FIELD POPULATION-DYNAMICS OF SOYBEAN CYST-NEMATODE ON RESISTANT AND SUSCEPTIBLE SOYBEANS AND THEIR BLENDS

Soybean cyst nematode (Heterodera glycines Ichinohe) (SCN) has become an increasing problem in the upper midwest. In order to develop approp riate practices to manage SCN populations it is important to understan d the population dynamics of SCN under held conditions. Soybean cyst n ematode population dynamics were determined in naturally infested comm ercial soybean [Glycine max (L.) Merr.] fields in southern Minnesota o n a monthly basis during 1991 and 1992 for an adapted susceptible (S) and a SCN Race 3 resistant (R) cultivar and three blends (25% R:75% S; 50% R:50% S; 75% R:25% S). Population dynamics differed between sites and years. In 1991, fields with similar initial cyst densities were c ompared. Densities of second-stage juveniles (J2) peaked in August at one site and gradually declined throughout the growing season at the o ther. Total cyst production was lower at the site where J2 densities d eclined. In 1992, fields with high and low initial egg densities were compared. SCN population densities increased when initial densities of J2 and eggs were low and decreased when initial J2 and egg densities were high. Physiological race of the nematode influenced population dy namics on resistant plants. The cyst index under field conditions for the resistant cultivar was 10 (moderately resistant) against Race 3 an d 32 (moderately susceptible) against Race 1. The blends generally low ered SCN population densities, but not significantly. Final egg densit y and cyst production for the resistant cultivar were significantly lo wer than the susceptible cultivar at each site. The use of SCN resista nt cultivars alone or in a 75:25% mixture with a susceptible cultivar (25%) is recommended as a SCN population management strategy to delay shifts from one race to another thus prolonging the effectiveness of r esistant cultivars.