Effect of crop rotation and cultivar resistance on seed yield and the soybean cyst nematode in full-season and double-cropped soybean

Soybean cyst nematode (Heterodera glycines Ichinohe) can cause considerable damage to soybean [Glycine mar (L.) Merr.], especially in western USA grow ing areas that are newly infested. The objective of this study was to rind cultural practices that reduce cyst nematode effect on soybean in this regi on. Four management systems, one having continuous susceptible soybean, a s econd consisting of a 3-yr rotation with 2 yr of a nonhost crop followed by susceptible soybean, and two 4-yr rotations alternating a susceptible and resistant cultivar with nonhost crops, were investigated at nematode-infest ed and noninfested locations. One 4-yr rotation had full-season soybean whi le the other had double-crop soybean. Data included soybean seed yield, yie ld components, and cyst nematode egg density. Grain yields of full-season, SCN-susceptible 'Stafford' were similar in all rotations at each location. Nonhost crops reduced nematode densities from 4000 eggs 100 cm(-3) in conti nuously cropped SCN-susceptible plots to 1000 and 500 eggs 100 cm(-3) in 3- and 4-yr rotations, respectively. However, nematode numbers rapidly increa sed to more than 5000 eggs 100 cm(-3) by seasons end when susceptible soybe an was grown. The SCN-susceptible cultivar produced 33% less grain and pods than did the SCN-resistant cultivar, Manokin, at the infested location whi le the cultivars yielded similarly at the noninfested site. Full-season and double-cropped soybean reacted similarly to SCN. Crop rotation reduced cys t nematode numbers, but this benefit did not reduce yield losses on subsequ ent SCN-susceptible soybean as numbers rapidly climbed to damaging levels.