The plant disease myrtle rust is heading inexorably towards mainland New Zealand, potentially threatening the multi-million dollar manuka honey industry and native species including pohutukawa and rata.
Originally from northern South America, it attacks young, growing leaves, shoots, fruits and flowers. Leaves and stems become deformed and in severe infections whole plants can die.
The damaging fungus has now been discovered on far flung Raoul Island, about 1000 kilometres from Cape Reinga in the Kermadec Island group.
Department of Conservation staff have found it on a small number of Kermadec pohutukawa trees.
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“It is a severe wake-up call about the threat of invasive plant pathogens to our productive and natural plant systems, and a reminder of the constant need for vigilance and preparedness,” said Dr David Teulon, director of Better Border Biosecurity (B3).
The rust rapidly spread from South America across to Queensland where it was first detected in 2010.
No-one knows for certain how it arrived in the Kermadeecs, but the Ministry for Primary Industries believes it probably floated across on the wind from Queensland because so few people access the islands.
If it ever arrived in New Zealand, myrtle rust would chiefly menace native plants. Pohutukawa, rata, ramarama, manuka and kanuka are all members of the myrtle family. Feijoas and eucalypts are also affected.
The rust easily attaches itself to clothing, raising fears that travellers returning from infected areas will bring spores of the fungus into New Zealand. There is no way to eradicate it.
Project Crimson director Gordon Hosking said New Zealand could guard against damage by growing plants in Australia to see if they were susceptible.
“It wouldn’t take too long to find out but the issue is what do you do if they are susceptible. These rusts are notoriously difficult to eradicate. It’s a matter of time before we get it. I’d hate to see some of our key species suffer, but if you raised some varieties that had some resistance, you are further down the track in preparing for its arrival.”
Hosking said because the same species of manuka (Leptospermum scoparium) occurred in Australia, research should also be carried out on the species there.
MPI’s director readiness and response Geoff Gwyn said a range of organisations was working on the threat, including Scion and B3 (Better Border Biosecurity) who were looking into the susceptibility of five native species, including mānuka, to the rust.
However he cautioned that any research carried out in Australia might not necessarily reflect what would happen in New Zealand, due to different environmental conditions. The fungus reacted differently every time it moved between countries or regions.
Teulon said to really understand the impact, there would need to be controlled innoculations under controlled conditions such as greenhouses or shadehouses.
He said it would be a “huge challenge” to breed up resistant strains in Australia which could then act as an insurance policy should the rust ever make it to mainland New Zealand.
MPI had changed the rules to stop any imports of myrtle flowers and foliage from areas with myrtle rust.
Modelling showed Northland, Bay of Plenty, Hawke’s Bay and coastal parts of the Canterbury Plains were the most climatically suitable for the pathogen. Auckland conditions would support year-round growth of myrtle rust, although it would do better in summer.
Gwyn said MPI’s Plant Health and Environment Laboratory had developed a highly-sensitive molecular test method for the detection of the rust. It is currently working with laboratories in the UK to research a generic antibody for the rapid detection of the rust in the field.
Anyone believing they have seen myrtle rust on plants in New Zealand should call MPI on 0800 80 99 66. Do not attempt to collect samples as this may aid in the spread of the disease.