SCN race test to be replaced
Researchers are working toward a time when a sample of soybean cyst nematode (SCN) will reveal — within days — which resistant varieties to plant.
To accomplish that, Terry Niblack, nematologist at the University of Illinois, along with several nematologists, soybean breeders, and geneticists, is setting up a new set of tests to replace the old SCN race system that classifies SCN resistance.
The old race test is conducted on three plant introduction (PI) lines and one variety. The new HG type test checks resistance on seven PI lines, and its offspring, the new SCN type test uses only three lines. The HG type test is more complete than the old SCN race test, while the new SCN type test narrows the test to the three PI lines breeders use to create SCN resistance in most commercially sold soybean varieties.
The seven indicator lines listed in the chart at left are used in the new HG type test. The three highlighted in green are lines checked in the condensed version, the SCN type test.
“The HG type test is an update. We made it more in line with what we know,” Niblack says. “The race test is 30 years old and we know a lot more about the nematode now.”
Greg Tylka, plant pathologist from Iowa State University, agrees. “I've learned that the race system for soybean cyst nematode is an oversimplification of the situation,” he says. “I think the HG type test is an improvement.”
Getting results from all three tests is slow. All are greenhouse bioassays that take 30 days to grow nematodes on soybean plants — that is, if enough nematodes were present in the sample. If they're not, it takes another 30 days to grow more nematodes before a test can even begin.
To combat that long timetable, Niblack has been studying nematodes that will and won't reproduce on three PI lines — Peking, PI88788 and PI437654 — to find out how they differ genetically.
“We can develop those genetic differences into markers we can use to test field populations,” Niblack says. “So that will be sort of equivalent to doing a female index test.” Once the markers are identified, a test will take days rather than months.
“The only farmers who actually need the SCN type test are ones who have been growing resistant varieties and think their yields are not what they should be,” Niblack notes. “If farmers have just found out they have SCN, they should just plant resistant varieties and the next time they grow soybeans, change varieties.”
In either case, Niblack says a farmer should know the varieties' source of resistance. To obtain that she recommends starting with an extension agent or seed dealer.
“It's not always easy to find,” she says. “If it says ‘resistant to race 3’ or ‘resistant to race 3 and 14,’ then its source of resistance is probably 88788. In Illinois that's true for 93% of the varieties available.”
In Tylka's opinion, seed companies should stop advertising soybean varieties as resistant to specific SCN races. “That's not completely true. Companies say a variety is resistant to race 3, and while that is mostly true, there are some race 3 SCN populations that can overcome that resistance.”
Companies also shouldn't advertise them as resistant to specific HG types, he says. “They simply need to tell growers what the source of resistance is.”
For a list of more than 1,200 varieties and their sources of resistance, spanning from maturity groups 0 to VIII, visit www.ag.uiuc.edu/~wardt/cover.htm .
Marion Shier, crop specialist, University of Illinois, compiles the list, which is updated annually. For a hard copy, send $5 to cover printing and postage to: Marion Shier, University of Illinois Extension, 1412 S. Locust St., Pontiac, IL 61764.
For a pdf of Iowa State Univer-sity's annual list of more than 650 varieties in maturity groups I-III, visit www.extension.iastate.edu/Publications/PM1649.pdf. 
The old SCN race and the new HG type tests are conducted in the same way, but on different soybean lines.
A greenhouse bioassay is conducted by taking an SCN sample from a field and infesting plants from a plant introduction (PI) line. The plants are grown for 30 days. The nematodes are then counted to arrive at a female index (FI) number. A low FI means the field's SCN population reproduces poorly on that line. A high number means a high rate of reproduction and that a variety from that line would be a bad choice for your field.
PI lines are used by breeders to cross desirable traits, such as SCN resistance, into varieties that will eventually make it to market. The SCN race test used three lines that had known resistance and one variety that was developed from one of the lines, says Terry Niblack, nematologist at the University of Illinois.
“Thirty years ago, there were only three known sources of resistance, three lines in the germplasm collection that had resistance to SCN — Peking, PI88788 and PI90763,” she says. “Researchers have now identified 118 sources of resistance, but only seven of them have actually been used in breeding programs.”
The HG type test is actually an expansion of the race test. Four new lines were added and the one variety that was previously tested was thrown out. A total of seven PI lines are now tested, Niblack says.
While all of that extra information is very helpful for breeders and researchers, Niblack notes that farmers may not need to know how their nematodes do on all seven lines.
“We're doing what's called an SCN type test in Illinois that includes only three lines from the HG type test lines. It's a bioassay too, but it includes only Peking, PI88788 and PI437654,” she says. “The reason is that those three lines account for 99% of the varieties marketed in Illinois.”
For growers outside of Illinois, those three lines are still going to cover most of the varieties offered, says Marion Shier, University of Illinois crop specialist.
“The vast majority are from PI88788, and most of those remaining are from Peking. There are a few that have both Peking and PI88788,” he says. “Starting last year, I had a handful with PI437654, the source of resistance in Hartwig and Cyst-X. That source is going to increase significantly in varieties offered over the next two or three years.”