The first soybean aphids, probably descendents of a stowaway from China, were discovered in Wisconsin in 2000. Since then, the pests have spread across the region, growing at rates one scientist calls “enormous.” In addition to damaging plants, the aphids are thought to spread viruses?including cucumber mosaic virus (CMV) and alfalfa mosaic virus (AMV)?among both soybeans (susceptible only to AMV) and snap beans, which caused $8 million in damage to Wisconsin crops during the 2000-2001 season.
Wisconsin produces 317,000 tons of snap beans, or 38 percent of U.S. production, and soybeans are one of the state?s top three crops. Researchers determined in 2001 that the aphids could cost state soybean farmers more than $50 an acre, and could cut yields by 10 to 15 percent. Aware of the threat to the state?s economy, University of Wisconsin-Madison scientists have been studying the soybean aphid since its presence was detected.
They know that soybeans serve as the primary host for the aphid. Once populations build up on soybeans, the aphids visit snap beans and spread CMV and AMV in the process. It takes time for aphid populations to grow, so snap beans planted early in the summer usually escape harm. Scientists believe that AMV comes from soybeans and other forage legumes, but are not yet clear on the source of CMV; this is one of the questions that investigators hope to answer as work in the College of Agricultural and Life Sciences continues this summer.
Focusing on snap beans, a team of researchers will examine how the virus spreads, whether other viruses are involved, whether any plant germplasm has natural resistance to the viruses, and whether insecticide can effectively control aphid populations. Work is funded by the Midwest Food Processors Association and also by the Wisconsin Potato & Vegetable Growers Association (indirectly through the USDA and the Wisconsin Department of Agriculture, Trade and Consumer Protection).
Tom German, chair of the entomology department, and Arne Thompson will survey snap beans, examine weeds as potential virus sources, and test for viruses other than CMV and AMV. Last summer, German surveyed snap beans from the second crop of the year in five regions across the state. He found that the southern regions fared the worst, with as many as 50 percent of the plants infected with CMV. German believes that weeds around fields may harbor the virus over the winter, making it available for aphids to spread in the spring. If this theory proves true, intensive weed management may be an effective way to combat the problem.
German is also looking at whether some seed may be arriving from commercial seed lots already infected with AMV. Last year he examined seed from four companies, and found that one had an infection rate of 24 percent. He?ll continue this line of investigation this summer. If it proves to be a significant problem, some form of seed certification may be advisable.
Michell Sass, a researcher in James Nienhuis? program in the horticulture department, will study USDA publicly available germplasm to determine if there is a genetic source of resistance to the viruses. Last year she obtained germplasm of 160 lines from the USDA Plant Introduction Phaseolus vulgaris collection. Phaseolus vulgaris includes snap beans (green beans) as well as dry beans (such as kidney, navy and pinto beans). She planted the lines at the College?s Arlington research station and inoculated the plants with CMV. All but 28 individual plants came up enzyme-linked immunosorbent assay (ELISA) positive for both CMV and AMV. She harvested the seed from those plants, increased the seed, and repeated the ELISA testing.
This summer, Sass will continue selecting for CMV and AMV resistance using more Phaseolus vulgaris PI germplasm. She will also continue to test and advance the 28 plants that show possible resistance to CMV and AMV. In many cases, plants from PI germplasm lack the horticultural traits that make them well-suited for use in commercial production. Traits such as indeterminate growth habit, the presence of pod fibers and suture string, non-white seed coat color and late maturity are common because much of the germplasm is from wild ancestors. Eventually, she hopes to use resistant plants as “fathers” to crossbreed with “mothers” from agricultural cultivars, but this is likely at least three years off.
Plant pathologists Craig Grau and Walt Stevenson are screening snap bean cultivars from industry for viral resistance. They?ve examined almost 200 lines so far, and this summer will study 20 or 30 that they consider “cream of the crop” for being less susceptible to CMV and AMV. According to Grau, some cultivars seem to have lower rates of infection when they are planted later in the summer. Grau and Stevenson have published the names of promising strains in the Midwest Food Processors Proceedings. Grau notes that farmers usually contract their services out to processors, and thus do not select what varieties they plant.
Jeff Wyman, an entomologist, will study ways to control aphid populations. Soybean aphids are not every effective at spreading disease, says Wyman, but their sheer numbers?as many as 50 aphids per leaf during peak times?often result in high levels of transmission.
Last year Chris Capozzi, one of Wyman?s graduate students, concluded that insecticides, which take minutes to work, cannot kill the aphids quickly enough to prevent virus transmission, which occurs in seconds. This summer, Wyman?s group will focus on using mineral oils to disrupt transmission. These have been used effectively in the southern United States and Mexico to reduce the spread of similar aphid-borne viruses on crops.
According to German, the CALS research team knows how important their work this summer will be to the state?s vegetable industry. “Wisconsin is the foremost producer of snap beans for processing,” he explains. “The losses in 2001 were devastating.” He hopes that results from this summer?s research may help to prevent such devastating losses in the future.