Myrtle rust: Just how bad could it get?

A worker sprays a roof at Kerikeri Plant Production where the fungal disease myrtle rust was first discovered. Photo / Peter de Graaf

Biosecurity teams are battling to contain fungal plant disease myrtle rust, this week detected in New Zealand for the first time. As officials scour Kerikeri for signs of further spread, Dr Amanda Black of Lincoln University and Dr Andrea Byrom of Landcare Research explain to science reporter Jamie Morton whether the scourge can actually be stopped - and what it will mean for our country.

What actually is myrtle rust and how does it infect and kill plants?

Myrtle rust (Austropuccinia psidii) has an origin as a tropical-sub tropical fungal disease of the Myrtaceae family (guava, feijoas, rata, pohutukawa, manuka) that "jumped" host range from natural Myrtaceae to planted Eucalypt forests in South and Central America and has spread primarily through biosecurity incursions.

It infects the leaves of the plants and it can be identified from its distinctive rust-like spots with yellow pustules.

Myrtle rust can distort the leaves of the plants affecting function, but more devastatingly, it makes the plant susceptible to secondary infections within days of the primary rust infection.

What has been driving its global spread?

It's primarily a wind borne pathogen that has been helped along considerably through global movement of people and plants, particularly via the nursery trade.

How did it likely arrive? And was it really inevitable that it was going to reach New Zealand?

On average one to two exotic windborne fungal rusts per annum have arrived autonomously in New Zealand from Australia since 1952.

New Zealand climate and environment is very favourable: we are subject to strong prevailing westerly weather systems, and have close economic and travel ties with Australia.

Given this and increasing cyclonic events that have been attributed to climate change, myrtle rust likely was blown here during one of these weather events.

Exactly how many plant species do we have here that are vulnerable to it: and what kind of damage could we be looking at in the worst-case scenario?

All New Zealand myrtaceae species, including indigenous plants such as our beloved pohutukawa, manuka, and rata, are at risk from myrtle rust infection but the extent of the impact on their health is largely unknown at this stage.

However, since the arrival of myrtle rust in Australia, its host range has doubled to over 336 known hosts globally, and at least nine native New Zealand myrtaceae in exotic cultivation in Australia have been confirmed to be infected.

MPI officials examine plants at Kerikeri Plant Production for signs of the fungal disease myrtle rust. Photo / Peter de Graaf

In Australia, we've seen myrtle rust devastate several indigenous plant species. Could the same happen here to some of our most cherished native species?

Yes definitely.

In Australia there is at least one species with just 12 individuals left in the wild.

All the plants are infected and no seed bank has been collected, which would have helped in saving the species.

All that can be done in that situation is containment and hygiene.

In New Zealand, precious threatened plants like Bartlett's rata will be under threat.

Species already under threat from other pests and diseases will also be especially vulnerable.

What could it mean for our $300 million manuka and kanuka honey industry? Is there a real threat here?

The threat is real, and the industries will be scrambling to react as soon as we have more information.

The extent of the effects for each species like New Zealand manuka are yet to be quantified (scientists have previously been unable to carry out extensive research on plants in New Zealand because myrtle rust was still undetected here).

Any loss in plant function would mean that the manuka honey industry would be impacted, because the window for honey production (flowering season) is short and manuka is not generally the preferred food for bees if they have other options available (like fields of clover).

How would you compare this incursion, in terms of potential scale and impact, both to our economy and cultural and biological heritage, to the 2010 PSA-V outbreak and the emergence of kauri dieback?

This incursion is very serious, mainly because of its potential widespread impact both for the conservation of threatened species and for our economy.

Myrtle rust is far more widespread and mobile than kauri dieback disease and Psa-V (both of which are soil-borne pathogens).

Myrtle rust on willow myrtle. Photo / File

It is a disease that can quickly jump hosts, and research has shown that the host range is ever-increasing.

Once established, eradication of myrtle rust will be extremely difficult, and determining host resistance will then be critical to maintaining ecosystem function and productivity.

Seed banking of rare and economically important plants must also be prioritised.

How have scientists here been preparing for its possible arrival?

Prior to its discovery in Kerikeri and Raoul Island, work with myrtle rust was not permitted in New Zealand for research purposes, which represented a conundrum; we couldn't carry out pre-emptive studies in New Zealand habitats to identify resistant genotypes, which will improve large-scale ecosystem resilience.

So far we've been relying on international collaborations to screen local host material offshore in very limited quantities.

Having said that, scientific research teams in Scion, Plant and Food Research, Landcare Research, the Auckland Botanic Gardens, Auckland Council, the Better Border Biosecurity consortium, the Maori Biosecurity Network, and the BioProtection Research Centre at Lincoln University have been working on many aspects of the pathogen including pathology, ecology and impacts, modelling pathways of potential spread, seed banking and genetic resistance, and using molecular diagnostic tools to "strain type" the rust which might help in identifying where it came from.

We are as well prepared as we can be.

Can we defeat it? Or, like kauri dieback and PSA-V, can we only try to limit its spread?

Realistically, unless the current incursion is limited to this one nursery, we won't be able to defeat it but we can try and manage the effects through containment and hygiene.

At this point in time global resistance screening and seed banking are our longer term options for ensuring the conservation of Myrtaceae plants.

People are also a big part of the picture: MPI has a goal in its new biosecurity strategy of "a biosecurity team of 4.7 million".

Surveillance by communities - followed by rapid response when needed - will be a big part of our future.

Biosecurity is important to everyone.