Myrtle rust

Myrtle rust is a disease caused by the fungus Austropuccinia psidii. It threatens plants in the Myrtaceae (myrtle) family. New Zealand has a variety of native plants that belong to this family including pōhutukawa, mānuka, kānuka, rātā, swamp maire and ramarama. Other plants in the Myrtaceae family include the introduced fruit species feijoa and guava and commercially grown species such as eucalyptus.

The fungus harms the young, soft, actively growing parts of plants – leaves, shoot tips, buds, flowers and fruit – and it can affect seed development. Severe or repeated infections can kill the plants.

Symptoms

Myrtle rust thrives in warm temperatures with high humidity. It first appears in bright yellow, powdery eruptions on the underside of young leaves. As the infection grows, the yellow, powdery eruptions appear on both sides of the leaves. The pustules darken with time, and the leaves may become twisted and die.

Myrtle rust origins

The disease is native to Brazil, but it is found in many parts of the world. The fungal spores are microscopic and are easily dispersed by wind, insects and birds or by human activity. Myrtle rust was first detected in Australia in 2010. Initially, the Australian Government tried to contain the disease, but it quickly spread and became established along Australia’s east coast and then other parts of the country. As a result, the New Zealand Government banned imported flowers and foliage from Australian Myrtaceae family plants. Biosecurity officials began to develop a range of measures in case the disease spread to New Zealand.

A co-ordinated biosecurity response

The first New Zealand myrtle rust discovery was in April 2017 on Raoul Island in the Kermadec Islands. The find was concerning as it affects Kermadec pōhutukawa trees, but the island’s strictly controlled access and long distance from New Zealand meant that the rest of the country was temporarily protected.

Unfortunately, within a month, the situation had changed. On 3 May 2017, the Ministry for Primary Industries (MPI) received notification that five pōhutukawa seedlings in a Kerikeri nursery showed signs of myrtle rust. Laboratory testing confirmed the results, and MPI immediately put a restricted place notice in effect. The nursery plants were sprayed with fungicide, movement of plants and people on the site was restricted and work began to track any plants that had left the site. MPI staff checked properties surrounding the nursery for signs of the disease. It also set up field headquarters with the Department of Conservation (DOC) and consulted iwi, industry and the local council. MPI staff began checking 800 high-risk sites across the upper North Island while DOC initiated searches on conservation land.

A second infected site was found a few days later at a nearby property. MPI began producing daily media updates, and the public were encouraged to contact MPI’s exotic pest and disease hotline and photograph plants they suspected of having myrtle rust symptoms. The plan was to confine and eradicate the disease, if possible.

By mid-May 2017, several sites in the Taranaki region had confirmation of myrtle rust in commercial and private properties. Scientists suspected that the infections were being caused by wind-blown spores from Australia rather than the movement of plants within New Zealand. A further find in the Bay of Plenty a few weeks later supported the windborne infection theory. MPI continued to contain and destroy the infected plants in places where they were found.

When myrtle rust was first discovered on mainland New Zealand in May last year, we said it would be a challenging disease to contain and eradicate but we would give it a good crack.

Dr Catherine Duthie, Ministry for Primary Industries myrtle rust response spokesperson

A new approach

Less than a year after the disease was first found on mainland New Zealand, myrtle rust appeared in the South Island’s Tasman region. This indicated that the disease had reached most of the country’s high-risk regions. MPI is no longer involved in field work to manage the disease. MPI offers advice and guidance to landowners with myrtle rust on their properties but allows the landowners to manage the plants themselves. MPI is continuing to collect, analyse and report myrtle rust data.

MPI is also funding scientific research to better understand the disease and explore treatment and management options. Projects include monitoring natural ecosystems, research regarding tolerance and resistance of myrtle species and seed banking for the future.

What we know

By March 2019, 988 sites with myrtle rust had been identified. It appears that the most susceptible native myrtle species are ramarama, pōhutukawa and rātā. Mature ramarama tree deaths in the East Cape region were reported in 2020.

Fortunately, only low levels of infection have been detected in mānuka, kānuka and orchard plants like feijoa. New Zealand Plant Producers Incorporated provides protocols and advice for nurseries and other plant retailers.

MPI still encourages the public to be vigilant regarding myrtle rust. If you think you’ve seen myrtle rust, don’t touch it – take a photo and call 0800 80 99 66.

See the useful links section for more information about myrtle rust, how to identify it and sector advice.