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Corn+Soybean Digest

Sudden Death Syndrome May Be Inevitable

Ask anyone who's seen it. There's no stopping soybean sudden death syndrome (SDS), and no cure, either, unless you move to an area where the disease hasn't yet shown up.

"Even that may not be a long-term cure," says Craig Grau, University of Wisconsin extension plant pathologist.

"We've not seen one case of sudden death in commercial soybeans in Wisconsin," he says. "There has been at least one case in a breeding nursery where soybeans have been planted every year for the past 20 years. This nursery is heavily irrigated and is used to screen soybean varieties for resistance to phytophthora. We also identified the fungus from roots of soybean plants that exhibited a root rot that was not typical of SDS."

Grau's point is that Fusarium solani, the fungus that causes SDS, may already be in some soils in areas not yet affected by the disease.

SDS tends to show up first and most severely in early planted beans, particularly in wet soils. Cool, damp conditions at pod set seem to favor SDS development. And infections are generally worse in fields with soybean cyst nematode (SCN).

However, extension plant pathologists at several universities point out that SDS can be a problem without SCN and not all fields with SCN have developed SDS - yet.

Because Fusarium solani lives in the soil and spreads as soil is moved, spread within infected fields is relatively slow.

If you're not privileged to live in a so-far SDS-free area, your best bet is to manage the disease. So far, there are no proven practices for managing this devastating disease.

Thoroughly cleaning equipment before moving from one field to another helps prevent the spread of other soilborne fungi, so it may help contain SDS, too.

"Waiting for soils to warm before planting and selecting varieties with as much resistance as possible are about the only management practices we have right now," says X.B. Yang, Iowa State University extension plant pathologist.

Wayne Pedersen, a University of Illinois research plant pathologist, says some varieties that show moderate or even greater leaf damage still tend to yield, while others lose yield with only minimal leaf damage.

Since SDS affects younger vegetation at the top of plants first, it often doesn't stop pod and seed formation at the bottoms. So plants can look severely damaged, but still produce an average yield.

Planning for 2000 could be important in limiting the impact of SDS, says Yang.

"We saw the worst infestation ever in 1998. If we assume a two-year rotation, growers will be planting those fields to soybeans again next spring. Our observations suggest that some newer varieties, both conventional and Roundup Ready, have better tolerance for SDS than older varieties."

Yang suggests that growers who have already experienced SDS pay close attention to disease ratings provided by seed companies.

And then what?

"Good question," says Pedersen. "First thing I'd do is improve drainage as much as possible, to get rid of wet spots in the field. These seem to be where the disease shows up first."

Yang notes that at least one chemical fungicide is being considered as a possible seed treatment. But it only protects soybeans in the seedling stage and is not sufficient to prevent later attacks.

Pedersen is looking at a possible biological control, but preliminary testing in the greenhouse suggests it may not help much.

"This product protects crops from some diseases by colonizing the roots and then fighting off further pathogen invasions. In a controlled environment, like a greenhouse, it's very effective at colonizing roots, and is also effective against some types of Fusarium. However, our initial greenhouse tests suggest it may not be effective against Fusarium solani."

Still, Pedersen sees some promise from the biological approach.

"University of Illinois research has shown, in previous years, that there are bacteria that can kill Fusarium solani," he says. "The problem in the past was that these bacteria were not aggressive enough to quickly colonize roots and thus protect a plant.

"If we could combine the fungicidal traits of the bacteria from the Illinois research with the aggressive root colonization traits of the product we're now studying, we'd have an effective biological control product that could be placed in the seed furrow with the seed and give season-long protection," Pedersen theorizes.

Yang points out, though, that developing such a product will require genetic engineering and, even then, could take some time.

And, Grau adds, getting all the governmental approvals could take even longer, given the current problems facing genetically engineered crops. ?

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