New varieties containing Peking resistance to soybean cyst nematode are beginning to filter into the commercial pipeline. (DTN photo by Pamela Smith)
The clock is ticking to find new ways to fight soybean cyst nematode (SCN).
Some 95% of the commercially available SCN-resistant varieties depend on a single source of resistance called PI 88788. While the use of PI 88788 is an agricultural success story, constant use of the single genetic source threatens continued use.
Soybean breeders funded by checkoff dollars from the United Soybean Board and North Central Soybean Research Program have been working to find new genetic sources to thwart the pest, which is estimated to have an economic impact in excess of $1 billion per year in North America. A recent release from the SCN Coalition details the work at www.thescncoalition.com
Public breeders have been working for years to insert other types of genetic resistance into elite soybean varieties. PI 548402 (Peking), PI 90763 and PI 437654 (Hartwig) are the most promising, but they are much more difficult to work with than PI 88788.
"With these unique resistance sources, you start with low-yielding backgrounds," said Brian Diers, University of Illinois plant breeder. "It takes time to breed resistance from these new sources into elite genetic backgrounds."
Diers said that's partly why the PI 88788 source of resistance has been overused. "It's worked really well, and breeders have been successful at incorporating it into high-yielding varieties," he said.
It is taking much longer to get to yield parity with the other sources, Diers noted. Part of the reason for that difficulty can be explained by the fact that PI 88788 involves one major gene, Rhg1. Peking, on the other hand, involves two genes, Rhg1 and Rhg4. It's simply harder for breeders to work with two genes.
"But as you continue cycles of breeding, you are able to incorporate these genes more readily into elite, high-yielding lines. Breeders have been working on this for a long time," Diers said. "Improved breeding technology is another factor. We have better genetic markers to select the genes we need, so we can develop varieties more quickly."
Soybean growers, particularly in the Midwest, are seeing more soybean varieties enter the market with the Peking source of resistance -- offering farmers more opportunities to rotate sources of varietal resistance.
"We have a large amount of evidence showing that this reduces selection pressure on SCN populations to continually adapt," Diers said.
REINTRODUCING PI 437654
Breeders at several universities have also been developing cultivars with the PI 437654 resistance source. The first variety released with this source of resistance was Hartwig, which was released in 1992. Breeders have continued to breed with resistance from this source and are now obtaining good yields.
"Our program has released two high-yielding lines with PI 437654 resistance, which were commercialized by companies through licenses from the university," Diers said. "George Graef, a plant breeder at the University of Nebraska-Lincoln, has recently developed top-yielding lines with PI 437654 resistance."
RESISTANCE STACKING UP
In addition, Diers' team recently released a variety with a three-gene stack that contains two new resistance genes from wild soybean (Glycine soja) that have proven very effective when bred into commercial soybean varieties.
"We combined the two resistance genes from wild soybean with Rhg1 from PI 88788 and have shown that this combination gives greater resistance than Rhg1 alone," he explained.
Diers has also developed a four-gene stack -- the two new resistance genes from wild soybean, stacked with Rhg1 from PI 88788 plus another resistance gene from PI 567516C.
"If you look in the literature, there are many SCN resistance genes that have been mapped," he said. "We worked on the gene from PI 567516C because it can give a greater increase in resistance than most other genes mapped."
ANOTHER TYPE OF ROTATION
Farmers battling aggressive nematode populations should also keep in mind that not all varieties containing PI 88788 are created equal, and rotating the variety you use might help, Diers noted. In other words, simply changing up varieties might provide some help against the pest.
"Varieties derived from PI 88788 resistance do not all have the same level of resistance, and this may be related to the number of copies of the Rhg1 gene. There are normally 10 copies of the Rhg1 gene in varieties with PI 88788 resistance, but some may have fewer copies. With PI 88788, the higher the copy number, the higher the resistance," he said.
Farmers have to rely on seed companies to tell them what type of resistance is in that seed bag. State variety trials can sometimes help sort through that information too.
One of the goals in the National Soybean Nematode Strategic Plan, a joint effort of the United Soybean Board and North Central Soybean Research Program, is to have varieties containing new SCN genetic resistance sources in all regions of the country.