Why myrtle rust is a threat
Shoots on pōhutukawa on Raoul Island affected by myrtle rust
New Zealand has a number of species in the myrtle family at risk, including:
- iconic natives such as pōhutukawa, kānuka, mānuka and rātā, and
- commercially-grown species such as eucalyptus, guava and feijoa.
It is likely that myrtle rust will continue to find new susceptible species in New Zealand.
Myrtle rust attacks young, soft, actively growing leaves, shoot tips and young stems. Initial symptoms are powdery, bright yellow or orange-yellow pustules on leaves, tips and stems.
The developing lesions may cause a deformation of the leaves and shoots, and twig dieback if the infection is severe. It can also affect flowers and fruit. Infection may result in plant death.
Myrtle rust is notoriously difficult to control. However, it was caught early on Lord Howe Island and may have been controlled there. Unfortunately by the time that the rust pustules are visible on plants, spores are already spreading.
It will be extremely difficult to eradicate in New Zealand.
Where it's found:
- in many parts of the world – the rust has rapidly spread on Australia's east coast
- on pōhutukawa trees on Raoul Island – found in March 2017
- on mainland New Zealand – first found in May 2017.
Myrtle rust (Austropuccinia psidii) is also known as guava rust and eucalyptus rust.
How it can spread
Myrtle rust spores are microscopic and can easily spread across large distances by wind. Myrtle rust is likely to be carried by wind to New Zealand from Australia, New Caledonia or Raoul Island.
We do not know where the rust has come from, and if it is the same type as that from Australia or New Caledonia. We need to still treat the rust in those countries as potentially different from that on Raoul Island or on mainland New Zealand.
Spores can also be transported on contaminated clothing, insects, rain splashes and equipment.
What it looks like
Myrtle rust generally attacks soft new growth including leaf surfaces, shoots, buds, flowers and fruit.
Symptoms to look out for are:
- bright yellow/orange powdery patches on leaves
- brown/grey rust pustules (older spores) on older lesions
- leaves that are buckled or twisted and dying off.
How the disease begins and forms - new yellow pustules and small brown spots
The legions get bigger and produce masses of bright yellow spores
Managing the threat
DOC is working closely with the Ministry for Primary Industries (MPI) to manage myrtle rust in New Zealand. Early detection of any outbreak in New Zealand will be vital to any attempt to manage it.
New Zealand already has stringent biosecurity measures to protect against myrtle rust introduction, including a complete ban on imports of cut flowers and foliage from myrtle species in New South Wales, Queensland and Victoria.
There are strict biosecurity procedures for DOC staff travelling between Raoul Island and the New Zealand mainland to prevent any people-assisted spread. Entry to the island is by permit only.
The Raoul outbreak will be used to learn more about myrtle rust – how it affects different species, trialing treatments and banking seed from local iconic species for future planting.
Seed banking: looking to the future
While MPI and DOC are dealing with the initial outbreak, we need to plan to secure the long term future of some taonga species by seed banking.
We know that internationally the effects of seed banking have varied between countries and plants.
Plans for seed banking in New Zealand:
- Seeds of vulnerable species need to be collected to store for future propagation – it's our insurance policy against the rust.
- Many of the priority seed sources are on public conservation land but we are aware that local knowledge of taonga species is with iwi Māori.
- We need to work closely with tangata whenua throughout the country to identify specific plants that are important to iwi.
- Some seeds have already been collected but there are many others to do.
- Time is of the essence because the disease is already here and many of the relevant species have ripe seed right now.
- Collection of seeds will continue at other times of year (when certain species have ripe seed) and over multiple years to ensure we have good seed diversity and fresh viable seed.
We must ensure whakapapa of these taonga are forever held. Whānau, hapū and iwi have been involved in development of protocols for plant protection.