September 16, 2005

Soybean nematodes out of mind, but not out of sight, in 2005

WEST LAFAYETTE, Ind. - An insidious soybean pest was not on most Indiana farmers' minds when the 2005 crop season began. Unfortunately for some growers, soybean cyst nematodes (SCN) sent an unwelcome reminder of what they are capable of doing.

Symptoms of SCN, a tiny soil-inhabiting worm that feeds on the nutrients within a soybean plant's roots, showed up in more Hoosier crop fields than was expected. Soybean acres in north central and southern Indiana suffered the most extensive damage.

Many farmers were caught off guard, said Jamal Faghihi, Purdue University Extension nematologist. Growers should take steps to reduce the risk of SCN problems next year, he said.

"In the past several years we haven't had a lot of soybean cyst nematode symptoms," Faghihi said. "A lot of farmers kind of forgot about it because resistant varieties were available, and everybody used them. But when I showed the symptoms and the cysts on the roots to some of the growers that I know, they were very surprised to see those cysts. They thought the problem was solved. They were surprised that cyst was back.

"I don't think they realized that soybean cyst nematode was never really gone - it was always there."

SCN infestations vary from year to year, Faghihi said. SCN activity was greater this season, in part, because of the dry, hot summer.

"Symptoms are more pronounced in a dry year," Faghihi said.

Those symptoms include yellow and stunted soybean plants with fibrous root systems, and smaller pods and beans. Because SCN consumes important plant nutrients, soybean crops infested with SCN also are weakened and vulnerable to other plant diseases.

SCN can reduce crop yields by nearly 50 percent. In severe cases, an entire crop can be lost.

Farmers should prepare for the 2006 crop season by checking the soil in their fields for SCN cysts, Faghihi said. Purdue offers a soil testing service, he said.

"They need to take soil samples so they know if they have cysts and how bad the problem is, before they start ordering seeds for next year," Faghihi said. "One additional service that we provide - and I don't think many people know about it - is we collect the varieties that growers are planning to plant next year, expose them to the soil that they send us from their fields and then we tell them if those varieties are going to be resistant."

Farmers can send soil samples for nematode analysis to: Nematology Laboratory, Purdue University, Department of Entomology, 901 W. State St., Smith Hall, West Lafayette, Ind. 47907-2089. Soil analysis is $10 per sample.

For decades farmers have met the SCN threat by planting nematode-resistant soybean varieties. One common source of resistance appears to be losing its effectiveness, Faghihi said.

"I've spoken to several Purdue Extension county educators and they noticed some of the cultivars that used to be resistant are no longer resistant," Faghihi said. "Those cultivars were showing a lot of susceptible symptoms. When we checked the roots we found a lot of cysts on the roots.

"I have set up some experiments to find out if the problem is with the resistant cultivars or the source of resistance in those cultivars. Ninety-seven percent of the resistant cultivars that are present in today's market rely on the same source of resistance - something called PI 88788. That source of resistance is so common that almost all the varieties have that source of resistance. This fall we're exposing every positive sample that we're getting in our laboratory to PI 88788."

Faghihi said he will publish his findings once his research is completed.

SCN females do the most feeding on soybean plants. Juvenile nematode worms use a short, hypodermic needle-like mouthpart to pierce soybean roots and suck out plant nutrients.

The SCN life cycle lasts about one month at a soil temperature of 75 degrees. When the soil reaches around 50 degrees, eggs hatch and release SCN juveniles. The juveniles enter soybean roots, find a feeding site, and stay there through adulthood. As the adult female ages, she fills with eggs, turns yellow and then brown to become the nematode cyst. The cyst protects the hundreds of eggs inside it.

Eggs can remain viable in the soil for many years, even if no soybean plants are present.

"In Indiana we might have two or three generations of soybean cyst nematode per season," Faghihi said. "They'll pump juice out of the plant 24-7. They feed all the time."

For more information, read Purdue Extension publication E-210-W, "Soybean Cyst Nematode," by Faghihi and fellow Purdue nematologist Virginia Ferris or the section on SCN in the Purdue Field Crops Management Manual.

Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu

Sources: Jamal Faghihi, (765) 494-5901, jamal@purdue.edu

Virginia Ferris, (765) 494-4609, vferris@purdue.edu

Ag Communications: (765) 494-2722;
Beth Forbes, forbes@purdue.edu
Agriculture News Page

Related Web sites:
Purdue University Nematology Home Page

Purdue University Field Crops Integrated Pest Management

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