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Field Population Dynamics of Soybean Cyst Nematode on Resistant and Susceptible Soybeans and Their Blends

Dep. of Plant Pathology, Univ. of Minnesota, St. Paul, MN 55108
Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108

^* Corresponding author (orfxx001{at}maroon.tc.umn.edu).

Soybean cyst nematode (Heterodera glycines Ichinohe) (SCN) become an increasing problem in the upper midwest. In order to develop appropriate practices to manage SCN populations it is important to understand the population dynamics of SCN under field conditions. Soybean cyst nematode population dynamics were determined in naturally infested commercial soybean [Glycine max (L.) Merr.] fields in southern Minnesota on 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 compared. Densities of second-stage juveniles (J2) peaked in August at one site and gradually declined throughout the growing season at the other. Total cyst production was lower at the site where J2 densities declined. 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 dynamics on resistant plants. The cyst index under field conditions for the resistant cultivar was 10 (moderately resistant) against Race and 32 (moderately susceptible) against Race 1. The blends generally lowered SCN population densities, but not significantly. Final egg density and cyst production for the resistant cultivar were significantly lower than the susceptible cultivar at each site. The use of SCN resistant 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 resistant cultivars.

Received for publication January 17, 1994.