Fighting a virus in plants and animals is like hitting a moving target. As soon as researchers find a solution to control a virus - it mutates. That's why researchers at Iowa State University decided to try a unique strategy to control soybean mosaic virus.

That strategy involved injecting a portion of the pathogen, in this case soybean mosaic virus, into the plant through genetic engineering. Its called pathogen-derived resistance.

"Everybody knows what happens when you make something immune. The pathogen works to overcome the immunity," said John Hill, Iowa State plant pathologist. "If you don't force the virus to mutate - you don't change the target."

The concept has been successfully used in several plants but it's an unusual success for soybeans. Hill said soybeans are one of the most difficult plants to genetically alter.

"This is the first time it's been done with soybeans," Hill said. "It's the first successful genetically engineered disease resistance in soybeans."

The transgenic plants were field-tested for two years at Iowa State research farms. The results indicate that the strategy reduced the damage caused by soybean mosaic virus. The key was finding a resistant plant that would delay the pathogen's growth until the plant approached maturity.

"We decided to look for rate-reducing resistance that allows a little infection in the plant and reduces the damage caused by the pathogen," Hill said. "The transgenic lines slow disease incidence until the plant is more developed."

The transgenic lines also offer other benefits. Hill said the virus causes changes in the seed coat that reduces grain quality at the elevator. "Because the disease is delayed until bloom or post-bloom time it cleans up the seed coat," Hill said.

Soybean mosaic virus is transmitted by 30 different species of aphids including the newly introduced soybean aphid. The soybean aphid came to North America in the last two years and is the only aphid that colonizes on soybeans. The aphid is a problem throughout the Midwest and is a significant problem in Wisconsin, Michigan and northeast Iowa. Hill predicts the aphid will continue to move west.

"When you get wall-to-wall aphids on the back of a soybean leaf it can suck the juice out of it. It's nice to have some resistance to some of the viruses the aphids transmit," Hill said.

Soybean mosaic virus can reduce yields by 8 to 35 percent depending on the cultivar planted and when the virus infects the plant. The virus also reduces seed quality, which can mean a dock in price for soybeans at the elevator. Hill said the transgenic lines will be a tool farmers can use in the future to reduce the damage caused by soybean mosaic virus.

The project was partially funded by the Iowa Soybean Promotion Board. More information about the soybean mosaic virus can be found at www.planthealth.info