Recovery of Phytophthora ramorum in plant tissue with mixed infections

Abstract

This study was performed to investigate the frequency with which P. ramorum would be isolated from host tissue co-infected with P. ramorum as well as an indigenous Phytophthora species or P. kernoviae. Three separate experiments were tested in a similar manner using different combinations of pathogens and hosts. Overall for any of the individual experiments, very few segments did not have any growth of Phytophthora spp. For mixed infections of P. ramorum and P. kernoviae, a difference was observed between isolating both of the species and P. ramorum only on rhododendron. The data showed that P. ramorum or P. kernoviae will not be detected 29 or 12% of the time, respectively, in infected Rhododendron sp. Phytophthora kernoviae was not detected alone in mixed infections with P. ramorum on Pieris japonica. When two different P. ramorum isolates were co-inoculated individually with one P. citricola isolate, there was a significant difference between isolating P. ramorum and isolating both species. These results confirm that choice of host species used for baiting can strongly influence detection results. For example, if P. japonica were used for baiting in mixed infections, there is a 55% chance that P. kernoviae would not be detected. This study highlights the difficulty in being confident in isolating and identifying an individual Phytophthora sp. from host material when mixed infections are present, and emphasizes the importance of a large sample size in order to increase the chances to recover all possible different species in a mixed infection.