No Direct Observational Evidence for Basidiospore-Derived Infection of Eucalypt and Rose Apple (Syzygium jambos) by the Myrtle Rust Pathogen, Austropuccinia psidii, From Brazil

Abstract

Myrtle rust caused by Austropuccinia psidii is one of the most important diseases affecting eucalypts (Eucalyptus spp. and Corymbia spp.) in Brazil. Asexual spores of the myrtle pathogen, urediniospores, give rise to infection of young tissues (e.g., leaf, stem, flower and fruit) of the myrtaceous hosts. Symptoms of myrtle rust disease are necrosis and leaf deformation, and death of the apical shoots, whereas the primary diagnostic signs of myrtle rust disease are yellow urediniospores, which are formed by the myrtle rust pathogen on the infected organs. Occasionally, teliospores are formed in brown pustules, usually at higher temperatures on more mature leaves of susceptible hosts, and these teliospores can germinate to produce basidia that produce basidiospores. Previous studies have demonstrated that genetic recombination in A. psidii was associated with basidiospores of the South African and Pandemic biotypes of A. psidii on rose apple (Syzygium jambos). However, it remains unconfirmed whether basidospore-derived infection and genetic recombination also occur within the most damaging biotype of A. psidii in Brazil. Furthermore, direct observation of basidiospore-associated infection by A. psidii has not been previously reported. In this study, we found no evidence of basidiospore germination and/or germ tube penetration into the host tissue of eucalypt (E. urophylla) and rose apple (S. jambos) using scanning electron microscopic observations, raising questions about the role of basidiospores in the life cycle of A. psidii in Brazil.