There's a now-you-see-it, now-you-don't aspect to sudden death syndrome (SDS). It all depends on the weather.
In 1998, growers from Mississippi to Minnesota didn't have to look far to find the soybean malady in their fields or nearby. Some SDS outbreaks were even detected in Ontario, Canada.
"The incidence of SDS was greater in Iowa in both 1997 and 1998," says X.B. Yang, Iowa State University extension plant pathologist.
"We found it in 31 counties by 1998, primarily in southeastern and central Iowa. That's 11 new counties. We don't know when or how the fungus spread into these areas, but conditions were right for SDS to express itself in 1998."
It was the same story in Missouri.
"We saw SDS over a wider area than we had seen before," says Laura Sweets, University of Missouri plant pathologist. "The first reports we got were from counties along the Missouri River. But by the middle of the season, we saw SDS in areas of northern Missouri where we hadn't seen it before."
In Illinois, SDS was detected in 91 of the state's 102 counties. The most severe outbreaks were in southern and central Illinois, but the syndrome was found all across the state.
"We looked at nearly 3,000 fields in 1998, and, frankly, we were staggered at how widespread SDS has become," says Oval Myers Jr., plant breeder, Southern Illinois University. "In the worst parts of fields, we had absolutely no yield because of the disease."
Why SDS spread - or at least showed up - over such a wide area in 1998 has some scientists puzzled. The syndrome is caused by a soil-borne microorganism, fusarium solani, a blue-pigmented fungus that attacks soybean roots.
"The organism lives in the soil; 1998 conditions were right to let it show up over a much wider area than before," believes Scott Abney, USDA-ARS plant pathologist at Purdue University.
"We've seen SDS in southwestern Indiana for several years. In 1993, and again in 1998, we detected SDS in central and northwestern Indiana," Abney adds.
He thinks the fungus has probably been in the soil for years. SDS ignites the plant's above-ground parts with unusually wet weather during the reproductive stage. However, it may enter the roots earlier.
"In 1998, conditions were just right - or just wrong - to let the disease manifest itself farther north," adds Yang.
For one thing, short-season soybeans typically have less bred-in tolerance to SDS than varieties developed for areas where the disease has been more prevalent.
Yang also believes changes in soybean germ plasm in recent years may have lowered the SDS tolerance in some beans. He suspects that, in the push for new traits, SDS tolerance may not have been a high priority among some plant breeders.
"There is a wide range of tolerance to SDS among soybean varieties," says Sweets. "I'd talk to seed companies to find those varieties that have tolerance to SDS."
Variety selection is a main defense against the disease, agrees Tom Essary, Asgrow Seeds field supervisor.
"If you had any symptoms of SDS this past year, you may want to look for varieties with good field tolerance," Essary says. "And I'd start looking early for varieties with SDS tolerance. With the widespread outbreaks this year, SDS tolerance will be on the minds of a lot of growers."
There's an all-out push to develop true resistance to the syndrome.
"If I had a field with severe SDS this year, I'd rotate two or more years out of soybeans. That doesn't eliminate the fungus, but rotation to other crops can reduce the volume of the fungus. And I'd use as good-quality seed as possible of a variety with as low susceptibility as I could find," says Essary.
Yang agrees that cropping methods and history are important.
"If you know a field is infested, wait and plant that field last to allow the soil time to warm up and dry out," says Yang. "No-till may not be very helpful in fighting SDS, because the soil doesn't warm up as fast. If you're set up for it, ridge-till may be the best cultural option for combating SDS."
Let soils dry out well before planting where SDS is a problem, agrees Missouri's Sweets. "That's easier said than done most years; in 1998, some fields never did dry out."