Investigating the Host-Range of the Rust Fungus Puccinia psidii sensu lato across Tribes of the Family Myrtaceae Present in Australia

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

Plant taxa grouped according to disease symptoms at three weeks after inoculation with <i>Puccinia psidii</i> s.l. (ex Australia, DAR81284).

a<p>Scores based on scoring system in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035434#pone-0035434-g001" target="_blank">Figure 1</a>.</p>b<p>Authorship of names presented in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035434#pone.0035434.s001" target="_blank">Table S1</a>.Number in parentheses correspond to tribe number based on Wilson et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035434#pone.0035434-Wilson1" target="_blank">[29]</a>.</p>c<p>Teliospores present on one or more replicates.</p

Scoring system for disease symptoms.

<p>Scoring system used to categorize visible symptoms observed on leaves three weeks after inoculation of plants with <i>Puccinia psidii</i> s.l. (ex Australia, DAR81284). The three leaves with the most severe symptoms on a plant were scored and the highest individual leaf score per plant was assigned to that replicate.</p