable and Forage Crops Research Laboratory at Prosser. The research was published in the journal Plant Disease.

The test, which yields results in less than a day versus weeks by traditional methods, has become a critical screening tool in the search for resistant bean germplasm. Only one snap bean variety out of 63 the researchers screened showed some resistance to chocolate pod.

Fortunately, a gene found in dry edible beans conferred stronger resistance. Even better, the gene "coexists" with another, dubbed bc-3, which confers resistance to other bean pathogens, including bean common mosaic virus and bean yellow mosaic virus.

Larsen and Miklas plan on crossing the resistant dry beans with the susceptible snap beans so that they, too, will reap the benefits of possessing multiple virus-resistance genes. Commercial cultivars developed from such crosses will be especially important for snap bean farmers in Wisconsin, Michigan and other Great Lakes states, where the first outbreak of chocolate pod occurred in 2001.

Read more about this research in the August 2009 issue of Agricultural Research magazine.

ARS is the principal intramural scientific research agency of the U.S. Department of Agriculture.