While soybean producers warily eye the arrival of Asian soybean rust in some U.S. fields this spring, pest control specialists want to remind growers to keep their guard up against a more pervasive problem: soybean cyst nematodes (SCN).
“I can't think of an easier thing for growers to do to recover yield loss,” says Greg Tylka, Iowa State University (ISU) plant pathologist. “We're talking an additional $55-60 an acre they could pocket.”
Tylka says 10 bu./acre yield losses to SCN are common and often still escape growers' notice, even though the pest has been a known factor for 20-plus years.
He attributes this to several reasons. Damage by the small parasitic worm occurs in the plant roots, so there are initially no aboveground symptoms to detect. The symptoms are also similar to numerous disease problems, weather related stresses and even compaction, so the problem goes undiagnosed or is misdiagnosed.
For those reasons, many growers don't realize SCN is present until they see yellowing, dying patches of plants contained within a circular patch of ground.
By this time, the pest, which can live in soil up to 10 years, has had several years to get established in that field. The result, Tylka says, is that SCN is the No. 1 disease-causing problem in Iowa and most other soybean growing states. More than 25 states have documented some level of SCN infestation.
“It's present in at least 75% of soybean fields in Iowa, and probably more in Illinois,” he notes.
Soybean grower Keith Bowden suspects the infestation levels in Iowa are even higher. “It wouldn't surprise me if it was more like 90%,” says Bowden of Alden.
Bowden, who farms in Hamilton County with his wife's father and brother, says they took proactive steps starting in the mid-1990s and began testing their fields and using specific seed varieties that could help them prevent and/or minimize the impact of SCN.
“The real shocker for us was that we never found a field that didn't have some presence of nematodes if we looked hard enough,” he recalls. “We've worked really hard to stay on top of it.”
Even so, Bowden says SCN problems coupled with other pest issues and unfavorable prices have brought his family to the point of looking for crop options to soybeans.
“We aren't seeing the yield advances in beans that we believe could be achieved if we didn't have all the pest problems,” he says. “Beans just aren't profitable enough for us anymore. We'll plant 75% corn this year and nearly 100% at some time in the future.”
Growers who do stick with soybeans need to monitor their fields and, depending upon the results, give consideration to soybean varieties that offer resistance.
There are two standard methods to check for SCN. Soil samples taken this spring are a good start. Ideally, soil samples are done in the fall; however, a sample taken once the ground thaws and fields drain somewhat can still point to the pest's presence.
“We're looking for the cysts at this point, which are dead female nematodes filled with eggs, and you can still identify them in the soil in the spring,” Tylka says.
ISU recommends growers limit the number of acres represented in each soil sample to 10-20 acres at most. The fewer acres represented in the sample, Tylka says the more accurate the results will be.
Growers can have their soil samples tested at a private soil fertility lab or forward them to ISU's Plant Disease Clinic. The cost is $15/sample for Iowans and $20 for out-of-state growers. The address is: ISU Plant Disease Clinic, Room 323, Bessey Hall, ISU, Ames, IA 50011. Growers also should be able to have soil samples tested for SCN by their state university plant disease clinic.
A second way for growers to identify the presence of SCN is to carefully check plant roots sometime between July and late August for the presence of the female cyst nematode. High-risk areas for SCN are in those plants near entryways, where equipment moves in and out of the field, low-lying spots, high pH soils and along fencelines.
“Walk through your field. About every 50 paces carefully dig up a plant and check its roots for small white or tan female cysts about the size of a pinhead,” Tylka says.
SCN-resistant soybean varieties also can help growers effectively battle the pest.
“People often ask me at what level you grow an SCN-resistant soybean variety, and I always tell them as soon as they detect it in their fields,” Tylka says. “It's easy to keep low SCN numbers low, but hard to drive high numbers back down.”
However, resistance is a somewhat vague term, Tylka adds, and not all “resistant” varieties offer the same level of protection against SCN or the same yield performance.
“There are an infinite number of nematode types, and they don't fall into neat categories,” he says. Unlike Phytophthora root rot, where a single gene placed in the soybean seed can provide resistance, SCN resistance requires the use of multiple genes.
The best approach to variety selection for resistance and yield performance is for growers to review yield-trial results, says USDA and University of Illinois Nematologist Greg Noel. He encourages growers in Illinois and surrounding states to review the Illinois Varietal Soybean Program results. This program annually addresses resistance and other performance factors in nearly 600 soybean varieties. It is available online at www.vipsoybeans.org.
Tylka oversees a unique SCN-resistant soybean variety testing program at Iowa State University where the agronomic performance of more than 100 SCN-resistant soybean varieties is evaluated every year.
An added aspect of this variety testing program is that SCN-resistant varieties also are assessed for the ability to control SCN soil population densities in field environments where yield data was obtained. The results of the variety evaluations since 1996 can be found at www.isuscnvarietytrials.info.
Noel offers some additional suggestions regarding SCN management. He says if growers are able to keep their SCN numbers low — below 150 eggs per 100 cc or one-half cup of soil — they can try planting a higher-yielding soybean variety that offers a lower level of resistance.
He adds that while nematicides are available, their control of SCN in his field experiments has been inconsistent, and the products are expensive. “I'm reluctant to recommend them because of that,” he says.
Some growers believe that early planting or delayed planting can repress SCN, but Tylka says he has seen no data that support either of those two planting approaches.
“You really can't outsmart the nematode,” he says. “Plant when it's time to plant, and make sure your soil fertility is what it should be so you get a healthy crop established.”