Population response of soybean cyst nematode to long term corn-soybean cropping sequences in Minnesota

Soybean cyst nematode [Heterodera glycines Ichinohe] (SCN) can reduce soybe an [Glycine max (L.) Merr.] yields. Rotating soybean with a nonhost crop us ually reduces SCN populations. Cropping sequence experiments initiated in t he early 1980s at two Minnesota locations were monitored in 1996, 1997, and 1998 for changes in SCN egg densities. Cropping sequences were: (i) 5-yr c onsecutive corn (Zea mays L.) alternated with 5-yr consecutive soybean, (ii ) continuous monoculture of each crop, (iii) annual alternation of two cult ivars within a continuous monoculture of each crop, and (iv) annual rotatio n of each crop. In 1989, SCN was detected in several of the plots at both l ocations. By 1996, all cropping sequences had detectable populations of SCN eggs at both locations, regardless of whether the land had been planted to continuous corn since the early 1980s. Lowest densities of SCN eggs were t ypically found in cropping sequences that involved continuous corn and wher e corn had been planted for the last three or more years, whereas highest l evels of SCN eggs were found in cropping sequences that involved continuous soybean and where soybean had been planted for the last two or more years. These results suggest planting a nonhost to SCN for as long as 5 Sr on inf ested land will not eliminate future problems with this pathogen. In additi on, the results suggest that at least a portion of the crop sequence effect s on yield has a soil microbiological basis involving mechanisms that are s pecific to a location.