Tackling the threat of invasive Ceratocystis species to Aotearoa New Zealand

Ceratocystis species continue to emerge as invasive pathogens with an ability to adapt to new regions and, in some instances, completely unrelated hosts. Should either of these pathogens be introduced into Aotearoa New Zealand, they have the potential to impact our Metrosideros species and possibly other native and exotic plants.

Rapid ‘ōhiʻa death (ROD), caused by two recently described fungal species, Ceratocystis lukuohia and C. huliohia, has caused widespread death of ‘ōhi’a lehua (Metrosideros polymorpha) in Hawaii. Recent testing on kiwifruit has revealed that kiwifruit is susceptible to C. lukuohia from ‘ōhiʻa and artificial inoculations have also confirmed that C. lukuohia is able to infect pōhutukawa, although symptoms and mortality were not observed.

Insects play an important role on the dispersal and transmission of Ceratocystis species. Importantly, the polyphagous ambrosia beetle Xyleborinus saxesenii, associated with the spread of ROD in Hawaiʻi, is well established and now widespread in Aotearoa NZ. New ambrosia beetles-Ceratocystis associations are also emerging, such as the native Platypus cylindrus vectoring C. platani in Greece and Xyleborus affinis transporting spores of C. manginecans in southeast Asia. Aotearoa NZ has at least a dozen of species of native and exotic ambrosia beetles, including representatives in the genera Platypus and Xyleborus, which could act as vectors for newly introduced Ceratocystis pathogens.

This project continues the work from B.21.2: “The risk of Rapid ‘ōhiʻa death to Aotearoa New Zealand and the South Pacific”. Our research addresses the threat posed by Ceratocystis pathogens, specifically C. lukuohia an C. huliohia, while also providing valuable insights applicable to other invasive Ceratocystis species. The cross-CRI project team have prioritised four key areas where research is needed:

RA1 Host testing of Aotearoa NZ Metrosideros spp. and horticultural crops (e.g. apple, grapes, stonefruit) against ROD pathogens. Host testing is critical to understand the risk these pathogens pose to Aotearoa NZ plant species, not only native species but also potentially to exotic species. Finalise the host testing already underway in Hawaiʻi. Results will inform risk assessment, surveillance, and response and will be prepared for publication.

RA2 Ceratocystis species (Aotearoa and South Pacific). We will update the international literature review on Ceratocystis species, emphasizing newly reported species and hosts. Targeted surveys will focus on trap/wounding on known host trees (non-taonga spp.) and those with frass/ambrosia beetle presence. Part of this work will be in collaboration with current Aotearoa NZ projects including sourcing beetles from RA3.

RA3. Ambrosia and bark beetles on Metrosideros to establish baseline knowledge. It is important to understand the potential primary vectors for spread, as this will inform management plans on ways to mitigate pathogen spread. A pilot study using interception and sticky traps at Christchurch Botanic Gardens has shown that ambrosia and bark beetles can be found in association with Metrosideros trees. We will refine sampling techniques to understand the bark and ambrosia beetle complex on selected Metrosideros spp. at representative sites across the country, aiming to create new insights in this area. We will also review literature, databases and collections for host ambrosia beetles-host associations.

RA4. Climate risk mapping would reveal what regions of Aotearoa NZ and the South Pacific could support establishment of the disease, both now and under future climates. Risk modelling for Aotearoa NZ is well underway with Luna Hasna’s MSc (B.21.2). This will be extended to areas of the South Pacific and for potential beetle vector(s). The outputs will be prepared for publication and an interactive tool-box to enable effective, easy interpretation for end-users. These tools are essential in targeting surveillance, biosecurity awareness, pathway management and developing resources for readiness and preparedness plans for use by end-users in Aotearoa NZ and the South Pacific.

Our research will contribute to science, biosecurity, and Māori outcomes by informing evidence-based management strategies to protect native biodiversity and horticultural crops in Aotearoa NZ and the wider Pacific region. Results will be shared through hui, popular articles, reports to stakeholders and peer reviewed research publications.

Contact Project Leader Virginia Marroni: [email protected]