Effects of Continuous Culture of Resistant Soybean Cultivars on Soybean Cyst Nematode Reproduction

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Abstract
Soybean cyst nematode (SCN) populations were evaluated from 17 commercial fields in which a cultivar resistant to SCN races 3 and 4 had been grown continuously for 3-6 yr. Index of parasitism (IP, number of cysts on a soybean line relative to 100 on ''Essex'') values for SCN reproduction ranging from 38 to 70 were obtained when the resistant cv. Bedford was grown in infested soil for 30 days. Seed yields of resistant cultivars grown in these fields and not decline measurably. These results may be compared with those from an earlier study in which ''Bedford'', grown for the 1st time in soil infested with SCN race 4, had an IP value of 32 when compared with 3 susceptible cultivars but gave no yield response from application of nematocides. Significantly less reproduction occurred on soybean strains J74-88, PI [plant introduction] 89772 and PI 90763 than on ''Bedford''; reproduction on ''Peking'', D72-8927, PI 88788 and PI 209332 was intermediate and varied among SCN populations. Reproduction of these SCN populations on the soybean strains was similar to that obtained from other greenhouse selection experiments involving the soybeans grown in SCN race 4 soil.