Abstract:
Phytophthora agathidicida (also known as PTA, or kauri dieback disease) is a virulent oomycete plant pathogen, which is currently known to only infect Agathis australis (kauri) in New Zealand. Phytophthora species rarely have a single plant host, however, so other hosts for P. agathidicida are likely but unknown. Phytophthora species are also often cryptic and sometimes asymptomatic on their host plants, making it a challenge to identify their true host range. Once a Phytophthora spp. is introduced to an area, it becomes virtually impossible to eliminate. A sound understanding of a Phytophthora spp. epidemiology is needed to prevent its spread onto uninfected hosts. In this thesis, I aim to determine whether P. agathidicida has a wider host range than currently recognised. Plant community composition was compared between healthy and infected A. australis forest stands to detect possible susceptible species, while detached leaf assays and soil inoculation were utilised as methods to further screen for possible hosts. Results showed a significant difference in species composition between sites infected with P. agathidicida and sites without P. agathidicida unrelated to other potential variables such as environment. Leaf assays indicated several native plant species other than A. australis as possible carriers or hosts, including Knightia excelsa, Leucopogon fasciculatus, Phyllocladus trichomanoides, Toronia toru, Dracophyllum latifolium, Dracophyllum sinclairii, and L. fasciculatus. Astelia trinervia leaves did not develop lesions as a result of P. agathidicida infection, but the pathogen was reisolated from leaf tissue at the end of this experiment, suggesting it may be a cryptic host. WinRhizo analysis did not find significantly less root tissue in infected K. excelsa compared with controls. However above ground symptoms and fine root damage was observed on infected P. trichomanoides seedlings. Taken in total, my results indicate that several species within A. australis dominated forest communities are also likely to be susceptible hosts to P. agathidicida. Further research is necessary, however, to delimit the full extent of the host range of this pathogen in New Zealand. Identifying whether the host range of P. agathidicida is broader than simply kauri is critically important to the design of future control strategies for this pathogen. If other species are also hosts, it will substantially increase the area of New Zealand and the ecosystem types potentially at risk from this pathogen.