Myrtle rust is having sex - what it means for New Zealand

The findings of a new study on invasive fungus myrtle rust has shown it could be more of a threat to some of our best loved native trees than first thought.

Myrtle rust infects iconic New Zealand trees in the Myrtaceae family, such as pōhutukawa and mānuka.

It was first detected in New Zealand in March 2017 and is now found throughout the upper North Island. It has currently been recorded as far south as Greymouth.

Austropuccinia psidii, the fungus that causes myrtle rust, is known to reproduce by cloning itself (making identical copies), but a new study has now found evidence that it is also able to reproduce sexually. 

Sexual recombination of genes means that the fungus has a better chance of overcoming natural plant resistance as well as being less easy to control by biological or chemical means.

"Sexual reproduction creates new individuals with new genotypes, allowing the fungus to adapt to host defences," said Stuart Fraser, a researcher from Scion who is one of the authors of the study, published in the European Journal of Plant Pathology.

"Sexual recombination also allows it to adapt to new environments and new host species," said Fraser.

Sexual recombination of genes means that the fungus has a better chance of overcoming natural plant resistance and is harder to control.
Sexual recombination of genes means that the fungus has a better chance of overcoming natural plant resistance and is harder to control. Photo credit: File

Although most fungi have a mixed mating system, meaning they can reproduce both clonally and sexually, previous research from other parts of the world has suggested that A. psidii reproduces clonally. 

However, this new study of samples of A. psidii from New Zealand and South Africa shows evidence of sexual recombination in addition to cloning.

Fraser said this has important implications for scientists who are working to protect New Zealand's native myrtles - such as pōhutukawa, mānuka and kānuka - from infection by myrtle rust.

"Sexual recombination makes myrtle rust more complicated to manage.

"It is important that we are aware of the impacts of the pathogen's sexual reproduction and plan our research around it."

A Beyond Myrtle Rust research programme is looking at the issue, with one of its key goals to improve understanding of A. psidii reproduction in New Zealand. 

This research area within the programme, which is being led by Fraser and Alistair McTaggart, the lead author on the paper, will investigate how sexual recombination is being driven by the environment and by hosts.

"We don't know the frequency or impact of sexual reproduction in natural populations," said McTaggart. 

"However, we hope our research improves knowledge about the biology of myrtle rust and informs strategies for combating the disease in the future."

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