Sadly, the discovery of more sites in New Zealand infected with myrtle rust suggests that it is here to stay. Originally from South America, myrtle rust invaded Australia in 2010 and rapidly spread. Botanist Lara Shepherd discusses what Australian scientists have discovered about myrtle rust over the last seven years.
What does myrtle rust infect?
Myrtle rust infects members of the Myrtaceae plant family. The Myrtaceae is Australia’s dominant plant family with over 1,600 native species, including over 500 species of Eucalyptus.
Experiments have shown that there is a big range in the vulnerability of different Australian species to myrtle rust infection. Some species are highly susceptible (all individuals infected), other species show no signs of infection.
Resistance to myrtle rust
Resistance to myrtle rust is linked to variation at a number of genes called ‘resistance genes’.
Plants which contain these ‘resistance genes’ are able to defend themselves against infection.
However, most Myrtaceae species show a range of infection rates – within these species there are both gene variants that protect them from myrtle rust as well as gene variants that make them susceptible.
The species that vary in their susceptibility often show differences in infection rates across the landscape with populations in one part of the range more susceptible to infection than populations elsewhere.
This finding emphasises the importance of conserving local populations of native species, even for species that are common with widespread distributions.
Conserving local populations of native species
Clearing a few hectares of kānuka (like this farmer decided to do) may seem minor, but if the population destroyed harbours the only resistance genes to myrtle rust, or another disease, then the results could be devastating to the long-term survival of the species.
Similarly, creating monocultures using a subset of a species that are selected for some desirable character, such as mānuka that produces high UMF honey, can similarly put a species at risk from new pests and diseases by reducing the overall genetic variation.
Infection trials of nine New Zealand Myrtaceae species in cultivation overseas have shown that all became infected with myrtle rust. But what we don’t yet know is whether any of our species contain resistance genes somewhere within their distributions.
Threat to the Bartlett’s rātā
Scientists are particularly concerned about the impact that myrtle rust may have on our Bartlett’s rātā – there are only 25 individuals remaining in the wild and genetic variation within the species is very limited. If one Bartlett’s rātā become infected it is likely that they all will.