Rust never sleeps – fighting a fungal killer

Department of Conservation —  24/02/2021

There is no doubt that 2020 was the year the term ‘pandemic’ became firmly lodged in our vocabulary. But for years, epidemics in plant communities have been silently spreading. One such pandemic putting 37 of our native species at risk is myrtle rust. This fungal disease affects species of the Myrtle family by attacking soft new growth of leaves, stems, flowers and fruit, with severe infections leading to death. Since arriving on the mainland in Kerikeri in 2017, the wind-bourne disease has spread north and south, affecting a range of our native and exotic myrtles.

Photographing suspected myrtle rust on ramarama.
📷: Suliana Teasdale

One quick look at the Myrtle Rust Reporter map on iNaturalist clearly shows that Tāmaki Makaurau / Auckland is a hotspot, where humid microclimates provide ideal conditions for the microscopic spores of myrtle rust to flourish and spread on the wind. Myrtle rust has been found in urban and natural Auckland habitats, including Waiheke Island in the Hauraki Gulf Marine Park/Ko te Pātaka kai o Tīkapa Moana, and more recently in the Waitākere Ranges. To date, it is predominantly attacking garden cultivars of our native ramarama (also known as ‘bubble leaf’) and the exotic brush cherry (known as ‘lilly pilly’), which are often planted domestically as a hedging.

So, what on earth can we do about it, and, what has been done to date? The answer to both these questions is, a lot. But before we launch into measures taken and how you can help, we need to understand what it is, where it came from and what is at stake.

Understanding the enemy

Myrtle rust (Austropuccinia psidii) originated in South America and has spread to many countries.  Following the discovery of myrtle rust in New Zealand in 2017, the Ministry for Primary Industries (MPI) and the Department of Conservation (DOC), with the help of iwi and other concerned individuals and groups, ran an operation to attempt to contain and control the disease. However, it quickly became apparent that the disease had spread too quickly, and the focus shifted to managing the myrtle rust in the long term.

Infected plants are usually identified by bright yellow powdery spots on the underside of leaves but can also show other symptoms such as grey powdery spots during the cooler months. To date, there is no known cure for myrtle rust.

But wait, what are myrtles?

Myrtle species are found in tropical and warm-temperate regions of the world. All myrtles are woody (possess stems reinforced with wood) with leathery evergreen leaves that contain oil glands, some of which provide those unmistakeable, woody and camphorous scents including clove, eucalyptus and mānuka. The flowers of myrtles have a base number of five petals, though some are minute or absent, while the stamens are usually very conspicuous, brightly coloured and numerous, such as our native pōhutukawa and rātā, and non-native species such as brush cherry (lilly pilly) and feijoa.

Rātā moehau (Metrosideros bartlettii), New Zealand’s most threatened tree.
📷: Jeremy Rolfe

New Zealand has 37 native myrtles ranging from vines, shrubs and trees, including pōhutukawa and rātā, various rātā vines, mānuka and kānuka, ramarama and maire tawake / swamp maire. All except mānuka (Leptospermum scoparium var. scoparium) are endemic, which means that they can’t be found anywhere else in the world. Several of these are threatened with extinction, or have a very limited distribution which places them at greater risk, such as the critically endangered white flowered rātā moehau (Metrosideros bartlettii), New Zealand’s most threatened tree.

Pohutukawa blossums.
📷: Tahu Taylor-Koolen

More commonly occurring plants such as mānuka, kānuka and pōhutukawa are keystone species in many native ecosystems. Mānuka and kānuka are among our most widespread early colonisers and important in restoration of native forest at a landscape scale. A range of other native plant species such as liverworts and mistletoes depend on our native myrtles, while insects, reptiles (such as our nectar-feeding geckos), and birds like tūī and kākāriki depend on such flowering and fruiting plants and trees for food and shelter.

Harlequin-gecko/tukutuku rakiurae, a nectar-feeding gecko.
📷: Sabine Bernert

Healing, health and money

Mānuka-flowering on the Cape Reinga coastal walkway.
📷: Jeremy Rolfe

Myrtles, both native and introduced, are also important to our economy. Honey made from manuka is a premium product that’s growing steadily as a high-value export for New Zealand, while eucalyptus, feijoa, guava and ornamental plants also contribute.

For Māori, many native myrtles have traditional and contemporary uses as materials for structures and art, are indicators of seasonal change and/or harvest times and are used in rongoā (traditional Māori healing).

Sometimes, we don’t understand quite what’s at risk until it’s gone – it’s difficult to imagine our coastlines without our ‘down under’ Christmas tree, the pōhutukawa, for example.

So, what’s the plan?

The Myrtle Rust Strategic Science Advisory Group (SSAG) was established in June 2019, which immediately set about identifying and prioritising research needs to help combat the spread of myrtle rust in New Zealand. The priority was to:

  • improve the surveillance and monitoring of myrtle rust
  • evaluate the ecological, economic, social and cultural impacts for New Zealand
  • learn about various myrtle species’ resistance and susceptibility
  • identify potential disease control tools likely to be effective against myrtle rust (in the short term)
  • develop a breeding resistance programme
  • provide a Te Ao Māori perspective of the cultural impacts of, responses to, and mātauranga to myrtle rust.

These findings can be found in the Myrtle Rust Science Stocktake, a living document detailing past and current research focussed on biology, impacts and management of myrtle rust.

Sowing the seeds for the future and getting involved yourself

At the same time, DOC undertook New Zealand’s largest targeted seed-collection to safeguard the future of the species. Seed-banking is an ‘insurance policy’ of sorts, in the event myrtle rust causes extinctions. The seed is stored in the New Zealand Indigenous Flora Seed Bank (NZIFSB) located in Palmerston North, where they are assessed, counted, sorted, sterilised and dried below 5% moisture content. Just one of our native species, maire tawake / swamp maire (Syzygium maire) is what is called ‘recalcitrant’ and can’t be stored in this way, requiring cryopreservation of isolated embryos.

Seed collection with a pole pruner.
📷: DOC

Another huge piece of the puzzle in combating myrtle rust is to understand the whereabouts of the disease. This is where you come in. There are easy, visual guides and a 2-hour online training programme for those taking a keen interest in identifying myrtle rust through its various seasonal changes. To report myrtle rust, iNaturalist hosts the Myrtle Rust Reporter project, where you can easily upload your observations.  

For more in-depth information, you may find these documents helpful:

Recent observations of myrtle rust in Tāmaki Makaurau// Auckland

Myrtle rust is predominantly affecting ramarama (Lophomyrtus bullata), rōhutu (Lophomyrtus obcordata), maire tawake (Syzygium maire) and brush cherry (Syzygium australe), including death of their fruit and flowers. Severely infected ramarama were found in urban and natural Auckland habitats, including restoration plantings, as well as in the Waitākere Ranges. Infected garden brush cherry (lilly pilly) hedges have also been found on Waiheke Island. The fungus does not seem to have a ‘dormant’ period here, unlike in other places it is active all year due to our temperate, humid climate.

If you think you see the symptoms of myrtle rust:

  • don’t touch it
  • if you have a camera or mobile phone, take clear photos, including the whole plant, the whole affected leaf, and a close-up of the spores or affected area of the plant, and submit it to the iNaturalist website or via the mobile app where experts can check to confirm whether your identification is correct.

Photographing suspected myrtle rust on ramarama.
📷: Suliana Teasdale

To find out more about myrtle rust, including how to identify it and report it, visit myrtlerust.org.nz