The response of soil microbial communities to the infection of kauri (Agathis australis) seedlings with Phytophthora agathidicida

New Zealand's ancient kauri (Agathis australis) forests are under threat from the spread of dieback disease, caused by the soil-borne pathogen Phytophthora agathidicida. Characterizing the response of the soil microbiota to the infection of kauri with P. agathidicida is essential to identify how they may form a protective response to pathogen invasion and disease expression. This study infected 18-month-old kauri seedlings with a standardized inoculum load of P. agathidicida for 6 weeks under controlled environmental conditions. Following this, changes in the diversity, composition and biomass of soil microbial communities associated with kauri seedlings were characterized using high-throughput 16S rRNA and ITS gene region sequencing and phospholipid fatty acid analysis. Significant differences were found in the composition of soil bacterial communities associated with inoculated and non-inoculated kauri seedlings. Furthermore, soils of inoculated seedlings had a significantly higher relative abundance of bacteria previously reported to be associated with plant disease suppression, which included several members of the Firmicutes. Significant reductions were found in the fungal: bacterial biomass of soils from inoculated seedlings. This finding contrasts to previous field-based research that observed an increased diversity of soil fungal communities associated with diseased kauri in old growth kauri forests. Further research that aims to isolate members of the kauri soil microbiota and study their interactions with P. agathidicida is required for us to begin selecting potential biocontrol agents against kauri dieback.