Myrtle rust: why local New Zealand species are under threat

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.

Myrtle rust is characterised by yellow pustules. Photograph by Scot Nelson

Myrtle rust is characterised by yellow pustules. Photograph by Scot Nelson

 

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.

Pohutukawa is a striking member of the Myrtaceae family.

Pōhutukawa tree, 2016. Photograph by Skeeze via pixabay. CC0 1.0

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.

12 Responses

  1. Peter Matthews

    I have two different kinds of guava tree growing at home in Auckland. Is guava more suceptible to myrtle rust than other trees? Should they be watched more closely? And removed immediately when infected? How to dispose of infected plants?

    Can photos of different infected species be published across all media, including local newspapers, to get feedback from the public on the occurrence and spread of this over time?

    What is the best way to recognise resistant populations? Repeated general surveys of absence and presence might provide baseline reference points that make it easier to recognise resistance sooner.

    Reply
    • Lara Shepherd

      Hi Peter,
      I’m not sure how susceptible guava is to myrtle rust – hopefully you won’t find out! If you do notice something on your trees that might be myrtle rust then don’t touch it – MPI says to do the following:
      Call the MPI Exotic Pest and Disease Hotline immediately on 0800 80 99 66.
      If you have a camera or phone camera, take clear photos, including the whole plant, the whole affected leaf, and a close-up of the spores/affected area of the plant.
      Don’t touch it or try to collect samples as this may increase the spread of the disease.

      Of course their advice may change if myrtle rust becomes widespread. Naturewatch (http://naturewatch.org.nz/home) would be a great way to track the spread of myrtle rust because photos, the date and location is recorded for each sighting. As of today no one has recorded it on that platform yet.

      In order to test for resistant populations in Australian plants scientists deliberately infected plants in cultivation. This would have required an available source of cultivated plants from a range of locations. I’m not sure if this kind of research is being done in New Zealand.
      Cheers,
      Lara

  2. Harm Linsen

    But what are the effects of Myrtle rust? Does it kill the plant or are the effects different per individual plant or species? What are the effects?
    PS Myrtle Rust, a great name for the main character in a novel.
    Yours,
    Harm

    Reply
    • Lara Shepherd

      Hi Harm,
      According to the Australian Department of Environment and Energy website:
      ‘The disease can cause deformed leaves, heavy defoliation of branches, reduced fertility, dieback, stunted growth, and plant death.’

      My understanding is that the severity of the effects depends on how susceptible the particular species is. Also seedlings are more likely to die that adults because of all the new growth they have.

      I agree about Myrtle Rust being a great character name – she’d have to be the villan though!
      Cheers,
      Lara

  3. Julia White

    Why is it only found in Nurseries so far, does that imply they swap plants.
    Is the Government going to provide funding to find a cure for it, so far, no plants with the rust samples have been sent to the scientists at Callaghan.

    Reply
  4. Denis Asher

    Did I miss the media release advising of extra funding for DoC to address this profound risk?

    Reply
  5. Peter Glover

    Genetic engineering would answer the question

    Reply
    • Nick Waipara

      Not necessarily true, there is actual NATURAL genetic resistance and tolerance in myrtaceae plants to this fungus, as we have seen in Australia over the past 7 years. It will be mch better R&D investment to explore existing resistance genes rather than a blunt costly tool of genetic engineering

    • Phillip Wilcox

      Completely agree with Nick on this one Peter – GE is NOT the best response. In 2015 I presented a review of naturally-occurring resistance in forest trees to newly introduced pathogens at the Kauri Dieback symposium held in Hokianga (it’s posted on YouTube). Bottom line is resistance virtually always occurs… just not often at high – or even moderate – frequency when the pathogen is a newly introduced pathogen. In fact the first resistance gene in a forest tree species to be characterised was in sugar pine – to western white pine blister rust. Also, GE is very expensive to develop and requires years of R&D to develop suitable plant delivery systems that are sufficiently robust and allow a wide range of genotypes to be transformed. Better off screening for naturally occurring resistance and propagating from those. In the case of Bartletts Rata, if all remaining individuals are susceptible then maybe a gene editing approach using resistance gene sequences from other myrtaceae would be more appropriate than transgenesis.

  6. Jack Craw

    I worry about spp found in primarily damp habitats, eg the subcanopy rata spp. Just hope that MPI/DOC collecting seed and cuttings from a wide range of habitats of all native Myrtaceae ie many thousands of samples.

    Reply
  7. Barbara Hammonds

    Thanks Lara . Very helpful. Sobering examples.

    Reply

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