Abstract
Puccinia psidii was long considered a significant threat to Australian plant industries and ecosystems. In 2010, P. psidii was detected for the first time in Australia on the NSW central coast. The fungus spread rapidly and is now widespread on the east coast of Australia. Our studies aimed to determine the host range and impact of P. psidii on species of environmental and commercial significance in Queensland. The effect of the disease on native Myrtaceae of Australia's ecosystems has yet to be determined as the full host and geographic ranges of the disease, and the susceptibility and damage caused to individual species, are assessed. This will depend on a range of factors including the rate of natural and human-assisted spread, and climatic conditions in potentially suitable areas. Commercial impacts are also difficult to measure. The nursery and garden industries have been impacted with losses of stock and subsequent necessity for disease management through plant selection and fungicide programs. The lemon myrtle (Backhousia citriodora) and forestry (Eucalyptus spp.) industries are screening for myrtle rust resistance. Disease development for Puccinia psidii is favoured following periods of rainfall and conditions of high humidity or fog when extended periods of leaf wetness are more likely to be achieved promoting spore germination and infection of the host. It is also necessary for spores to encounter a host plant during stages of active growth or flush, which can occur several times and at different times throughout the year depending on the host species and climatic conditions, with periods of rainfall promoting more active plant growth. The host range of P. psidii has increased rapidly exceeding 160 species from 37 different genera in Queensland. Puccinia psidii has now been identified from a range of native forest ecosystems including coastal heath, coastal and river wetlands, sand island ecosystems, and littoral, montane, subtropical and tropical rainforests. The impact of P. psidii on individual trees and shrubs has ranged from minor leaf spots to dieback and reduced fecundity. Tree death as a result of repeated infection has also been recorded for some species, with regenerating seedlings becoming infected and killed by P. psidii. Puccinia psidii infection has been recorded on flower buds, flowers and fruits of 27 host species. The most highly susceptible species recorded to date are Syzygium jambos, Eugenia reinwardtiana, Agonis flexuosa, Gossia inophloia, Melaleuca quinquenervia, Rhodamnia rubescens, R. maideniana, R. angustifolia, Chamelaucium uncinatum and Decaspermum humile.