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Jason Jenkins
Senior Information Specialist
573-882-2980
JenkinsJL@missouri.edu
Published: March 2, 2006
Story Source: Bob Heinz (573) 884-9118
false sense of security to producers
COLUMBIA, Mo. - Soybean producers who have unexplained yield losses
shouldn’t dismiss soybean cyst nematode as the possible culprit, even
if they’re planting SCN-resistant varieties, a University of Missouri
researcher said.
"Many farmers think the SCN problem is licked (with resistant
soybeans)," said Bob Heinz, coordinator of the MU Nematology
Laboratory. "I would think it’s licked, too, if I didn’t see the high
samples come in."
During the 2005 season, Heinz, in cooperation with producers and
assistance from MU Extension regional agronomists, collected 122 soil
samples from 47 Missouri counties and tested them for soybean cyst
nematode, a parasitic roundworm that feeds on the roots of soybeans
and can cause up to 30 percent yield loss.
Participating producers also were asked about their use of
SCN-resistant soybean varieties and their perceptions about the impact
of SCN on their operations.
Analysis revealed that while 61 percent of the producers’ samples
contained SCN egg levels that exceeded the economic damage threshold,
62 percent of the producers did not believe they had any yield loss
attributable to SCN.
"We asked how long they had been planting resistant soybean varieties,
and we found that 85 to 92 percent of producers currently plant
SCN-resistant beans," Heinz said. "Producers are losing yield, but
they’re not attributing the loss to SCN because they are growing
resistant beans and don’t think they have a problem."
Heinz explained that just as bacteria can become tolerant of
antibiotics and weeds can become tolerant to herbicides, soybean cyst
nematodes can overcome the defenses of a once-resistant soybean
variety.
"Around 90 percent of all SCN-resistant beans in Missouri derive their
resistance from the same source," he said. "By only having one source
of resistance, you’re just asking for trouble because it could
eventually break down. A farmer thinks he’s growing resistant beans,
but really he’s developing a population of nematodes that may grow
well on his resistant line."
Heinz said scientists are uncertain why resistant lines seem to remain
effective for one producer, while the same resistance is overcome by
nematodes in another producer’s fields.
"It’d be nice to make generalizations for everyone to use, but we
don’t fully understand all the genetic and environmental variables in
the different field populations," he said. "The best action producers
can take is to monitor their fields by sending in a soil sample for an
egg count at least once every three years."
Heinz added that researchers such as MU plant pathologist Melissa
Mitchum are studying how soybean cyst nematodes break resistance and
are seeking new strategies for resistance through biotechnology.
Of those producers participating in the 2005 survey, 64 percent had
never submitted a sample for an SCN egg count test. Only 6 percent of
producers had submitted a sample within the past five years. "If you
have a problem field, a $15 SCN egg count can buy a lot of peace of
mind," Heinz said.
He recommended sampling fields that have slipped in yield or have had
a history of high egg counts. Proper sampling technique will insure an
accurate count.
For those who do find high egg counts in fields where resistant lines
have been grown for years, the lab also offers an HG Type race test
that indicates the race of SCN in the field, Heinz said. Producers
also can include up to two soybean varieties with the test, providing
them with information on how those varieties fare against the SCN
types in their fields and helping them choose a soybean variety with a
different source of resistance.
"Races can shift and decrease the effectiveness of resistant soybeans,
but the process of races shifting and the breakdown of resistance may
vary greatly from one farm to another," he said. "If a field is not
yielding up to par, a farmer’s first step should be to get an SCN egg
count test. If the test reveals a problem, then a HG Type race test
may be needed. However, unless a field has a high egg count, the SCN
race isn’t very important."
More nematology lab information including submission forms, sampling
techniques and fee structures can be found online at
http://soilplantlab.missouri.edu. Submission forms also are available
at local extension offices throughout the state, he said.