Myrtle rust (Austropuccinia psidii) is a serious plant disease that impacts native species like pōhutukawa, rātā, maire tawake and ramarama. In New Zealand, we only have one biotype – pandemic – present but there are several different biotypes of the myrtle rust fungus that infect different host species overseas. A biotype is a group of organisms having the same or nearly the same genotype. These biotypes found overseas pose a significant biosecurity threat.

In a paper published in January, Scion researchers collected seed (with permission) from North Island populations of kānuka, mānuka and pōhutukawa and plants were grown overseas to investigate their susceptibility. There was only a Iimited amount of Lophomyrtus (ramarama, rōhutu) seed available, so it wasn't tested.

Seedlings were screened against Eucalyptus and other biotypes to check for susceptibility and resistance. All of the myrtle species screened developed symptoms following exposure to each of the biotypes tested. A similar pattern of relative susceptibility was found among the myrtle species tested. Pōhutukawa was the most susceptible species tested, followed by mānuka, then kānuka.

Some seedlings showed evidence of full resistance (more so in kānuka and mānuka). The myrtle rust fungus produced its sexual spore stage on all the NZ myrtles tested, indicating that sexual reproduction between different biotypes of the pathogen might be possible.

Why this research is important

These findings highlight the need for development of tools to differentiate between strains and biotypes of myrtle rust for rapid diagnostics to monitor for and respond to incursions.

Identifying host ranges for different biotypes could be used to investigate the connection between host associations and virulence and improve our understanding of the evolution of virulence towards or away from multiple hosts.

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