Field observations in Ohio suggested a possible interaction between soybean cyst nematode (SCN) and glyphosate in Countrymark 316 soybean, a transgenic glyphosate-resistant (R) variety that also expresses SCN resistance derived from ‘PI88788’ soybean. To investigate this possible interaction under controlled conditions, greenhouse experiments were conducted in which Countrymark 316 (R) and Corsoy 79 (S = susceptible to glyphosate and SCN) soybean root and shoot weights were measured in response to three concentrations of race 3 SCN inoculum and five glyphosate doses. Experiments were conducted in which glyphosate was applied 9, 18, or 27 d after inoculation (DAI). Data from SCN treatments in the (S) soybean experiments were regressed on glyphosate dosage and fit to a log-logistic dose–response model. SCN–glyphosate interaction in the (S) soybean variety reduced root dry weight of SCN-inoculated plants 6% compared with noninoculated plants when 1.0 kg/ha glyphosate was applied 18 DAI. The most significant interaction of SCN and glyphosate in the (S) variety occurred when glyphosate was applied 27 DAI; the glyphosate dose required to reduce shoot fresh weight 25% was 0.55 kg/ha in the noninoculated control compared with 0.32 kg/ha in plants inoculated with 1,000 SCN eggs/200 cm3 soil. Glyphosate rates of 0.84 and 1.69 kg/ha reduced root dry weights of Countrymark 316 (R) soybean 10 to 13% at the 18 DAI application timing only. Inoculation with SCN reduced shoot fresh weight of (R) soybean 8 to 29% across all experiments, but there was no interaction of glyphosate and SCN in (R) soybean.
Nomenclature: Glyphosate; soybean, Glycine max (L.) Merr. ‘Countrymark 316’ and ‘Corsoy 79’; soybean cyst nematode, Heterodera glycines Ichinohe.
Additional index words: Herbicide–nematode interaction, integrated pest management.
Abbreviations: DAI, days after inoculation; EPSPS, 5-enolpyruvylshikimate-3-phosphate synthase (EC 126.96.36.199); GR25, dose required to reduce root or shoot growth by 25%; I50, dose corresponding to biomass halfway between upper- and lower-response limits; POST, postemergence; R, glyphosate SCN resistant; S, glyphosate SCN susceptible; SCN, soybean cyst nematode.