RESUMEN
A severe outbreak of rust disease was observed on Prunus species, P. persica (peach), P. persica var. nectarina (nectarine), and P. salicina (Japanese plum) cultivated in northern Thailand in the rainy season. Previous reports have identified the causal agents as Tranzschelia discolor and T. pruni-spinosae based on only morphological characteristics. Thus, the aim of this study was to identify rust fungi of Prunus spp. based on morphology and molecular analyses. Between May and July in 2020 and 2022, 18 isolates were collected from five areas in Chiang Mai and Chiang Rai provinces. Symptoms of rust consisted of cinnamon brown pustules of uredinia that were hypophyllous and visible as pale greenish to chlorotic yellowish angular spots on the upper leaf surfaces. Urediniospore shape, size, and color were similar to T. discolor. Molecular analysis of internal transcribed spacer (ITS) and partial 28S large subunit (LSU) region rRNA genes confirmed the isolates to be T. discolor. This is the first report of P. salicina as a host of T. discolor in Thailand.