Myrtle rust is a disease caused by the fungus Austropuccinia psidii. The spores of this fungus are wind-borne, which allowed the disease to jump across the Tasman from Australia in 2017.
Myrtle rust attacks plants in the Myrtaceae family, causing damage and (for some species) death. Pōhutukawa, mānuka and kānuka are some of Aotearoa New Zealand’s most iconic Myrtaceaeous species.
Other plants, like ramarama (Lophomyrtus bullata) and rōhutu (Lophomyrtus obcordata) and their natural hybrids are less well-known but no less important to our ecosystems. Lophomyrtus species are also used as garden plants.
Myrtle rust is a choosy disease. It attacks new growth, including leaves, stems, buds, and flowers. The first signs of infection are often pustules of yellow spores erupting through the leaf’s surface. As the infection progresses, the new growth starts to die, and the pustules turn a grey-ish color. The whole plant can look burned around the edges.
Some plants are more susceptible than others. Susceptibility depends on age as well as species. For example, pōhutukawa seedlings can be hard hit by disease. However, adults are not as hard hit, with infections generally occurring on new growth at the base of the tree instead of in the canopy.
At a species level, ramarama and rōhutu are most susceptible to the disease while mānuka and kanuka are less susceptible.
As with any disease, all this could change. A warmer, wetter New Zealand could result in an explosion of myrtle rust. With an increased level of spores in the environment, species that are currently doing okay could become infected more frequently.
It may seem a bit hopeless, but there are things you can do to help:
- Report suspected infections on iNaturalist
- If you go tramping, keep your gear clean
- If you find it in your garden, dispose of infected material safely
- If you are choosing new plants for your garden, stay away from exotic Myrtaceae and choose alternative plants
If you’d like to stay up to date on the research into myrtle rust, follow Beyond Myrtle Rust on social media. Beyond Myrtle Rust is a research programme that is investigating the fungus and the plants it infects for clues to understand and combat the disease.