Myrtle rust is the rust fungus Austropuccinia psidii, formerly Puccinia psidii. The fungus has origins in South America where multiple strains/biotypes have been identified. In Australia, only a single type, the pandemic strain, has been identified.

Symptoms of Myrtle Rust (photos courtesy of the Department of Agriculture and Fisheries)

Myrtaceae are the dominant iconic and ecologically important plant species in Australia, with approximately 2,250 native species within 88 genera, we have more than half of the global number of Myrtaceae.

Myrtaceae dominate many fragile and essential ecosystems and there are more species of Myrtaceae than any other plant family in Australia. Species of Myrtaceae are present in 11 out of the 13 major vegetation formations in Australia. Myrtaceae are also culturally significant as well as commercially important including the iconic eucalypts as a timber resource and the developing native food and oil industries.

First detected on guava in Brazil in 1884, myrtle rust has now spread to multiple locations including Australia in 2010.  Since the first detection of A.psidii in Australia (central coast NSW), the spread has been rapid and myrtle rust is now well established in native ecosystems in Queensland and New South Wales, primarily east of the Great Dividing Range.

Spores are readily spread by wind, people, infected plant material, clothing, insects and birds.  The rust requires leaf wetness or moisture to trigger the infection process and when conditions are good for plant growth (including regenerating plants post-fire) then they are good for rust.  Decline in tree health mainly progresses with a “bottom up” progression of foliage loss occurring.  Foliage size of any new growth is dramatically reduced as a result of repeated infection. This has been observed on a range of species.

In Australia, myrtle rust has now been observed in over 350 species from 57 different genera, with 163 species assessed under field conditions.  48 species considered highly or extremely susceptible with severe dieback and tree deaths recorded.

In 2013, surveys of Fraser Island (K’gari) identified rust on five species: Austromyrtus dulcis, Backhousia myrtifolia, Homoranthus virgatus, Melaleuca quinquenervia, and Rhodamnia acuminate.  Thankfully, no evidence of rust was found on Syncarpia hillii (Fraser Island turpentine).

Current research suggests that in just ten years Myrtle Rust could devastate key Myrtaceae communities on the island, notably Melaleuca swamps and rainforest pioneer taxa, and consequently the natural and cultural landscapes dependent on these ecosystems.

There’s a need for urgent research, field surveys and emergency conservation actions (including preservation of collection and preservation of germplasm/seed of key species); identification of potential cultural impacts on both flora and fauna associated with rainforest and wetland Myrtaceae; and impact monitoring of affected species/sites.

Article prepared by Sue Sargent with acknowledgement to Dr Geoff Pegg, Forest Pathologist, Department of Agriculture and Fisheries and Bob Makinson, Plant Biosecurity Cooperative Research Centre.