Phyllosphere of Agathis australis Leaves and the Impact of the Soil-Borne Pathogen Phytophthora agathidicida

Leaf surface microbial communities play an important role in forest ecosystems and are known to be affected by environmental and host conditions, including diseases impacting the host. Phytophthora agathidicida is a soil-borne pathogen that causes severe disease (kauri dieback) in one of New Zealand’s endemic trees, Agathis australis (kauri). This research characterised the microbial communities of the A. australis phyllosphere (i.e. leaf surface) using modern molecular techniques and explored the effects of P. agathidicida on those communities. Fresh leaves were collected from trees where P. agathidicida was and was not detected in the soil and characterisation of the leaf surface microbial community was carried out via high-throughput amplicon sequencing of the internal transcribed spacer (ITS) and 16S ribosomal RNA regions. Nutrients in leaf leachates were also measured to identify other possible drivers of microbial diversity. The dominant phyllosphere microbial phylum was Proteobacteria followed by Acidobacteria. The phyllosphere microbial richness of A. agathis associated with P. agathidicida- infected soils was found to be generally lower than where the pathogen was not detected for both prokaryote (bacterial) and fungal phyla. Leaf leachate pH as well as boron and silicon had significant associations with bacterial and fungal community structure. These findings contribute to the development of a comprehensive understanding of A. australis leaf surface microbial communities and the effects of the soil pathogen P. agathidicida on those communities.