How destructive could myrtle rust be to our trees?

Being able to quantify how important Myrtaceae are in AoNZ is important because some Myrtaceae species continue to be threatened by myrtle rust, an invasive disease caused by the fungal pathogen (Austropuccinia psidii), that found its way here from its native South America in 2017.

This is just what Manaaki Whenua ecologist Dr Insu Jo has set out to do. With the help of researchers including Drs Peter Bellingham, James McCarthy, Tomas Easdale, Maj Padamsee, Susan Wiser, and Sarah Richardson, the team analysed data from a nationwide forest and shrubland inventory collected from 2009 to 2014.

“Myrtaceae occurred in 74% of the plots we looked at,” says Dr Jo.

“And we found its importance value, determined by abundance and species richness, was the second highest after Nothofagaceae, also known as the southern beeches.” The results of the study, published in the Journal of Vegetation Science, also discussed the potential role of Myrtaceae as a relatively stable carbon store in local woody ecosystems. The study compared functional plant traits such as wood density with other co-occurring woody families and the significance of Myrtaceae woody climbers, of which six of the eight known Myrtaceae climbing species in the world are endemic to AoNZ.

Researchers are only beginning to understand the differences in susceptibility at a species or even individual level – like with COVID-19, some individuals are more hard-hit than others.

“We cannot definitively predict how myrtle rust will impact forest composition and ecosystem processes yet,” says Dr Jo. “It’s important to remember there is no functional equivalent, especially in terms of carbon storage, if they are lost. There will potentially be large and deleterious outcomes in forest ecosystems if taxon-specific pathogens, such as A. psidii, spread and significantly reduce the species.” In other work linked to myrtle rust, Dr James McCarthy and colleagues have been assessing the extent of the disease in the Taranaki area as part of the Beyond Myrtle Rust MBIE programme, using dataloggers to measure thousands of individual leaves of myrtle species. Work is also in the early stages looking for mycoparasites that might use myrtle rust spores as a food source.