PATHOGENICITY OF Pythium deliense ISOLATED FROM THE RHIZOSPHERE SOIL OF ORANGE IN VIETNAM

Orange (Citrus sinensis), a major crop in Vietnam, has been suffering from a disease with symptoms of leaf yellowing, gummosis, and root rot. This study identified the associated microorganism of the disease and evaluated its pathogenicity to citrus species. In 2021, two Pythium isolates, assigned as VN-Oo16 and VN-Oo29, were isolated from the rhizosphere soil of symptomatic orange plants in Tuyen Quang province in northern Vietnam, and were identified as Pythium deliense based on morphological and molecular characteristics. The two isolates formed numerous inflated, lobulate, and toruloid sporangia, smooth oogonia, and aplerotic, thick-walled oospores. In the phylogenetic trees based on the Internal Transcribed Spacers (ITS) and cytochrome c oxidase subunit 1 (cox1) sequences, these two isolates and other GenBank P. deliense isolates formed a distinct species cluster. Pathogenicity assays of P. deliense (VN-Oo29 isolate) were conducted by inoculation of mycelium plugs or zoospore suspension on the detached fruits, stems of grafted seedlings, and roots of seedlings grown from seeds. In pomelo and orange, P. deliense produced typical brown rot on wounded fruits and gummosis on wounded stems. P. deliense was not able to infect the roots of pomelo, orange, and lime seedlings. To our knowledge, this is the first report of P. deliense in association with citrus in the world as well as in Vietnam. © 2023, International Society for Southeast Asian Agricultural Sciences. All rights reserved.