The first-ever survey of soybean cyst nematode infestations in Missouri indicates that 63% of the state's soybean fields are infested with the destructive pest.
University of Missouri researchers analyzed 382 random soil samples from 194 fields in nine Missouri regions to test for the presence of the soybean cyst nematode (SCN).
In Northwest Missouri, the largest soybean-growing region with 994,200 acres harvested in 1998, SCN was not observed until the late 1970s. The survey showed SCN infestations in 18% of the random samples.
In Southeast Missouri, close behind with 970,700 acres, SCN was first detected in the mid-1950s. MU researchers found 72% of the samples contained SCN, although the nematode counts were not as high as in some other regions.
"We did find definite differences between the regions, and we expected that," said MU nematologist Pat Donald, one of the principal investigators for the survey. "It doesn't correlate to how long they've been around. The places where they've had it longest are not the places with the highest infestations."
One reason might be the buildup of biological control organisms, she said. Also, producers in Southeast Missouri used to apply nematicides to their fields. Although nematicides are now banned, their use decades ago might account for lower nematode counts today.
In Northeast Missouri, there were 748,000 acres of soybeans harvested and 77% of the random samples infested. Previously, samples from that region submitted by farmers to the MU Nematology Lab had shown only 42% of the fields were infested.
Donald said previous estimates that SCN was present in 60% of Missouri soybean fields were "based on the samples sent into the lab. We didn't know how many fields those samples truly represented." The recent random survey, she said, indicated that a greater percentage of fields are SCN-infested.
"The bottom line is that 14% of producers are not sampling for SCN when they should be checking for infested fields," she said. "Higher levels of sampling are reflected in areas where there have been extensive educational efforts."
The other areas:
--North-Central Missouri harvested 705,900 acres of soybeans, and 63% of the samples were infested.
--In West-Central Missouri, 574,400 acres were harvested with 75% of the samples infested.
--In Central Missouri, 515,800 acres were harvested with 61% of the samples infested.
--In East Central Missouri, 291,500 acres were harvested with 83% of the samples infested.
--In Southwest Missouri, 163,400 acres were harvested with 44% of the samples infested.
--South Central Missouri had 35,600 acres harvested with 50% of the samples infested.
Infestation is determined by counting the nematode eggs in a cup of soil, Donald said. "We recommend they switch to resistant varieties if there are more than 500 eggs per cup of soil." The survey found 60% of the infested fields had at least that level of infestation. Now, MU researchers are running tests on many of those samples to determine the races of the nematodes present in Missouri.
The survey also asked growers whether they planted SCN-resistant varieties.
The responses indicated 82.5% of Missouri soybean acreage is planted with resistant varieties, but Donald suspects that number is too high. "I've been involved in these situations before, where people say they planted resistant varieties; then, when you ask them which variety, it turns out they planted non-resistant varieties."
Iowa State University researchers have found that in SCN-infested fields planted with non-resistant varieties, the yield loss is about one third of the entire crop, she said. In fields with resistant varieties, "there is yield loss, but we have no way of measuring what the yield loss is from SCN. There are too many other variables."
Donald and her colleagues did find that yields start dropping when the egg counts reach the 2,000 to 5,000-per-cup stage. "If they have an egg count of 50,000 per cup or higher, I encourage them not even to grow soybeans."