Selection Pressure on Soybean Cyst Nematode from Soybean Cropping Sequences

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Abstract
It has been recommended that soybean [Glycine max (L.) Merr.] cultivars resistant to soybean cyst nematode (SCN), Heterodera glycines Ichinohe, be rotated with susceptible cultivars in order to delay or avoid development of populations capable of damaging resistant cultivars. The objective of this study was to determine if evidence for stabilizing selection of SCN populations with use of susceptible cultivars existed following 10 yr of such cropping sequences. Cropping sequences included susceptible ‘Tracy’ and race 3 resistant ‘Centennial’ grown consecutively with and without nematicide. Sequences of 2 yr of Centennial and 1 yr of Tracy, a blend of 80% Centennial and 20% Tracy, and a breeding line resistant to SCN races 3 and 4 planted consecutively were the other treatments. ‘Lee’, ‘Pickett 71’, ‘Peking’, PI 88788, and PI 90763 were grown in the greenhouse in soil from each field plot in 1985. Cyst indices (percent of cysts on a soybean line compared to number on Lee) were calculated for these race differentials. Race 3 was predominant following Tracy planted consecutively and the breeding line resistant to races 3 and 4. Race 2 was predominant in one of three treatments involving rotation with 2 yr of Centennial and 1 yr of Tracy. Cyst indices on Pickett 71 were large for cropping sequences continuing Centennial and the blend of Centennial and Tracy. Cyst indices were variable on Peking, PI 88788, and PI 90763 for sequences involving Centennial, but indices were within a standard deviation of the description for race 4. No evidence for stabilizing selection sufficient to prevent race shifts in SCN populations through use of susceptible cultivars was obtained.