Suppression of Phytophthora Root Rot by a Composted Eucalyptus Bark Mix.

A potting medium containing composted Eucalyptus (Eucalyptus calophylla and E. diversicolor) barks (CEB) was suppressive in decreasing order to Phytophthora cryptogea, P. nicotianae var. nicotianae, P. citricola, P. drechsleri and P. cinnamomi infections of waratah (Telopea speciosissima) and Banksia occidentalis. Suppressiveness of the CEB appears to be biological, as the CEB became conducive to root rot after steam-sterilisation. A non-composted commercial mix (nurseryman's mix, NM) was conducive to all the Phytophthora spp. tested. Pathogenicity of each Phytophthora sp. varied between the container media. In the NM, P. citricola was the most pathogenic species. Steaming the NM resulted in pathogenicity being similar for all Phytophthora spp. However, this did not happen when the CEB was steamed.