A much faster method of screening soybean lines for brown stem rot (BSR) resistance has been developed by Iowa State University plant pathologists.

The method, conceived by Charlotte Bronson and Ned Crane, is now in final testing by X.B. Yang and Michael Uphoff to determine its reliability compared to the traditional field assay method of screening for the disease.

The conventional method requires plants to be grown outdoors in BSR-infested soil until mature. Then researchers split the stem of each plant to measure the amount of internal browning compared with a susceptible control. The more browning the plant has, the more susceptible it is.

The new method involves planting soybeans in an environmentally controlled growth chamber. Soon after emergence, young seedling-stage plants are infected. Their stems are pricked with a toothpick that has been dipped into a suspension of fungal spores that cause the disease.

Plants are then evaluated by inspecting leaf symptoms - at a much earlier stage than with the old method, explain Yang and Bronson.

Foliar symptoms the scientists check for include dead leaves or leaves that have turned yellow or brown and/or have fallen off, points out Bronson. Those symptoms indicate the plants aren't resistant and have the disease.

"In about a month, you have the results," says Yang. "It's a very short time compared with the traditional method. And you can do a large quantity many times during the year. You're not limited by the season. You can do it in winter.

"In terms of BSR screening," Yang adds, "this is definitely a significant advance. Its results correlate well with those from the much slower field evaluation method. But the method is still pretty time- and space-consuming for mass screening. Nevertheless, relative to the older method, this is a quick and reliable approach to developing BSR resistance in soybeans."

Bronson agrees that "it's definitely a big time saver." But, she says, Iowa State scientists are looking at several approaches to further refine the BSR test to speed it up and do it without a growth chamber.

"We are also testing molecular markers as a way to screen for resistance to BSR," Bronson explains. "We've identified some molecular markers, which are just little DNA sequences that are linked to the genes for resistance in the most common soybean varieties.

"We are now testing those molecular markers to see how well they predict resistance and how expensive it is to use the molecular markers compared to the expense of growth chamber and field assay work."

The bottom line: Public and private breeders and other scientists will be able to use this new and faster method to make certain that the resistance gene for BSR has successfully been introduced into a new line or variety, say Yang and Bronson.