Interactions between Phytophthora cinnamomi and Rhizobium isolates

The effect of Rhizobium isolates on sporulation, hyphal lysis and zoospore survival of P. cinnamomi was examined in extracts of suppressive and conducive soils. Varying degrees of inhibition of P. cinnamomi growth by fifteen Rhizobium isolates were observed on agar media. Inhibition of mycelial growth was enhanced when the rhizobia were inoculated into the agar as compared to the surface streak method. An isolate from nodules of Acacia pulchella was found to be the most antagonistic to mycelial growth. Sporulation and hyphal lysis were stimulated by three of the rhizobia tested, with both suppressive and conducive soils. There was no significant difference between the effects of type of soil on sporangial formation or hyphal lysis. Rhizobia isolated from root nodules of A. pulchella significantly reduced zoospore survival in both the non-sterile suppressive and conducive soil extracts. There was a slightly greater zoospore survival in the conducive yellow podsolic extract than in the suppressive red earth extract, possibly attributable to microbial components of the extracts. These results indicate that Rhizobium , in concert with other soil micro-organisms, actively suppresses the pathogen P. cinnamomi in vitro .