Abstract
Agathis australis (kauri), the only member of the ancient conifer family Araucariaceae in New Zealand, is colonised by arbuscular mycorrhizal (AM) fungi. The survival of A. australis is threatened by an Oomycota, Phytophthora agathidicida , the causal agent of kauri dieback. Phosphite injections into the trunk are the primary treatment for kauri dieback and have proven effective in reducing disease symptoms. However, the potential impact of phosphite treatment on AM fungal colonisation within A. australis roots and nodules has not been investigated. We conducted a preliminary investigation into whether AM fungi colonise phosphite‐treated trees. Using a combination of microscopy and metabarcoding of AM fungi 18S nuclear ribosomal small subunit genes, we demonstrate that AM fungi colonise the roots and nodules of A. australis trees three to 4 years post‐treatment. Glomeraceae was found to dominate, and AM fungi community composition and colonisation rates were comparable to previously reported values in trees not treated with phosphite. Soil pH, but not date of phosphite treatment or PA infection status, was found to significantly contribute to AM fungal community composition. This work highlights the importance in investigating colonisation events immediately after phosphite treatment of A. australis and a comparison with untreated trees. AM fungi are likely integral for A. australis forests to thrive, thus, ensuring they are protected in the treatment of kauri dieback is essential.