Monitoring Austropuccinia psidii (myrtle rust) on New Zealand Myrtaceae in native forest

Austropuccinia psidii (myrtle rust) was first detected on mainland Aotearoa (New Zealand) in 2017 and has established in various urban areas and native forests. To understand the spread of this pathogen and its effect on host species, surveillance for myrtle rust on Myrtaceae in native forests was undertaken in central Te Ika a Māui (North Island). In one site, with confirmed A. psidii infection, the rust infected up to 90% of new flush stem and leaves of some ramarama and rōhutu (Lophomyrtus spp.), with the pathogen eventually causing dieback of these shoots. The rust also infected developing fruit, causing it to prematurely drop, and infected all seedlings monitored in the site. It is likely that heavily infected trees will die and natural regeneration of Lophomyrtus spp. is unlikely; localised extinction is probable. Other Myrtaceae species in the stand, white rātā (Metrosideros diffusa) and mānuka (Leptospermum scoparium), were also infected but the severity of infection was less on these species than observed on Lophomyrtus spp. However, the long-term impact on these species from increasing or sustained disease pressure is unknown. Highly infected plants had decreased insect activity and diversity, highlighting the multi-tropic risk this invasive disease poses. A second site, approximately 15 km from known infected areas, which also contained Lophomyrtus spp., remained myrtle rust-free, showing that spread of this disease across landscapes is variable. This is the first monitoring study of myrtle rust on native forests in New Zealand. Continued monitoring is critical to provide information for effective management of this disease.