Myrtle rust is a particularly damaging and invasive form of rust because it can infect the entire myrtle family, and there are hundreds of known susceptible hosts.
The first visible symptoms are powdery, bright yellow or orange-yellow pustules that develop on leaves, tips and stems. The pustules can progress into wounds on the plant and may cause leaves and shoots to become misshapen or disfigured and die off. The disease can cause deformed leaves, heavy leaf loss from branches, damaged fruits, canopy dieback, stunted plant growth, and eventually may lead to plant death.
Myrtle rust symptoms on plants can look like:
Myrtle rust on a Ramarama plant
Rusts are a kind of fungal disease that affect plants. They get the name rust because they produce large numbers of tiny rust-coloured spores. Myrtle rust is a fungal disease caused by the fungus Austropuccinia psidii that only affects plants in the 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.
When a plant becomes infected with myrtle rust, the disease affects the young, soft, actively growing leaves, shoot tips and young stems. At the very early stages symptoms are invisible to the naked eye. Even in the later stages of infection myrtle rust can be very difficult to detect, as some symptoms can look similar to insect or other types of damage.
As 2019, the most susceptible native myrtle species in New Zealand is ramarama, also known as bubble leaf (Lophomyrtus bullata and its hybrids). This attractive native species is used widely in large-scale plantings as well as in residential and commercial hedging, where the dense young growth that results from repeated pruning may help to create an ideal environment for the fungus to rapidly grow and spread. Relatively high levels of infection have also been found in Metrosideros species (pōhutukawa and a range of related hybrids).
The photos below show how myrtle rust can affect plants over time.