RA4 PAPER: NZ Myrtaceae susceptible to myrtle ...

New Zealand Myrtaceae are susceptible to a strain from the Eucalyptus biotype of Austropuccinia psidii present in South America

January 2025

**Soewarto J, Pérez C, Bartlett M, Somchit C. Ganley B, Sutherland R, Simeto S, Stewart JE, Ibarra Caballero JR, Fraser S, Scott PM, Nadrajan J, Waipara N, Marsh A, Ryan J, Miller E, Smith GR. 2025. New Zealand Myrtaceae are susceptible to a strain from the Eucalyptus biotype of Austropuccinia psidii present in South America. Biol Invasions 27, 72,

ABSTRACT

The international spread of the myrtle rust pathogen, Austropuccinia psidii, can be largely attributed to the “pandemic” biotype that has more than 450 host species. However, within South America, the putative native range of A. psidii, multiple biotypes have been characterised, each with a restricted known number of hosts. These biotypes may pose a significant biosecurity threat to countries already affected by the pandemic biotype. Here, we report the susceptibility of four species of Myrtaceae from New Zealand, pōhutukawa (Metrosideros excelsa), mānuka (Leptospermum scoparium), kānuka (Kunzea ericoides), and rawiri mānuka (Kunzea ericoides ‘gumland ecotype’), to a strain of the Eucalyptus biotype of A. psidii. Symptoms and signs developed on inoculated plants of all species. Qualitative resistance phenotypes, with no disease development, were observed for all four species. However, no hypersensitive responses were observed. As seen for other biotypes, pōhutukawa had the greatest susceptibility, while kānuka had the lowest. These findings are consistent with prior work, showing that the Eucalyptus biotype can infect a broader range of species than its field host association implies. As well as uredinia (asexual spores), telia (sexual spores) developed on several plants, indicating that these species could provide a universal host for sexual reproduction and outcrossing between biotypes. Knowledge that the Eucalyptus biotype of A. psidii is virulent on several indigenous New Zealand Myrtaceae will inform future biosecurity risk assessments. These findings highlight the need to develop diagnostics tools to differentiate between biotypes and allow rapid responses to potential future incursions.

KEYWORDS

biological invasion; biosecurity risk assessment; disease resistance and susceptibility screening; host adaptation; pathogen virulence