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RELATIONSHIP BETWEEN STRAINS OF RHIZOBIUM JAPONICUM AND THEIR BACTERIOPHAGES FROM SOIL AND NODULES OF FIELD‐GROWN SOYBEANS

The phage particles which attack Rhizobium japonicum were isolated from soybean (Glycine max L. Merrill) nodules and rhizosphere soil to study the relationship to the serological groups of R. japonicum present in these samples. Phages were isolated from chloroform-treated samples using eight host strains of R. japonicum in a double-layer agar method. These strains belonged to seven serogroups common in Iowa soils. Phages were found in nearly all soil and nodule samples. Active (red) nodules usually showed few phage, but senescent (green-black) nodules had up to 3 X 105 particles/g. Distribution of phages in soil and nodules was found to be similar to the occurrence of R. japonicum serogroups in the nodules; both phage and rhizobia were related to soil pH. Among 51 phage isolates 45 lysed only rhizobial strains from the same serological group as the strain on which the phage was isolated. Phage types were well correlated with serological grouping of the strain. However, up to 50 percent of the rhizobial strains in a serological group were not sensitive to the phage isolated; thus, more exact identification of strains may be possible by use of phage types. With the electron microscope, rhizobial phages SB4 and SB5 were seen to have hexagonal heads with very thin and short tails.