Alina S. Puig, PhD
USDA Agricultural Research Service, Foreign Disease – Weed Science Research Unit
Fort Detrick, MD, USA
Emily Pfeufer, PhD
Risk Analyst
USDA Animal and Plant Health Inspection Service (APHIS)
Frederick, Maryland, United States
Timothy Widmer
USDA-ARS
Beltsville, Maryland, United States
Glen Groben
ORISE
Frederick, Maryland, United States
Lindsay Harrison
USDA-ARS
Frederick, Maryland, United States
Pham Thu
Vietnamese Academy of Forest Sciences
Bac Tu Liem District, Ha Noi, Vietnam
To determine the diversity of oomycetes in Vietnam, which is within the presumed center of origin of most Phytophthora taxa, isolations were done from rivers, agricultural soil, and forested areas. Identifications were made using sequences from the internal transcribed spacer (ITS) and the cytochrome 2 oxidase (cox2) regions of the genome. Of the 245 isolates included in this study, most (66.5%) were identified as Phytopythium spp., followed by Phytophthora spp. (31%) and Pythium (2.4%). The most prevalent species were Phytopythium vexans and Phytophthora cinnamomi, making up 51.8 and 24.5% of all organisms obtained. Seventeen isolates appear to belong to multiple undescribed species. A single isolate of Phytophthora and Pythium were obtained from agricultural soils, compared with 17.2% of Phytopythium isolates. An undescribed Phytopythium sp. isolated numerous times from soil in commercial citrus production areas was tested for pathogenicity on citrus roots alongside known oomycete pathogens of the crop. Inoculated plants developed 42.5% root necrosis after 5 weeks, compared with 33.2% and 28.8% following inoculation with Phytophthora palmivora and P. nicotianae, respectively. These differences were not statistically significant. The relevance of these results to pathogen emergence and the potential impact on the citrus industry will be discussed.