The Jedi knights of Star Wars weren't the only people fighting a phantom menace last spring.

While fans flocked to theaters to see the movie, Iowa State University researchers Forrest Nutter, a plant pathologist, and John Hill, a plant virologist, were planting test plots to do battle with a more terrestrial threat: soybean mosaic virus.

The disease earns stealth status because it doesn't create visible symptoms on newer soybean varieties. And symptoms such as leaf crinkling that appear on older varieties are usually blamed on drift from 2,4-D or other herbicides.

"It's sort of a Typhoid Mary," Nutter says. "The plants are infected and diseased but you can't see the damage."

While not visible in the field, the results of the damage appear in the combine grain tank and your bank account.

In Iowa, this unseen and therefore ignored virus has shown it can cut yield by as much as 24%.

Stopping that loss is a goal of Iowa State researchers. They're developing a new, more effective strategy that protects yield by delaying mosaic virus infection until after pod set.

"We're changing the strategy from resistance to affecting the rate of the epidemic. We're not trying to annihilate the virus, just cripple it," plant pathologist Nutter points out.

The researchers are using the virus against itself by testing four soybean lines bioengineered to include a protein taken from the mosaic virus. This protein slows the replication and movement of the virus, meaning that the plant can still be infected but takes longer to become infectious.

That's important because the only way the seedborne disease moves from plant to plant is by aphid feeding. The longer it takes a plant to become infectious, the longer it takes aphid feeding to spread the disease.

The researchers expect the transgenic lines will require more than 21 days to develop the virus instead of the normal five days. With a two-week head start, the growing crop can outrun the disease. "If we reduce the rate of seed infection by half, no yield loss occurs. Little yield is lost if infection occurs after pod set," Nutter says.

"We're not looking for near immunity or extreme resistance," Nutter adds.

Such a traditional approach works for awhile, until the pathogens fight back. A gene that defends against one strain of the disease often allows other strains to become numerous enough to overpower the resistance. Already, researchers have identified 13 strains of the soybean mosaic virus.

The field study has been measuring how fast a known strain of the virus spreads from the center of the plot. Every seven to 10 days the researchers have collected leaves from each foot of row and tested for the presence of the virus using a lab technique pioneered by Hill.

Because the study is evaluating the technique and the protein, rather than developing a specific variety, it's unlikely the transgenic types from Maturity Groups III and IV used in the field trials will be released for commercial production.

While the objective of the strategy isn't to obliterate the disease, that could still happen by keeping the seed infection rate low, he adds.

"No one else is looking for resistance like this," he concludes. "Others have been looking only for immunity, and a lot of material that could have been used has been thrown out. This (method) may be better."