URBANA, Ill. — As with weeds developing resistance to whatever herbicide is thrown at them, soybean cyst nematodes also are evolving to overcome resistant varieties.
“Soybean cyst nematode causes the greatest yield loss of any soybean disease or pest in the U.S. It’s basically present wherever soybeans are grown, including every county in Illinois. It’s estimated to be in 80 percent of the soybean fields in Illinois,” said Brian Diers, University of Illinois Department of Crop Sciences professor.
The university’s soybean breeding program is developing varieties that combine high yields with multiple SCN resistant genes to combat the pest.
SCN often is referred to as the silent killer, particularly in soils with high fertility where there can be yield losses with no above-ground symptoms.
“You can have a 25 percent yield loss from SCN and the soybeans look healthy. So, you really need to verify whether or not the nematodes are in the field,” Diers said.
The SCN Coalition, a public/checkoff/private partnership formed to increase the number of farmers who are actively managing SCN, recommends sending soil samples to a lab to determine the infestation levels in a field.
The group also recommends rotating resistant varieties, and rotating to a non-host crop such as corn to manage SCN.
“Also consider using a nematode protectant seed treatment. There are a lot on the market. There have been some mixed results on how well they work, but that’s another option,” he said.
When considering a soybean variety, consider varieties with PI 88788, PI 437654 (Hartwig) or Peking resistance to SCN.
The issue is that over 95 percent of the varieties available only have the PI 88788 resistance.
“The problem is breeders have worked on this source for 20, 30 years but they’ve had a hard time getting high yield with the Hartwig source of resistance relative to 88788 resistance. Nobody wants a variety that yields poorly even if it has great levels of resistance.
“This has been a struggle. People are making progress. We released a variety with Hartwig type resistance and it has comparable yield to what we see with 88788 resistance.”
Soybean breeders have said the reason 437654 has such a high level of resistance is that it’s “agronomically so horrible the nematodes don’t even recognize the roots as soybean roots,” he said.
“That and the fact that 437654 grows viny across the ground. It’s got black little seeds and just terrible to work with. So, it’s been really hard to basically breed the gene out of that and get into high yield.”
With some documented breakdown of SCN resistance in 88788 varieties, it can’t be depended upon as a source forever.
Data from tests of over 1,200 SCN resistant and susceptible experimental lines in over 400 field locations from near the Canadian border in North Dakota to southern Missouri over 11 years were analyzed.
The analysis showed that SCN resistance could provide a yield benefit even in fields that had low SCN pressure, and the advantage increased over higher infestation levels.
“We also found the 88788 resistance and resistance from 437654 gave us about the same yield protection. We didn’t see where at the higher infestation levels this 437654 resistance gave us better yield than 88788,” Diers said.
“It showed that there’s a yield advantage for using resistant varieties even at low SCN pressure. You need to grow SCN resistant varieties if you have SCN in the field, and on average the resistance from 88788 is working pretty well.
“It’s not to say it works in all fields. Some people have fields where they have high SCN populations that can overcome 88788 resistance. In those fields you really need to find varieties that have Peking or 437654 type resistance.”
Researchers at the U of I are utilizing difference sources of the resistant genes, including 88788 together with genes mapped from wild soybeans and another soybean variety, making crosses in the breeding and combined those genes together to develop a population of experimental lines.
“We tested all of those lines for genetic markers so we could figure out what genes each one of those lines had and then we tested the population with a very nasty strain of SCN that basically can overcome most sources of resistance. We looked to see how well those genes are working,” Diers said.
“When we combined all four genes we had the highest level of SCN resistance.”
Tom C. Doran can be reached at 815-780-7894 or email@example.com. Follow him on Twitter at: @AgNews_Doran.