Nematode-resistant soybean  varieties aren't always living up to that billing. And it might be time to think about more use of nematicides along with a sound crop rotation to control the crop-crippling creatures.
If you're counting on nematode-resistant varieties to prevent all nematode damage to your crop, you might be disappointed, says Loren Giesler, University of Nebraska-Lincoln Extension plant pathologist. He sees a stronger future for nematicides on beans, even though they haven't been cost effective for most growers in the past.
Giesler is among numerous plant pathologists, entomologists and other university soybean specialists who lend their talents to the North Central Soybean Research Program (NCSRP) . Among their biggest targets is management of soybean cyst nematode (SCN) .
SCN feeds on soybean plant roots, robbing the plant of nutrients. It also creates root wounds that allow disease fungus to invade the plant. The severity of crop damage and yield loss is dependent on crop rotation and the soybean variety planted.
“It's the most yield-robbing pest in soybeans across the U.S.,” stresses Giesler, who monitors SCN development in the field and the laboratory. “We've seen a gradual increase of SCN over several years and they are difficult to manage.
“New nematicides being developed can be a tool to help manage SCN populations, especially as a tool to help new resistance sources be effective for more years after they are released,” Giesler says.
IT HAS BEEN common for growers to skip nematicide use to control SCN. Plant pathologists often advise against using them. Giesler says they haven't been as effective or as economical as other control methods, like crop rotation and resistant varieties. “In the past nematicides have provided early season protection, but not season-long control,” he says.
“Growers received great early season control and good root development. But after the chemical residual is gone, the root system was great for late-season SCN invasions. Many research studies demonstrated this with susceptible soybean varieties, so we didn't recommend nematicides,” he adds.
Giesler says the idea with the new seed treatments would be to couple them with resistant varieties to reduce the number of nematodes that occur in a season.
But that may change, a prospect that has growers enthused about new SCN and other nematode control measures over the horizon. “There's enough going on with new nematicides being labeled that growers know it will be soon,”says Giesler.
Nematicides that have been on the market for soybeans include Telone II, a fumigant from Dow AgroSciences, and Temik brand 15G Aldicarb Pesticide from Bayer CropScience. (For regional product labeling questions and applications rates, contact your local Extension plant pathologist or consultant.)
Among new products being considered for nematode control in soybeans is Votivo from Bayer CropScience. Expected to be available to growers for the 2011 planting season, Votivo is designed to block nematodes from reaching plant roots, says Paul Hewitt, a Bayer plant pathologist. It would come in combinations with Poncho and other seed treatments.
“Votivo is a biological product, a bacteria,” says Hewitt. “As seed germinates and roots begin to grow, the bacteria surround the roots. When nematodes come looking for things to eat, there are no signs of the roots available because it is surrounded by Votivo.”
Hewitt says it provides protection long enough for the root system to develop well enough to handle nematode pressure. He adds when Votivo is added as a seed treatment, research shows growers can seea 4-8 bu./acre increase in yields.
Syngenta Crop Protection, which recently introduced Avicta Complete seed treatment for corn, is also looking at having the product available for nematode control in soybeans. David Long, Syngenta technical crop manager for seed care, says Avicta Complete is tentatively scheduled to be launched on a limited basis this March or April, “probably in the southern U.S.
“It will be registered for all nematodes except SCN,” he adds, noting that research is still ongoing for SCN and other nematode control for northern growing areas. “We're probably at least two years out(for an SCN label),” says Long.
AVICTA COMPLETE IS a combination of fungicides, insecticides and nematicides, “and research shows a 3-5-bu./acre soybean yield increase over Cruiser Maxx seed treatment,” he says.
However, until improved products and treatments are available, Giesler encourages growers to use rotation, soil testing and other production practices to manage the tiny worm-like critters:
- Soil tests
Just like the need to check soil for fertility, soil tests are needed to determine the magnitude of SCN or other nematode infestations. “A standard soil test of 20-25 cores, 6-8 in. deep should be taken,” says Giesler, noting that fields should be soil sampled at least every four or five years, even if they haven't had nematode pressure in the past. “Mix the samples, then place them in a quart bag and send or take it to your Extension office or other testing source.”
Yield-loss threshold begins at 200 eggs/cup of soil. One mature female cyst can contain between 200 and 500 eggs. At 2,000 eggs/cup of soil, equivalent to about 10 females, most soybean varieties susceptible to SCN will be damaged to the point where it's uneconomical to grow them.
- Plant resistant varieties
There are many varieties with nematode-resistance traits. “A rotation of resistance sources is recommended for SCN-infested fields,” adds Giesler, pointing out that the PI88788 line is the most common resistant trait in soybeans. “As we see new resistant sources being developed, this will be a key area for use of the new seed treatment nematicides.”
He adds, however, that not all varieties with PI88788 control SCN equally well. “In Illinois, for example, 50% of the SCN populations in ‘resistant varieties’ reproduced effectively,” says Giesler. “If the level of reproduction on the resistant variety is over 10% of the reproduction on a susceptible check, that shows the variety is not resistant.”
- Rotate, rotate, rotate
Giesler recommends that a good rotation with non-host crops be used to help control SCN and other nematodes.
“Growers should use a rotation similar to this: Rotate from soybeans to a non-host crop, followed by resistant soybean, followed by a non-host crop the third year, then resistant soybean again the fourth year. Additional years of the non-host crop will reduce the SCN population further,” Giesler says.
According to NCSRP, there are also natural enemies of SCN often found in soils that could suppress SCN populations. Certain fungi, bacteria and predaceous nematodes are known to destroy SCN, but they have been very difficult to develop into commercial products. Nonetheless, progress is being made in this area. NCSRP says to check with local sources for more information on SCN control biological agents as research progresses.