An infected juvenile pōhutukawa leaf (Photo: Peter de Lange / Design: Archi Banal)
An infected juvenile pōhutukawa leaf (Photo: Peter de Lange / Design: Archi Banal)

ScienceJune 6, 2023

The race to stop the spread of myrtle rust and save some of our most iconic plants

An infected juvenile pōhutukawa leaf (Photo: Peter de Lange / Design: Archi Banal)
An infected juvenile pōhutukawa leaf (Photo: Peter de Lange / Design: Archi Banal)

This invasive fungal pathogen is decimating forests – but research and field trials offer hope.

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It’s been six years since myrtle rust was first detected in mainland New Zealand, after winds carried the invasive fungal pathogen across the Tasman Sea from Australia. Since then, the orange spores have infected trees across the North Island and into parts of the South Island. The fungus attacks soft new leaves, shoot-tips and stems, and can result in plant death. Iconic species such as pōhutukawa, ramarama, rōhutu and rātā – which are all part of the myrtle family – are susceptible.

Dr Stuart Fraser, a forest pathologist, says that Scion Research monitoring sites in the Bay of Plenty are being hammered by myrtle rust. “You see a pretty devastating impact on Lophomytrus species [e.g. ramarama]. You just see them in a terrible state.” Pōhutukawa also got hit pretty hard for the first time this summer, Fraser says, “I saw one tree and it was the worst I’ve ever seen.”

An infected NZ myrtle plant (Photo: Peter de Lange)

We will lose populations if we don’t do something, according to Stuart – but luckily, plenty of people are doing lots of brilliant research and field trials to stop the spread.

The Āwhitu Peninsula is home to one of the last remaining stands of rōhutu in the Auckland region – a “nationally critical” species. Here, Dr Michael Bartlett from Scion has been monitoring rōhutu since 2019, watching as the plants steadily decline. But in 2022, funding from Auckland Council allowed Āwhitu Landcare to undertake a trial with 27 plants, applying a rotating regimen of fungicides over spring and summer.

By December last year, when Fraser visited the site, the trees were “a lot happier and healthier,” he says. Specimens were in flower and fruit was beginning to set, with very little sign of myrtle rust. “If nothing had been done, they would’ve just died,” says Fraser. The trials are set to continue this year.

Myrtle rust infection can now be detected even before the bright orange rust appears, thanks to Scion researchers led by data scientist Elizaveta Graevskaya. Using a rose apple plant deliberately inoculated with the fungus, the Scion team deployed thermal imaging to detect a dip in leaf temperature up to one day before the infection showed its orange spots. A hyperspectral sensor, which detects changes in the colour of the leaf, could pick up infections even earlier: up to three days before visible symptoms.

Researchers David Cajes and Dilshan de Silva in the lab (Photo: Scion Research/Supplied)

Early detection is comparable to Covid-19 testing: the sooner you pick up sick plants, the better your chances of stopping the spread. While the sensors aren’t ready for wider roll-out yet, the Scion team are excited about the possibilities of handheld myrtle rust detectors in nurseries, and even adapting the tech for use in the field.

There’s plenty more in the works, says Fraser, including research into myrtle rust’s genetics, life cycle and reproduction – and the little fly that appears to munch on myrtle rust too. While we’ll never be able to eradicate myrtle rust, all this activity gives Fraser hope that we can save species: “If we can mobilise people, if we can give people tools and resources and funding, then I definitely have hope.”

Keep going!