Myrtle rust, caused by Austropuccinia psidii, is an invasive fungal disease that affects plants in the myrtle family, the Myrtaceae. Of the almost 6,000 Myrtaceae species worldwide about 480 are currently recorded as hosts, but the host range is considered to be expanding. Myrtle rust originated in South America and has spread across the world at an increasing rate since the 1930s; it is now in more than 20 countries. Across the Pacific Ocean, it arrived in Hawaii in 2005, Australia in 2010, New Caledonia in 2013 and New Zealand in 2017. It is believed to have been carried to mainland New Zealand by wind from Australia across the Tasman Sea. The first New Zealand detections were on Raoul Island in the Kermadec Group in March 2017 and the first detection on the mainland was in Northland in May 2017.
Myrtle rust epidemics are caused by the uredinial stage of A. psidiiand the strain in New Zealand is the ‘pandemic biotype’, which is the same as that found in Australia and Hawaii. Signs of infection are pustules (uredinia) containing bright yellow, powdery uredinio spores on infected leaves and stems, and dieback of young growing shoots. In some species it also infects flowers and fruit. Myrtle rust survives only on living plants and only infects new expanding shoots, buds, flowers and fruits. When warm moist conditions occur with a high spore load and a flush of new growth, myrtle rust can destroy all the growing shoots on highly susceptible plants. It has been reported in Australia that repeated infections over several years have killed mature trees, and species extinction in some natural habitats is reported to be occurring there.