Soybean cyst nematode (SCN) is a silent partner that amplifies the devastating effects of brown stem rot (BSR) and sudden death syndrome (SDS) on soybean fields.

SCN paired with BSR can even make BSR-resistant varieties susceptible.

When trying to control BSR or SDS, you first need to know if you have SCN, says Greg Tylka, plant pathologist at Iowa State University. The damage SCN does by itself should be enough incentive to find out if you have it. But it's doubly important if you are trying to control one of the other two diseases, he says.

For the past three years, Tylka's checkoff-funded research, with colleagues Charlotte Bronson and Girma Tabor, has focused on BSR/SCN interactions. His greenhouse studies have shown significant increases in BSR infection when SCN is present. “Sometimes we've seen up to triple or quadruple the amount of browning,” he says.

“We're seeing SCN make what was once a very effective control strategy (BSR-resistant varieties) not very effective,” he says.

His greenhouse testing has conclusively shown that SCN increases the symptoms of BSR and the spread of the fungus in almost every tested variety.

“In testing, it didn't matter if the varieties had resistance to brown stem rot or to soybean cyst nematode — they both showed damage. You need to find something with double resistance,” he notes.

Mike Schmidt, plant breeder, agrees. “Genetics are a farmer's best control measure,” he says. Ask your seed dealer or county extension office for the most resistant varieties.

Schmidt has been studying SDS and SCN with nematologist Jason Bond. The two Southern Illinois University researchers have been conducting microplot studies to pick apart the interactions that occur between the two diseases.

“SCN seems to increase the impact of SDS,” says Bond. “You'll have greater damage caused by SDS if SCN is co-infecting the same plant.”

Nematodes feed on the root system of a plant, creating more portals for fungi to get in, he explains.

If you have SDS and SCN, you'd better manage for both, says Schmidt. Unfortunately, there aren't many varieties that have resistance to both. Notable exceptions are available and should be sought out by the grower, he adds. Visit www.planthealth.info/SDS.htm for more information on that research.

“In terms of yield, you need to take care of SCN first,” warns Terry Niblack, University of Illinois nematologist.

If you have BSR or SDS, the fungus that causes each disease is always present in the field. Both are subject to environmental conditions like rainfall and temperature, so they won't affect yield every year, Niblack explains. However, SCN will.

“If a farmer lets nematodes get out of hand and plants are stressed enough, that increases the likelihood that those other diseases will show up,” Niblack says.

While you can have SDS or BSR without having SCN, it's very likely that if you have one, you have SCN, too. “I've never found a field with SDS or BSR that's not also had SCN,” says Niblack.

A 1995-1996 research survey conducted by Iowa State's Tylka and colleague X.B. Yang found high percentages of BSR and SCN in the 1,737 fields they tested in six Midwestern states. (See above table.)

Niblack says that the old rule of thumb that BSR is a Northern problem and SDS is a Southern one is breaking down. Both diseases are becoming more widespread and migrating.