Changes in the Heterodera glycines Female Index as Affected by Ten-year Cropping Sequences.

The objective of this experiment was to measure the change in female index (FI) of Heterodera glycines from bioassays on Bedford, Peking, PI 89772, and PI 90763 soybean (Glycine max) for 12 cropping sequence treatments over a 10-year period. Cropping sequences included continuous plantings of Forrest, Peking, and D72-8927 soybean (all resistant to race 3); Bedford, Nathan, and D75-10710 soybean (all resistant to races 3 and 14); a Bedford-corn (Zea mays) rotation; a rotation of Bedford, Essex (susceptible), and Forrest; and a 70:30 blend of Bedford and Forrest. The FI from bioassays with PI 89772 and PI 90763 decreased over time from 24.3 to 1.6 with treatments involving continuous Bedford, Nathan, and D75-10710 and the Bedford-corn rotation. The FI increased in bioassays using Bedford with treatments involving Bedford, Nathan, D75-10710, the Bedford-Forrest blend, and the two rotations. Results of this field experiment confirm greenhouse experiments in which reciprocal changes occur in FI on PI 89772 and PI 90673 compared with FI on Bedford.