A screening method for a fungal seed infection, being developed by the University of Arkansas Division of Agriculture, may provide quicker and more consistent results to help soybean breeders develop resistant varieties.
Phomopsis is a fungus that 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.”
Phomopsis is especially prevalent in areas where rainfall and heavy dews keep soybean plants wet for long periods. “It makes ideal conditions for infection and growth of the fungus,” he said.
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 is an expensive procedure that requires plating the pods and seeds. It takes an entire season to test an inoculated breeding line or variety for resistance because you have to wait for the seeds to mature.”
Throw in a bad year of drought and a whole growing season may be lost from variety screening data.
“I felt there ought to be a way to speed this up,” Fenn said. With support from the Soybean Promotion Board, he began looking for a way to screen seedlings. “It turns out, at the first trifoliates — 10 to 12 days out of the seed — there's an opportunity to run inoculations that are giving us pretty reliable results.”
Fenn, Ph.D. student Eric Jackson and research technologist Pam Miller cut disks of tissue from the trifoliates — the first three-lobed leaves to grow on the plants — and stem segments from seedlings inoculated with phomopsis to culture in the lab. They compare the results with seed and pod cultures from mature plants.
“The procedure used to take leaf tissue samples is non-destructive,” he said. “We can grow these same plants to maturity for comparison.”
He said the leaf disks give more-consistent results than the stem segments. “In our early research, we've found a correlation between infection of the trifoliate leaf tissue and the mature seeds.”
Advantages of screening seedlings will include faster results and the ability to screen varieties in a greenhouse in the winter.
“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 before planting test plots in the spring.
“We're showing that this technique is promising, but we want to improve some of the methodology and learn to control some of the variables,” he said. “We want to show that this test is a reliable predictor of infection that will provide a reliable means of breeding new varieties with phomopsis resistance.”
Fred Miller is science editor for the Arkansas Agricultural Experiment Station. e-mail: [email protected].