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The soybean cyst nematode (SCN) continues to be a serious threat to soybean
production in Iowa and the Midwest, occurring commonly throughout fields in
all soybean-producing areas of the region, February 2007.

There are hundreds of SCN-resistant soybean varieties available to
growers. The Iowa State University (ISU) SCN-resistant Soybean Variety Trial
program evaluates many of these varieties for agronomic performance and for
nematode control at numerous locations throughout Iowa each year. The
results of the 2006 evaluations were released recently.

?In general, the SCN-resistant soybean varieties yielded 1.7 to 17.6 bushels
per acre more than susceptible varieties in the trials in 2006,? said Greg
Tylka, ISU research and extension nematologist who oversees the
SCN-resistant soybean variety evaluations at ISU. ?SCN population densities
increased 2- to 15-fold on susceptible varieties but were kept in check or
even decreased a little with the SCN-resistant varieties. But the results
from our 2006 trials again show that not all SCN-resistant soybean varieties
are the same - in terms of yield and also in terms of nematode control.?

The yield of SCN-resistant soybean varieties in fields without SCN does not
indicate yield in SCN-infested fields, so ISU tests the varieties in
SCN-infested fields. The presence of SCN is confirmed in each plot in all of
the variety trial locations because the nematode can occur in patches and
not uniformly throughout some infested fields. Soil samples also are
collected from each variety trial plot at the end of the season to determine
how well the resistant varieties controlled the nematode throughout the
season. Results from previous years revealed that high-yielding
SCN-resistant soybean varieties do not necessarily keep SCN from increasing
in numbers.

?The yield and nematode control that occur in the field are the result of a
season-long, complicated interaction of the resistance genes in the soybean
varieties with genes in the SCN populations,? states Tylka. ?Preventing
increases in SCN population densities is necessary for continued profitable
soybean production in Iowa because SCN is extremely long-lived in the soil.
Once a field is infested with SCN, it is basically there forever, so we must
manage SCN numbers as well as achieve high yields to ensure profitable
soybean production for future years.?

Tylka also explains that combined data from multiple years and numerous
locations often are used to select soybean varieties that have a high
probability of yielding well in many different environments. ?But SCN is not
an environmental factor, like soil type or weather, that allows the data
from SCN-infested research locations to be averaged with other locations,
particularly non-infested locations,? says Tylka. ?The individual location
data reveal the amount of SCN reproduction that a resistant variety
supports.?

Copies of the 2006 Iowa State University SCN-resistant Soybean Variety Trial
results (ISU Extension Publication IPM 52) can be obtained at ISU Extension
county offices or on the Web at www.isuscntrials.info or
www.extension.iastate.edu/store. New in the 2006 report are color-coded bar
graphs that complement the traditional data tables to illustrate, at a
glance, the yield of the SCN-resistant soybean varieties as well as the
extent of SCN reproduction on the varieties.

?Growers and agronomists are encouraged to look carefully at the data in the
tables, including the statistical data,? says Tylka. But the color graphs
give a first indication of the overall yield and nematode control of the
numerous varieties evaluated at each of the nine variety trial sites. The
ISU SCN-resistant Soybean Variety Trial program is funded, in part, by
soybean checkoff dollars from the Iowa Soybean Association.

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