A screening method for phomopsis, developed by University of Arkansas researchers, has helped identify sources of resistance to the fungal disease. The technique has opened other avenues of research to help soybean breeders develop resistant varieties.
Phomopsis is especially prevalent when plants are wet for long periods. It causes such diseases as pod and stem blight, seed decay and stem canker, but the biggest problem from phomopsis is latent seed infection, said Pat Fenn, plant pathologist for the Arkansas Agricultural Experiment Station.
“Soybeans with latent infection appear healthy and process successfully,” Fenn said. “If they're processed for food, there's no problem. But if they are used for seed, you'll often get a low germination rate. Seed will rot in the ground or sprout into low-vigor plants.”
There are no visible symptoms that permit plants to be rated in the field for latent infection. “Plants must be cultured to find out if they're infected,” Fenn said. “The existing method takes an entire season to test an inoculated breeding line or variety for resistance because you have to wait for the seeds to mature.”
With support from the Soybean Promotion Board, Fenn developed a new method that screens seedlings instead of mature plants. Fenn, Ph.D. student Eric Jackson and research technologist Pam Miller cut disks of tissue from the trifoliates — the first three-lobed leaves that grow on the plants — from seedlings inoculated with phomopsis. They cultured them in the lab, compared the results with seed and pod cultures from mature plants and found they were getting fast, reliable information about resistance or susceptibility.
“We can have results in three to four weeks, instead of a whole growing season, and 10 seedlings gives us all the information we need,” Fenn said. “Breeders can eliminate susceptible lines in a greenhouse over winter before planting test plots in the spring.”
Fenn continues to compare the trifoliate screening method with the traditional method, building data to support its reliability. He also has begun using it to move closer to breeding phomopsis resistant cultivars.
“The work has branched off into four areas we're concentrating on now,” he said.
He has been screening plants for sources of phomopsis resistance that can be bred into Arkansas-adapted soybeans. “We've found five sources of germplasm that have low rates of infection,” he said. “We're testing them in the field and in the greenhouse to verify resistance.”
At the same time, he is doing DNA studies to see how many genetic traits control the resistance. This determines heritability, or how easily the resistance can be crossed to adapted breeding lines.
“If resistance is controlled by one or two genes, it shouldn't be too difficult to cross it using conventional breeding techniques,” Fenn said. “If it's controlled by three or more genes, crossing becomes more difficult and time-consuming.”
Fenn also is working with UA soybean breeder Pengyin Chen to begin incorporating phomopsis resistance into high-yielding Arkansas breeding lines. The two plants with resistance are being crossed into two Arkansas varieties and two breeding lines.
They are also working together to find genetic markers that can assist them in screening the crosses to see if the resistance genes are present. “Marker-assisted selection dramatically accelerates the breeding process,” Fenn said.
Breeding phomopsis resistance into soybeans is important, but Fenn said no plant has complete immunity from the disease.
“Even resistant varieties are vulnerable to low levels of infection,” he said. “Over time, the fungus can adapt to and overcome the resistance in the plant. That's why breeders are always working to develop improved varieties with resistance bred from new sources.”
Fenn and other UA researchers are studying natural variation in virulence among populations of phomopsis. “We want to know, what happens if we put resistant varieties out there now? How long will the protection last?”
He said building in resistance from more than one source can help sustain resistance for a longer period. “If we can pyramid in multiple sources of resistance, we can provide a longer life for a resistant variety.”
Fred Miller is science editor for the Arkansas Agricultural Experiment Station. e-mail: firstname.lastname@example.org.