New Phytopythium species associated with water and soil in Vietnam and USA.

Phytopythium is a recently established genus with sporangial morphology similar to Phytophthora and zoospore discharge similar to Pythium. It contains 27 officially recognized species including plant pathogens as well as aquatic saprophytes. Among the most widely distributed are P. helicoides and P. vexans, followed by P. oedochilum and P. litorale, each having been reported on six continents. This study uses morphological and molecular analyses to describe two novel Phytopythium species: P. phamii from river water in Vietnam and P. virginianum from an irrigation pond in the USA and citrus orchard soil in Vietnam. Based on the maximum likelihood and Bayesian phylogenies, P. virginianum and P. phamii generally grouped with members of the Phytopythium clade 2, near P. chamaehyphon and P. helicoides. Isolates of both species grew at temperatures between 7 and 35 C, with an optimum temperature of 30 C for P. phamii and 35 C for P. virginianum. Although abundant oospore production was observed in cultures of P. phamii, none was observed in cultures of P. virginianum on any of the media used. The formal description of these new species helps scientists and regulators understand the geographic distributions of organisms and identify potential emerging pathogens that can become a threat to agricultural crops.