Identification of Radopholus duriophilus from Laos with remarks on R. similis and its phylogenetic relationship within the genus Radopholus

Summary Burrowing nematodes, Radopholus spp., have been consistently detected in the rhizosphere medium and root samples of various plant seedlings imported from Ecuador, Laos, Indonesia and Thailand in Yunnan province, P.R. China. Two isolates of Radopholus duriophilus were intercepted in the rhizosphere medium and root samples of Durio zibethinus and Syzygium hainanense seedlings from Laos, respectively. Six isolates of R. similis were intercepted in the rhizosphere medium and root samples of various seedling plants from three countries. They were characterised using morphological, morphometric and molecular criteria. The two newly recorded isolates of R. duriophilus share common morphological and morphometric characteristics but differ to some degree from their original description in Vietnam. They are characterised by: sexual adults with four lateral lines, with the central band mostly distinctly narrower than two outer bands, female labial region with 3-4 marked annuli of equal width, excretory pore located at, or slightly posterior to, the level of the pharyngo-intestinal junction, spermatheca filled with round or oval-shaped sperm depending on the viewing angle, rarely empty and post-rectal intestine sac present. The population ratio of females to males was approximately 20:1. The six newly recorded isolates of R. similis share common morphological characteristics with slight morphometric differences from some isolates in the literature. Phylogenetic analyses were performed using ITS and 28S D2-D3 rRNA gene sequences, as well as mitochondrial cytochrome oxidase subunit I (mt COI ) sequences from burrowing nematode species, including newly obtained sequences of two isolates of R. duriophilus and six isolates of R. similis , along with other related genera. All Bayesian trees indicate that two populations of R. duriophilus from Laos have the closest phylogenetic relationship with those from Vietnam, while six populations of R. similis exhibit the closest phylogenetic relationships with related populations of R. similis . Furthermore, R. duriophilus is documented as a new geographical record for Laos and as a new host record for Syzygium hainanense .