Phage coating of soybean seed reduces nodulation by indigenous soil bradyrhizobia

Inoculation of soybean with Bradyrhizobium japonicum is often unsuccessful owing to the failure of inoculum strains to nodulate soybeans (Glycine max (L.) Merr.) in the presence of indigenous strains of rhizobia in soil. Previous studies have shown that it is possible to reduce nodulation with indigenous strains of rhizobia by amending the soil with a bacteriophage specific for the indigenous strain. The objective of the current study was to determine whether the coating of seed with phage affected nodule occupancy and soybean growth. A phage specific for B. japonicum USDA 469 and a symbiotically superior strain of rhizobium (B. japonicum USDA 110) were coated together onto soybean seed and planted into both greenhouse and field soil previously inoculated with B. japonicum USDA 469. The phage coated onto seed reduced nodulation by B. japonicum USDA 469 to 48% occupancy, compared with 64% for the untreated control value. Nodulation by the superior inoculum strain was increased from 48 to 82% occupancy by coating seed with the homologous phage and B. japonicum USDA 110. The rate of nitrogenase activity (on a per plant basis) was increased by coating seed with the phage and B. japonicum USDA 110. No other plant or symbiotic parameters were affected by phage coating of seed. These results indicate that the nodulation of soybeans can be significantly affected by the coating of seed with phage specific for undesirable strains of rhizobia in soil and the concurrent coating of seed with desirable strains of rhizobia. Key words: competition, rhizobiophage, rhizobia, soybeans.