First report of myrtle rust caused by Austropuccinia psidii on Rhodomyrtus tomentosa (Myrtaceae) from Singapore

Austropuccinia psidii (formerly Puccinia psidii) causes rust on species of Myrtaceae. Since 2006, different genotypes of A. psidii have spread to countries in the Pacific and to South Africa (Roux et al. 2016). The pathogen was recently discovered on Eucalyptus pellita and Melaleuca leucadendra in Indonesia (McTaggart et al. 2015). Three suspected specimens of A. psidii were collected in August 2016 from Singapore on leaves of Rhodomyrtus tomentosa (Myrtaceae), a tree native to southeastern Asia. The symptoms were the same as those caused by this pathogen with unmistakable yellow pustules of uredinia on the young leaves and inflorescences. The specimens were lodged in the PREM herbarium, South Africa (PREM 61592, 61593, 61594). Uredinia and telia were present on the abaxial leaf surface of PREM 61592. Urediniospores were ellipsoid to obovoid, 19–24 × 16–21 µm; wall 1.5‒2.0 µm thick, finely echinulate. Teliospores were cylindrical or ellipsoidal, 2-celled, apex rounded, pale yellowish brown, (29–) 35–45 (–50) × 16–23 µm; wall 1‒2 µm thick, smooth; pedicel up to 15 µm long. This was morphologically similar to the description of A. psidii in Australia (available at: http://collections.daff.qld.gov.au/web/key/rustfungi/Media/Html/pucciniapsidii.html) (Shivas et al. 2014). A DNA sequence-based identification was made using the internal transcribed spacer (ITS) and large subunit (LSU) regions of ribosomal DNA. The ITS sequences were 99% identical (551 out of 555 base pairs) to KM282154, the epitype sequence of A. psidii (Machado et al. 2015). The ITS and LSU sequences had 99–100% identity to specimens from Indonesia, with one degenerate base pair difference in the ITS region (McTaggart et al. 2015). The sequences obtained were deposited in GenBank as KY816929 and KY816930. The genotype of the isolates from Singapore was not determined, but may be the same as in Indonesia (McTaggart et al. 2015), which has the pandemic strain, so named due to its spread through the Pacific. This report of A. psidii in Singapore may indicate the rust will spread further through Southeast Asia, into countries such as Malaysia and Thailand