Current status and characterization of Phytophthora species associated with gummosis of citrus in Northern Vietnam

Abstract Phytophthora species have long been reported to cause Phytophthora disease in citrus with symptoms of gummosis, root rot and fruit brown rot. Due to the lack of an up‐to‐date investigation of Phytophthora species currently causing the disease in Northern Vietnam, this study aimed to determine the current Phytophthora species associated with the disease in this region. An extensive collection of diseased samples was conducted in the main citrus‐growing areas in Northern Vietnam from 2021 to 2022. Fifty‐four Phytophthora isolates were obtained and initially divided into four groups based on morphological characteristics, including colony morphology, mycelial growth rate and reproductive structures. Representative Phytophthora isolates from each group were then identified as P. palmivora , P. nicotianae , P. citrophthora and P. mekongensis based on the phylogenetic analysis of ITS and cox 1 gene sequences. Pathogenicity tests showed that these four Phytophthora species induce gummosis and brown rot on pomelo, orange and lime, with pomelo being the most susceptible, followed by orange and lime, respectively. Koch's postulates were confirmed. All the Phytophthora species were sensitive to various fungicides including mancozeb, metalaxyl, chlorothalonil, fosetyl aluminium, potassium phosphonate and dimethomorph, of which metalaxyl was the most potent inhibitor of colony growth. Our findings provide updated information on the current Phytophthora species associated with gummosis in citrus in Northern Vietnam, with P. mekongensis being recorded for the first time in this region. This will serve as a reference for Phytophthora studies on citrus in the future and for developing an integrated management strategy for Phytophthora disease.