Prevalence of Phytophthora species in macadamia orchards in Australia and their ability to cause stem canker

In Australia, Phytophthora cinnamomi is the only species reported as the causal agent of stem canker and root rot in macadamia. In other countries, five Phytophthora species have been reported to cause disease in macadamia, which led us to question if more than one Phytophthora species is responsible for poor tree health, in macadamia orchards in Australia. To investigate this, samples were collected from the rhizosphere, stem and root tissues of symptomatic and asymptomatic trees, nursery and water sources from 70 commercial macadamia orchards in Australia. Phytophthora isolates were identified based on morphological characteristics and DNA sequencing. Phytophthora cinnamomi was the most predominant, widely distributed and was obtained from the different types of samples including asymptomatic root tissues. In addition to P. cinnamomi, only P. multivora was isolated from diseased tissue (stem canker) samples. Six other Phytophthora species were obtained from the rhizosphere samples including P. pseudocryptogea, P. citrophthora, P. nicotianae, P. gondwanense, P. sojae and a new Phytophthora taxon. Only P. cinnamomi was obtained from macadamia nursery samples while five Phytophthora species were obtained from water sources. Of the heterothallic Phytophthora species, mating type A2 isolates were dominant in P. cinnamomi isolates, whereas only mating type A1 isolates were obtained for P. nicotianae, P. pseudocryptogea and P. citrophthora. Pathogenicity assays revealed that P. cinnamomi and P. multivora caused significantly larger stem and leaf lesions than P. citrophthora, P. nicotianae and P. pseudocryptogea. Phytophthora sp. and P. sojae were non-pathogenic towards leaf and stems.