Myrtle rust

An invader in New Zealand’s ecosystems

Figure 1 Various stages of myrtle rust infection on myrtle plants in New Zealand. Images (a), (b) and (c) by Peter de Lange/pjd1 via iNaturalist (CC0) and image (d) by jacqui-nz via iNaturalist (CC BY-NC 4.0).

Myrtle rust is thought to have originated in South and Central America, and since reaching Hawaii in 2005, the spread has increased exponentially.[1] Recent research confirms that Aotearoa New Zealand natives are susceptible to both the pandemic and South African strains.[2] The arrival of myrtle rust to Aotearoa New Zealand was expected given the severity of infection in eastern Australia combined with the ability for the spores to disperse extremely long distances by wind.

Rusts are fungal diseases that typically affect just one species of plant. Myrtle rust is unusual compared to other rusts as it affects hundreds of species globally [3] and is expected to impact 37 native taxa, including species of pōhutukawa, mānuka, kānuka and rātā.[4] It can attack the new growth, flowers and fruits of infected plants. Repeated infections reduce the plant’s ability to regenerate and mature.[5]

Figure 2 Two iconic native species threatened by myrtle rust in Aotearoa New Zealand: (a) Mānuka (Leptospermum scoparium) by jacqui-nz (CC-BY-NC-SA 4.0) via iNaturalist; (b) Pōhutukawa (Metrosideros excelsa) by E. Rykers.

The fungus can reproduce quickly (between two and four weeks) in conditions of high humidity and temperatures around 25°C. After infection, it will take up to two weeks for the disease symptoms to present, which makes it hard to identify in the early stages. Most fungi reproduce asexually, but myrtle rust can reproduce sexually. Sexual reproduction introduces more genetic variability, meaning myrtle rust may adapt more rapidly—with the potential it might infect new host plants as a result.[6] If the South African strain arrived in Aotearoa New Zealand, in addition to the damage it would cause, there is also concern that the two strains could sexually reproduce to potentially form more aggressive new strains.[2] Myrtle rust is mostly transmitted by wind, but spores can also be spread via contaminated clothing, insects and equipment.[7]

The species that are affected by myrtle rust support our environment through ecosystem services. These include providing habitat for other organisms, producing food for pollinators and the process of nutrient turnover. They also provide benefits for humans such as food, medicine and construction materials, and some species were extensively used by Māori.[8, 9] Owing to the cultural and practical values, some of these species are formally recognised as taonga (treasures).[10, 11] MPI suggest that ramarama and pōhutukawa appear to be the most susceptible species.[12] While mānuka previously showed few symptoms of infection, recent research has found that myrtle rust can infect mānuka fruit and seeds. A lack of symptoms does not equal resistance, however, and research is ongoing to identify variants across the myrtle family with resistance.[13] There is particular concern for already threatened species, such as the critically endangered rātā moehau (Bartlett’s rātā, Metrosideros bartletti) which could be vulnerable to myrtle rust.

​To safeguard native myrtle species, seed collection began in 2017 led by the Department of Conservation (DOC) with the assistance of the New Zealand Indigenous Flora Seed Bank (NZIFSB). These collections aim to act as insurance policy against regional or national extinctions. As of July 2019, 49% of the seed required has been collected.[16] Work is now being undertaken to assess the quality and quantity of collections, including the viability of the oldest collections of stored seed. A nationwide strategy for preserving native Myrtaceae co-designed with mana whenua is still required with adequate resources and infrastructure to implement it.[4]

How myrtle rust will behave in our largely temperate climate and how it will impact ecosystems is not well understood. Nationwide data are no longer being collected on its spread, intensification and impacts, with limited resource available to carry out robust, long-term, national surveillance and monitoring in the future.[4] MPI-funded research has allowed the development of a monitoring form to assist community-led tracking. Furthermore, the societal implications of myrtle rust in Aotearoa New Zealand, including those for Māori communities, have only been partially explored.[8, 17] In particular, Māori are at risk of losing cultural identity as some species affected by myrtle rust are threatened with local extinction in their rohe. Myrtle rust is expected to have considerable negative impact on the economy with losses in GDP between $144 and $411 million estimated by 2030.[13]

There is a Strategic Science Advisory Group (SSAG) in place to support and guide the development of a prioritised research program through the recent ‘Myrtle Rust Science Plan’.[20] The plan has identified the areas requiring investment as:

• Exploring tools for surveillance, monitoring and understanding the impact of disease
• Research focusing on the epidemiology of myrtle rust, how this affects ecosystems and how to encourage resilience to myrtle rust
• Developing a Te Ao Māori and Mātauranga Māori framework and strong co-design and co-implementation practices
• Understanding the social links between people and ecosystems, and the associated socioeconomic factors
• Implementation of effective species conservation, disease control and management

Last updated: 30 April 2021