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Corn+Soybean Digest

Rotation Whips SCN

You can write it in concrete. To whip soybean cyst nematode (SCN) into submission, you need to rotate crops and soybean varieties.

That means starting with a non-host crop such as corn once soil tests have shown you have cyst nematodes, then following with an SCN-resistant variety. Then go back to a non-host crop, followed by a different resistant variety.

Some experts even recommend working in two consecutive years of non-host crops, if practical. Then, when nematode egg counts get low enough, throw in a susceptible variety for one year, just to keep the destructive buggers off balance.

Most nematologists think it's best to rotate soybean varieties with different sources of resistance, though choices are still limited in that area.

Some think it's also important to know the SCN race or races that you have in your fields before selecting the source of resistance to combat those races. That would involve growers paying for race testing.

"I think there is quite good agreement among most nematologists that rotating soybean varieties and sources of resistance is good management," says Greg Tylka, Iowa State University nematologist.

However, only four sources of resistance are available in today's commercial varieties, with three more sources still in the breeding pipeline. The four sources commercially available are PI88788 (Fayette), Peking, PI209332 and PI437654 (Hartwig).

Consider these examples. In Iowa, Tylka estimates that 98% of resistant varieties utilize the PI88788 resistance. Only three have Peking resistance. About 95% of the varieties in Indiana also have PI88788 as their resistance source, says Purdue University nematologist John Ferris.

It's an acceptable source of resistance. But in the fairly near future, growers in that latitude will have varieties with an excellent resistance source - PI437654 (Hartwig). It has worked well in the South but hasn't offered agronomic characteristics that work well for the Corn Belt.

After screening 220 lines, Purdue scientists found one that had this resistance in a Williams variety background. It has been released to soybean breeders, is being increased and should be available to growers in about two or three years. It will be sold under the CystX trade name.

"I think that coming down the pike there are going to be some very good varieties with very high cyst nematode resistance," notes Ferris.

Greg Noel, USDA nematologist at the University of Illinois, has recently completed an 11-year study in cooperation with U of I nematologist Dale Edwards. It showed cyst nematodes can be reduced to undetectable levels by rotating sources of resistance. They started with soybeans with the Peking source of resistance.

"By the fourth and fifth years, the plots began to show a marked increase in nematode populations," Noel says. "At that point, we changed to the variety Fayette, with a different source of resistance. We carefully measured the number of cysts in the soil each year. After five years of planting Fayette, the SCN populations were reduced to non-detectable levels."

That means it's important for growers to know what races of nematodes are present in their fields, Noel says. That involves race testing, which presently is time-consuming and expensive.

That's partly why many nematologists, including Tylka at Iowa State and Ferris at Purdue, don't recommend that growers pursue race testing. In fact, Iowa State no longer offers the service, and though it's still available at Purdue, Ferris strongly discourages it.

Tylka says: "In my experience, any SCN resistance, coming from Peking or PI88788, will have some suppressive effect on virtually any race, even if it doesn't meet the breeder's definition for resistance that says it suppresses 90% of reproduction. So, in general, I tell Iowa farmers that they don't need to know what race they have in order to utilize genetic resistance and manage it properly."

Ferris doesn't believe growers should even worry about trying to rotate sources of resistance at this point.

"We found in our research that when a resistant variety breaks down, it's usually the first time it's planted in the infested field," he explains. "So that population in the field was capable of taking down the variety right from the very beginning.

"When you rotate to soybeans, our recommendation is to select a resistant variety, plant it, and if it doesn't do well, shift to another one the next time. And don't worry about the source of resistance - at least for now until we get some of these new resistant varieties."

University of Illinois nematologists collaborated with U of I soybean breeder Cecil Nickell to release five varieties and germplasm lines that are resistant to races that attack both Peking and Fayette.

"One of our newest releases, named Ina, incorporates the SCN resistance from Hartwig, which was developed at the University of Missouri, into a high-yielding variety from maturity Group IV," Noel says. "This is especially well-adapted for use in southern Illinois and is resistant to virtually all the SCN races found in the state."

For the most complete list of SCN-resistant varieties available, check out Marion Shier's Soybean Varieties With Soybean Cyst Nematode Resistance, an 18-page publication.

The pamphlet covers Maturity Groups I through VIII. Send a $2 check to: Livingston County Extension Office, 1412 S. Locust St., Pontiac, IL 61764. Make checks payable to the Livingston County Extension Office. You can also check out the listing on the Internet at: ~wardt/cover.htm.

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