RA3 Report: Lophomyrtus spp reproductive ...

Zhulanov M, Soewarto J, Bartlett M, Sen D, Sutherland R, Gillard K, Miller E, Fraser S. 2022. Monitoring myrtle rust disease effects on Lophomyrtus spp. reproductive capability in the natural environment Aotearoa New Zealand. A report for New Zealand’s Biological Heritage National Science Challenge: Ngā Rākau Taketake. 27 p.

This report is embargoed until 31/01/2025 or until a paper is published. For further information please contact Stuart Fraser at Scion Research (stuart.fraser@scionresearch.com)

Executive Summary

The problem

Lophomyrtus species are some of the most susceptible native host species under threat from myrtle rust in New Zealand. Previous studies have shown that all the reproductive structures of Lophomyrtus spp. are vulnerable to the disease, including flower buds, flowers, and fruits. While myrtle rust has the potential to kill mature Lophomyrtus trees (e.g. East Cape), it could also be interfering with the regeneration of Lophomyrtus species populations in the natural environment. Little is known about how myrtle rust impacts floral reproductive potential and seed germination. Quantitative characterisation of the impact of myrtle rust on Lophomyrtus species reproduction will directly inform conservation activities.

Client initiatives

New Zealand’s BioHeritage National Science Challenge Ngā Rākau Taketake-Saving our Iconic Trees provided essential funding to monitor the impact of myrtle rust on the reproductive potential of Lophomyrtus spp. over the 2019-2020, 2020-2021 and 2021-2022 summer seasons.

This project

This study aimed to investigate whether myrtle rust could impact the reproductive capacity of Lophomyrtus spp. in the natural forest environment. The development of flowers and fruits and the incidence of myrtle rust on these were assessed at three sites in the Bay of Plenty region.

Key results

• Lophomyrtus spp. reproductive phenology overlapped with the myrtle rust seasonal epidemic.

• Symptoms and signs of disease can develop on all reproductive stages, (from flower buds to developing fruit), but predominantly on developing fruits.

• Regularly infected Lophomyrtus spp. trees had a lower rate of fruit production, likely due to the indirect effect of repeated myrtle rust infections on plant health rather than direct infection of flower buds and flowers.

• Along with the increase of myrtle rust incidence on developing fruits, the production of fruits per tree during the fruiting peak declined at each site. Between 2021 and 2022, fruit production decreased 73-93% at the three locations assessed.

Further work

• Statistical analysis is required to confirm the impact of fruit infection severity on seed germination rate.

• Continuing the current monitoring and extending it to other regions in New Zealand with different climatic conditions and forest composition would determine how variation in environmental conditions impacts host susceptibility or tolerance over consecutive years.

• Extending the seed germination test would enable the evaluation of the longer-term impacts of fruit infection on seedling fitness.

• Active conservation practices such as encouragement of reproduction and identification of resistant phenotypes of Lophomyrtus spp. will support a long-term disease resistance breeding programme.