Effects of the soil-borne pathogen Phytophthora agathidicida on the kauri (Agathis australis) phyllosphere

Abstract

The temperate forests of Aotearoa’s upper North Island are threatened by the soil-borne oomycete Phytophthora agathidicida, a severe pathogen of the endemic keystone species, kauri (Agathis australis), and causal agent of kauri dieback disease. Infection of kauri by P. agathidicida causes damage to the tree through the colonisation of the host’s roots and vascular tissue, and the consequent restriction of water and nutrient transport through the tree. In the canopy, symptoms of disease from P. agathidicida infection include foliage discolouration and thinning, which suggest that fundamental biochemical changes occur within the leaves of infected kauri. Given the role of leaves in ecosystem processes such as nutrient cycling, it is important to consider the implications of P. agathidicida infection for kauri foliage. This research explored the effects of P. agathidicida on kauri fresh leaf nutrient concentrations and the composition of the phylloplane microbial community. To this end, fresh leaves were collected from three kauri-dominant sites in the Waitākere Ranges and analysed for their macronutrient (C, N, P, K, S, Ca, Mg, and Na) and micronutrient (Fe, Mn, Zn, Cu, B, and Mo) content. Concurrently, characterisation of the leaf surface microbial community was carried out via high throughput amplicon sequencing of ITS and 16S regions. To determine the presence of P. agathidicida, samples of the associated soil of the study trees were tested using loop-mediated isothermal amplification (LAMP). Phytophthora agathidicida was detected in the associated soil of 62% of the trees sampled. Linear mixed effect modelling revealed a significant effect of the pathogen on the concentrations of potassium (percent difference = 18.3%, p = 0.02) and phosphorous (percent difference = 7.5%, p = 0.05) in the kauri fresh leaf, but on no other leaf nutrient concentrations. The dominant microbial phyla identified on the kauri phylloplane were Proteobacteria and Acidobacteria, and richness of microbiota where P. agathidicida was detected was found to be generally lower than where the pathogen was not detected for both prokaryote (bacterial) and fungal phyla. The findings of this thesis contribute to the development of a comprehensive understanding of the effects of P. agathidicida on kauri and their forest ecosystems.