Considering predictions that Iowa may have a high chance of a severe, statewide drought in 2008, Iowa State University Extension Plant Pathologist Greg Tylka is cautioning farmers and agronomists that they need to be careful with part of ISU's soybean cyst nematode management recommendations. "The part I'm referring to is where we suggest using a SCN-susceptible variety in infested fields after resistant varieties have been grown there a few times," says Tylka.
The soybean cyst nematode can be managed effectively by growing SCN-resistant soybean varieties. There currently are more than 700 SCN-resistant soybean varieties available in maturity groups 1, 2 and 3 for Iowa growers. Information on the varieties is available in ISU Extension publication Pm-1649, "Soybean Cyst Nematode-Resistant Soybean Varieties for Iowa" available online at www.extension.iastate.edu/Publications/PM1649.pdf
"Although not 100% effective at preventing reproduction of the nematode, SCN-resistant soybean varieties usually prevent increases in SCN population densities and can even decrease the nematode's numbers throughout a growing season," explains Tylka. "But since some nematode reproduction occurs on resistant varieties, there is the potential for an SCN population to become 'resistant to the resistance' as resistant bean varieties are repeatedly grown."
Rotating bean varieties with SCN resistance
Soybean varieties that are resistant to SCN possess resistance genes from one of four sources of resistance (which are breeding lines). To reduce the chance of a SCN population being selected that can readily reproduce on resistant varieties, Iowa State University specialists recommend growers use varieties with different sources of resistance in different years.
However, almost all SCN-resistant soybean varieties available for Iowa growers have the PI 88788 source of resistance ("PI" stands for plant introduction). "So rotating soybean varieties with different sources of SCN resistance is difficult, if not impossible," notes Tylka.
There is another tactic that ISU recommends farmers and crop consultants consider using to slow the development of an SCN population that reproduces well on resistant varieties. That tactic is to grow a susceptible (non-resistant) variety periodically in a field after resistant varieties have been grown a few times in that same field.
Be sure SCN population is low enough
Tylka cautions that SCN population densities must be low (1,000 eggs per 100 cc soil or less) before a susceptible variety should be grown in an SCN-infested field. "A good, representative soil sample should be taken from the field prior to deciding to grow the susceptible soybean variety," says Tylka. "You should have the soil sample tested to determine the SCN population density before considering growing a susceptible variety in the field."
SCN causes much greater damage and seems to reproduce at a greater rate in hot, dry growing seasons than in years of adequate or excess rainfall.
"So if a severe drought is anticipated, growers might opt not to grow a SCN-susceptible variety in an SCN-infested field in 2008, even if SCN population densities are low," says Tylka. "Growing SCN-resistant soybeans three or four times in a row in a corn-soybean crop rotation should not result in complete breakdown of the effectiveness of the resistance."
ISU management recommendations for SCN are available online at www.plantpath.iastate.edu/soybeancyst/files/IPM63.pdf