Research brings white mold-resistant varieties  a step closer

OSU 5630 bush blue lake variety beans are grown in Willamette Valley, Ore.
Photos courtesy of Dr. James Myers.

Some 93,000 tons of mostly blue lake green beans are harvested annually in the Willamette Valley of Oregon. Oregon is second in the nation’s production of processing green beans, with only Wisconsin producing more. Growers cope not only with fluctuating market prices, as in all vegetable production, but also with white mold, a culprit that can result in entire fields being rejected for market. Oregon State University (OSU) research is helping green bean and dry bean growers move a step closer to avoiding such losses as Dr. James R. Myers leads a green bean breeding research program. OSU research has recently identified three chromosomal regions of the bean genome that bring the development of white mold-resistant varieties a step closer. The same research is benefiting dry bean growers in various parts of the country where white mold is a major concern.

White mold has been an issue for about 30 years in the U.S., but has gradually increased over that time. The very attributes that make Oregon an ideal growing location—mild climate and moist conditions—actually contribute to the survival of the pathogenic fungus that causes white mold. The disease affects beans across the spectrum as well as other crops, and is a significant threat to Oregon’s green bean industry, but integrated research is making progress toward developing beans that carry resistance to white mold.

Wilted bean canopy is typical of white mold infection.

“Progress is incremental, and there is no magic bullet,” said Myers, OSU horticulture professor. Myers came to OSU in 1996 as the Baggett-Frazier professor, in which capacity he works toward developing improved blue lake bush beans and other vegetables adapted to the Pacific Northwest. The endowed professorship is named for researchers William “Tex” Frazier and James Baggett, who developed bush blue lake bean varieties in the 1950s, paving the way to Oregon’s $23 million green bean industry. The beans had originally been grown in the Blue Lake District of Ukiah, Calif., making their way to Oregon in the early 1920s. Baggett and Frazier’s development of the bush blue lake variety eliminated the costly and time-consuming staking and hand-harvesting required in the pole variety. Bush blue lake green beans can be harvested by machine.

Work continues to develop better-tasting green beans that meet industry processing requirements, and much of the work focuses on disease resistance. While Myers works with a number of vegetables, he focuses on green bean production issues and is participating in the National Sclerotinia Initiative addressing white mold issues.

Although the research entails using the most advanced techniques of identifying molecular markers, his students also learn basic field work. Myers said, “I try to make sure that my students are well-versed in plant breeding field techniques. As a consequence, they are in high demand by vegetable seed companies.”

With a special emphasis on white mold issues within the green bean industry, Myers continues to work toward identifying and breeding beans with resistance to white mold. A number of genes carry small effects of resistance, but combined, the genes are important.

“We have now identified three chromosomal regions of the bean genome with resistance,” Myers said. This identification is an important step toward developing bean varieties that carry resistance to white mold.

Whitish appearance of stems and pods is characteristic of white mold.

Emergence of white mold concern

White mold, identified in Europe in the mid-1800s, is a major concern in production of edible dry beans, soybeans, canola and sunflower seeds, as well as in the green bean crops grown in the Willamette Valley. While white mold was identified about 150 years ago, it became problematic in the U.S. only about 30 years ago. White mold was mostly controlled by the use of fungicides, both as preventative and as-needed, until the most effective fungicides used in control were withdrawn from the market following an EPA ban on their use. Currently available fungicides are less effective, are more time-sensitive and require two applications instead of one over the growing season. “Most processors allow only 2 to 3 percent of moldy bean pods or they reject the entire load,” Myers said. If that occurs and growers are unable to market entire fields, grave financial hardships result for growers.

White mold-infected plants grow in field.

Understanding white mold

White mold is caused by the pathogenic fungus Sclerotinia sclerotiorum and infects more than 400 plant species. Crops can be infected by ascospores falling onto adjacent plants from infected plants or the ascospores can be carried long distances; wind can transport debris or contaminated machinery can introduce the disease; or surface or irrigation water can move the disease from one field to another. Additionally, contaminated seed can be the cause of the disease.

Sclerotinia overwinter in the ground. They germinate to produce small, mushroom-like structures called apothecia, which produce the ascospores. The ascospores must land on a part of the plant that can provide a food source, such as a dead blossom or other dead plant part. The pathogen produces oxalic acid and other enzymes that damage green bean and dry bean plants, as well as other crops. Germination usually requires that the Sclerotinia be in wet soil one to two weeks. Studies have not determined how long Sclerotinia can survive in soil or the effect of tillage on its survival. Environmental conditions, though, play a significant role in its survival, and major shifts in temperature have a negative impact.

Looking for resistance

In 2004, USDA established within its Agricultural Research Service (ARS) the National Sclerotinia Research Initiative to address white mold concerns across the spectrum of bean production. Initiative goals include gaining a better understanding of the development and control of white mold, and developing bean varieties with white mold resistance. Research is conducted on a number of crops as a gene with resistance is sought. Myers has crossed blue lake green beans as well as dry beans with the scarlet runner bean, native to Central America and grown ornamentally in the U.S.

Maturing plants of a resistant line that shows no white mold infection.

“We screened all available varieties of the scarlet runner in the USDA plant germplasm collection for white mold resistance,” Myers said. Two particular lines were identified as having white mold resistance and have provided the plants for subsequent crossing and backcrossing with the traditional selection process.

A genetic linkage map of molecular markers has been developed, allowing determination of where on the bean chromosomes factors for resistance reside. This information can also be used to determine which progeny in the population carry the molecular markers for white mold resistance. Myers will now be crossing into different bean varieties with pinto beans representing the first cross.

Close-up photo shows white mold on bean pod.

“Mapping white mold genes is complicated because their individual effects are hard to measure,” Myers said. “The easiest way to map these is to take each molecular marker on the map and split the population into those individuals that have one version of the marker and another group that has the other version of the marker. We then ask the question whether the individuals with one version of the molecular marker are more resistant than the individuals with the other marker. If they are, then we know that a gene for resistance is near that molecular marker. These findings need to be validated by looking in other populations and in different environments. We are now validating what we’ve found,” Myers said. Three field trials are being conducted to look at populations with the most resistance.

Myers’ green bean breeding research is funded by the Oregon Processed Vegetable Commission, and OSU has participated in a project funded by the National Sclerotinia Initiative since 2002. “We’re participating in a second project funded by the Initiative and coordinated by the University of Nebraska to screen potentially resistant breeding lines and varieties at a number of locations around the U.S. annually. We’re conducting field and greenhouse screening in Oregon and beginning to contribute lines from our scarlet runner bean populations.

“It’s a way to validate resistance from different genetic sources and to share germplasm with other bean breeders,” Myers said.

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