Soybean Cyst Nematode and Rhizobium Strain Influences on Soybean Nodulation and N2‐fixation1

Abstract The spread of the soybean cyst nematode (SCN) (Heterodera glycines Ichinhoe) has required the breeding of resistant soybean [Glycine max (L.) Merr.] cultivars. The SCN causes a reduction in nodulation and N2‐fixation which may explain part of the yield reductions associated with SCN infestations. This study was designed to determine the interaction effects of SCN resistant and susceptible cultivars, Rhizobium japonicum (Kirch.) Buchanan strains, and SCN races on nodulation and N2‐fixation. The susceptible cultivar ‘Williams’ and resistant cultivars ‘Bedford’, ‘Forrest’, and ‘Franklin’ were grown on sterile sand culture in the greenhouse and inoculated with all possible combinations of SCN races 3 and 4 and R. japonicum strains 3, 110, and 123. Bedford and Forrest inoculated with strain 110 had the highest specific N2(C2H2)‐fixation activity (SA), but Bedford, Forrest, and Franklin inoculated with strain 123 had the largest nodule mass (NM). SCN race 3 did not reduce NM, but reduced both total N2(C2H2)‐fixation activity (TA) and SA. Race 4 did not influence TA and was inconsistent in its influence on SA and NM. Total N2(C2H2)‐fixation activity and SA was estimated at the R2 (determinate) and R4 (indeterminate) stages on several field‐grown soybean cultivars. The number of SCN cysts per 250 cc of soil (cyst index) also was determined. Resistant cultivars had low cyst indices and high N2‐fixation potentials. Susceptible cultivars had higher cyst indices and generally lower N2‐fixation rates but ‘Williams’, ‘Wayne’, and ‘York’ exhibited high fixation potentials despite high cyst indices. Nodule serology indicated a significant cultivar ✕ strain interaction which may account for the TA and SA differences. These results indicate that nodulation and N2‐fixation are influenced by cultivar ✕ Rhizobium strain and SCN race interactions and that the SCN is not equally deleterious to all susceptible cultivars. Williams and Wayne may be especially valuable in breeding resistant cultivars since they exhibited good nodulation and N2‐fixation despite high cyst indices.