No abstract

A rust affecting Myrtaceae was recently detected in New South Wales, Australia. Based on urediniospore morphology and host range, it was identified as Uredo rangelii, a taxon regarded as a member of the eucalyptus/ guava rust (Puccinia psidii sensu lato) complex, although confusion currently surrounds its taxonomy. The exotic rust was given the common name of myrtle rust to distinguish it from eucalyptus/guava rust. The more recent discovery of teliospores in NSW that match those of P. psidii sensu stricto indicates the rust in Australia is a strain (with tonsured urediniospores) of P. psidii s.l. Outside Australia, P. psidii has a wide host range within Myrtaceae, being reported from 129 species in 33 genera, and is very damaging in South and Central America-including in eucalypt plantations in Brazil-the Caribbean and in Florida and Hawaii. To ascertain the potential threat to forestry in Australia posed by the introduced rust, we tested key forestry species, as well as key known hosts of eucalyptus/ guava rust, in artificial inoculation experiments. We showed that several species of Eucalyptus are susceptible (viz. E. pilularis, E. cloeziana, E. agglomerata and E. grandis), as is Melaleuca quinquenervia. Observations during testing revealed a lengthened latent period (from inoculation until pustule formation and eruption) of four to five weeks during winter. Here we also report on observations on new hosts from surveys in NSW under the emergency response that followed the detection of the exotic rust, and surveys in NSW and Queensland following the cessation of the emergency response. In Australia, P. psidii s.l. has currently been found on 107 host species in 30 genera during surveys, including species in Angophora, Asteromyrtus, Tristaniopsis, Ugni, Uromyrtus and Xanthostemon. Species under cultivation (in nurseries and gardens) that are severely affected include Gossia inophloia, Agonis flexuosa, Syzygium jambos and S. anisatum while species that are severely damaged in native bushland include Rhodamnia rubescens, Rhodomyrtus psidioides, Choricarpia leptopetala and Melaleuca quinquenervia.

The exotic rust fungus Puccinia psidii sensu lato was first detected in Australia in April 2010. This study aimed to determine the host-range potential of this accession of the rust by testing its pathogenicity on plants of 122 taxa, representative of the 15 tribes of the subfamily Myrtoideae in the family Myrtaceae. Each taxon was tested in two separate trials (unless indicated otherwise) that comprised up to five replicates per taxon and six replicates of a positive control (Syzygium jambos). No visible symptoms were observed on the following four taxa in either trial: Eucalyptus grandis×camaldulensis, E. moluccana, Lophostemon confertus and Sannantha angusta. Only small chlorotic or necrotic flecks without any uredinia (rust fruiting bodies) were observed on inoculated leaves of seven other taxa (Acca sellowiana, Corymbia calophylla ‘Rosea’, Lophostemon suaveolens, Psidium cattleyanum, P. guajava ‘Hawaiian’ and ‘Indian’, Syzygium unipunctatum). Fully-developed uredinia were observed on all replicates across both trials of 28 taxa from 8 tribes belonging to the following 17 genera: Agonis, Austromyrtus, Beaufortia, Callistemon, Calothamnus, Chamelaucium, Darwinia, Eucalyptus, Gossia, Kunzea, Leptospermum, Melaleuca, Metrosideros, Syzygium, Thryptomene, Tristania, Verticordia. In contrast, the remaining 83 taxa inoculated, including the majority of Corymbia and Eucalyptus species, developed a broad range of symptoms, often across the full spectrum, from fully-developed uredinia to no visible symptoms. These results were encouraging as they indicate that some levels of genetic resistance to the rust possibly exist in these taxa. Overall, our results indicated no apparent association between the presence or absence of disease symptoms and the phylogenetic relatedness of taxa. It is most likely that the majority of the thousands of Myrtaceae species found in Australia have the potential to become infected to some degree by the rust, although this wide host range may not be fully realized in the field.