PŌHUTUKAWA, ramarama and possibly mānuka and kānuka are all under dire threat as myrtle rust spreads out of control through much of the North Island and has been recently discovered in Christchurch.
This discovery is the latest in a series since myrtle rust was first discovered on the New Zealand mainland in a Kerikeri nursery in May 2017. It is generally believed to have come from Australia.
Two dominant myrtaceae species on Australia’s east coast are now functionally extinct from their natural host ranges as a result of the rust.
Working collaboratively with their Australian counterparts, the Kiwi scientific team sequenced the genome of Austropuccinia psidii, the myrtle rust fungus, and assembled the sequences to produce the world’s largest assembled fungal genome.
In a case of needing to know one’s enemy before being able to challenge it, the group used almost 360,000 CPU hours at the University of Sydney’s high-performance computing facility to assemble the sequence.
The machine worked 24/7 for over four months to deliver the genomic sequence of a genome that had just over a billion base pairs of DNA, 10 times more than some other rust fungi and 34,000 times more than the covid virus.
Plant & Food Research principal scientist Dr Grant Smith, a research leader in both the Beyond Myrtle Rust and Ngā Rākau Taketake research programmes, compares the work to assembling a three-dimensional jigsaw, without a good picture on the jigsaw box to look at for guidance.
But the work also relied as much upon scientific intuition and talent, as it did raw computing power.
“Once we had the data we needed to ask ourselves ‘does this data even make sense?’ given it had not been mapped before,” Smith said.
Now that they have a map of the deadly fungi’s genome, the team are in a position to better understand options to target the pathogen to reduce or stop its devastating impact.
Only 10 years after being found in Australia, the rust has resulted in a massive decline in some of the country’s myrtle species. This is causing significant concern as myrtles are the dominant native species in most Australian landscapes.
Bush regrowth is significantly affected, with myrtle species being replaced by vigorous weed species, including wild tobacco and lantana, changing the profile of Australian bush for good.
The scientists identified 367 “effectors” in the genome. These specific proteins are used by the fungi to manipulate the host’s metabolism and take it over.
Smith likens the fungi to a vampire, having to keep its host alive enough to continue to harvest its nutrients for itself.
“The pathogen also suppresses the host’s defence mechanism, weakening its ability to respond to the fungi’s presence,” he said.
There are published reports of pōhutukawa and ramarama being hammered by the fungi, with greenhouse trials finding only one resistant pōhutukawa and no resistant ramarama.
Scientists are reserved about its likely impact upon mānuka and kānuka, with some findings of flowers and seed capsules that have been infected.
“At this point in time it does not seem to become infected as much as other species; we are not quite certain why yet,” he said.
It is thought the genetics of resistance in mānuka have a level of complexity that is not yet fully understood.
“But there are a lot of plants out there that our Australian colleagues had initially believed were field resistant that are now susceptible,” he said.
Here on the East Cape conservationists have been collecting seeds of apparently resistant trees, as the rust spreads quickly from Gisborne right through to Opotiki in Eastern Bay of Plenty.
Australia has 2253 native myrtle species, compared to 27 native species in NZ. However, the number of myrtle species in NZ is over 120, as feijoa, guava and other exotic species have been introduced over the years.
Chemical control of the rust using fungicides is possible but far from practical, and has ecological consequences.
Having also mapped the mānuka genome, the scientists are able to understand and distinguish which pathogen genes and which host genes are interacting at the earliest point of the infection process.
Smith says the work has proven the value of collaborative scientific strength, with the shadow of possible species extinction adding a sharp reminder about its value.
