March 01, 2005

Soybean rotation strategy needs tweaking

by K.C. Jaehnig

CARBONDALE, Ill. -- An old standby farmers use to battle a destructive soybean pest needs some tweaking to perform at its best.

"Twenty years ago, scientists all agreed that rotating two or three varieties that have different types of resistance with a susceptible variety was probably the best strategy for managing soybean cyst nematode," said David A. Lightfoot, a biotechnologist from Southern Illinois University Carbondale's College of Agricultural Sciences.

"Now we say you have to know your resistance alleles (type of the gene), then rotate, rotate, rotate!"

The rotation strategy runs on a three-year cycle, Lightfoot said. Farmers plant varieties resistant to SCN for two years, following with a susceptible variety in the third. The idea was that because some worms remain in the soil after each crop is harvested, those that attacked susceptible varieties would breed with others that attacked the resistant varieties. The resulting offspring wouldn't be as good at attacking the resistant varieties, or so they thought.

Although it is a reasonable tactic — that kind of rotation works well for farmers fighting off cotton boll worms or corn ear worms — soybean cyst nematodes turned out to be … well, a different can of worms.

"The pathologists started to look at (that kind of rotation) and said, ‘Wait a minute — it's not working!'" Lightfoot said.

"That was very surprising to me as a geneticist."

Intrigued, Lightfoot and his research team began taking a closer look at why the strategy failed.

"What we discovered was that what we called ‘susceptibles' actually had resistance genes in them —they weren't fully non-resistant," he said.

"These resistance genes maintained pressure on the worms to get round a particular defense mechanism that's in a lot of the susceptible varieties."

But the susceptible varieties can't take all the blame. The resistant cultivars have their own problem.

"The resistance gene doesn't seem to do what resistance genes normally do — it doesn't dominate," Lightfoot said.

"Instead, it behaves as a recessive or co-dominant gene does. It's not normal."

Although funding and licensing problems are causing some difficulties, Lightfoot hopes to have his team begin working in the not-so-distant future to catalog resistance genes in all 900 cultivars used by Illinois growers.

"Then we could find out if you really could use purifying selection (breed for a weaker worm) with proper rotation," he said. "Right now, we don't know what's in what cultivar (in terms of resistance gene alleles)."

In the meantime, Lightfoot offers this advice to farmers using rotation to deal with SCN.

"Use as many different resistant and susceptible varieties as you can — never go back to the same ones you used before," he said. "At least that way you have a 10 percent chance of hitting the right combination."