Myrtle Rust Can Infect Mānuka Fruit And Seeds

Researchers investigating the susceptibility of several plant species to myrtle rust as part of Ngā Rākau Taketake, the BioHeritage National Science Challenge’s MBIE funded programme on kauri dieback and myrtle rust, have observed myrtle rust infection on mānuka seed capsules.

In 2019, scientists from Scion and Plant & Food Research with the partnership and support of mana whenua, Ngāti Tamateatutahi and Ngāti Kawiti, established field monitoring sites in Auckland and Rotorua to investigate how natural myrtle rust infection affects plant growth or survival, and how environmental factors contribute to myrtle rust development. The sites included five native species: mānuka, kānuka, pōhutukawa, ramarama, rōhutu; plus the exotic rose apple.

During the spring of 2020, an outbreak of myrtle rust on ramarama, rōhutu and pōhutukawa was producing abundant myrtle rust spores at the Auckland monitoring site. At the Auckland site, leaves and stems of mānuka were not affected by myrtle rust during spring, but in December the first mānuka fruit (seed capsule) infection was discovered during routine monthly monitoring. The infected seed capsules were also observed in January and February 2021, but by mid-March, seed capsule infection was no longer found. The researchers think the infection probably established during November 2020 while the mānuka plants were flowering. The plants had not been artifically inoculated, but became naturally infected by myrtle rust.

Infection was observed on four of 18 mānuka seedlings at the observation site. On infected seedlings the number of infected seed capsules was very low (only 0.5%).

Further monitoring in January, February and March this year showed that by late February there were fewer new infected seed capsules. The outside of older infected seed capsules had died and turned black, but yellow spores could still be found inside some of the infected capsules. By 17 March, infected seed capsules were no longer found.

According to Dr Rob Beresford, who co-led the study, this is a new discovery of the effects of myrtle rust in New Zealand. “The more we know, the better we can prepare for myrtle rust. As yet, we don’t know the impact of seed infection on the plant or the production of flowers and pollen, so more research is needed to understand how this finding will impact mānuka.”

The public is urged to remain vigilant in monitoring the spread of myrtle rust. If an infection is spotted, follow the directions at www.myrtlerust.org.nz/what-to-do-if-you-find-myrtle-rust. The NZ Myrtaceae Key is also available to help users identify plants in the myrtle family that grow here in New Zealand.

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