Quantification of inoculum density of Phytophthora palmivora in soil and its relation to disease incidence in papaw in far northern Queensland

A dilution-plate technique using five media selective for Phytophthora was evaluated at 16 and 26°C to develop a direct quantitative isolation method for Phytophthora palmivora Butler from naturally infested soil. P10ARP+H medium at 26°C was found to be the most effective. This method was used to examine the relationship between inoculum density of P. palmivora and disease in papaw seedlings in the glasshouse. Results showed 100% plant mortality at an initial inoculum level of 100.4 cfu g−1 and significant primary root damage (P < 0.05) at >= 2.9 cfu g−1 after 10 weeks in naturally infested soil. Low to medium initial inoculum levels increased during the experiment by four to six times and the highest initial inoculum level increased by two-fold. A survey of 35 papawgrowing sites showed populations of P. palmivora were highest where growers followed papaw with papaw. In most cases, lengthy rotations with other crops and fallows reduced both inoculum levels and the incidence of tree lodging due to root rot.