Morphological and molecular characterisations of Helicotylenchus laichauensis sp. n. associated with Lai Chau ginseng (Panax vietnamensis var. fuscidiscus) from Vietnam.

A new spiral nematode species, Helicotylenchus laichauensis sp. n., is described from the rhizosphere of Panax vietnamensis var. fuscidiscus in northern Vietnam. The new species is characterised by a hemispherical lip region with 4-5 annuli, a lateral field with four incisures whose inner lines fuse distally into a distinct m-shaped pattern on the tail, and a well-developed stylet 24.5-26.7 μm long, with knobs flattened to slightly indented anteriorly and a conus forming 50.1-53.5% of the total stylet length. The spermatheca is round and filled with sperm, and males are common. The tail is conical, dorsally convex, tapering to a prominent ventral projection bearing a variable mucro; phasmids are located 3-7 annuli anterior to the anus. Based particularly on similarities in the lip region structure, stylet morphology, lateral field pattern, phasmid position, and tail configuration, the H. laichauensis sp. n. is close to the known species namely H. microcephalus, H. craigi, H. oleae, and H. amplius. Phylogenetic analyses based on the D2-D3 expansion segments of 28S, partial 18S, and ITS rRNA sequences revealed that the newly generated sequences of the new species form a maximally supported clade with corresponding sequences of Helicotylenchus microcephalus in all constructed trees.