First Report of <i>Meloidogyne incognita</i> Infecting <i>Siegesbeckia orientalis</i> in China: Guizhou Province

ABSTRACT In September 2024, the medicinal plant Siegesbeckia orientalis was discovered growing as a weed in tobacco fields in Weining County, Guizhou Province, China, showing symptoms of stunting and root knot formation. To identify the root‐knot nematode species infecting S. orientalis , a combination of morphological observation, sodium hypochlorite‐acid fuchsin staining, molecular identification (SCAR marker and 28S rDNA D2‐D3 region sequence alignment) and pathogenicity tests was used. Staining observations revealed the presence of purplish‐red pear‐shaped adult females, slender second‐stage juveniles, and oval eggs within the roots. The female stylet measured 15.4 ± 0.8 μm in length, with the dorsal oesophageal gland orifice located 3.0 ± 0.5 μm from the stylet base. The perineal pattern was rounded to oval, featuring a high dorsal arch and fine, dense striations without distinct lateral lines. Second‐stage juveniles had a body length of 378.5 ± 22.4 μm, a stylet length of 11.2 ± 0.9 μm, a dorsal oesophageal gland orifice situated 2.8 ± 0.4 μm from the stylet base, and a tail length of 46.7 ± 4.3 μm. Molecular identification, based on SCAR markers and sequencing of the 28S rDNA D2—D3 region (GenBank accession numbers PQ885372.1 and PX710698.1), further confirmed the species, showing sequence similarities of 100% and 99.40%, respectively. The pathogen was identified as Meloidogyne incognita . This study reports the first incidence of M. incognita infecting S. orientalis in Guizhou Province, China. This finding extends the known host range of this nematode and highlights the ecological risk associated with its persistence and dispersal through weed hosts within agricultural ecosystems, which provides a critical basis for developing integrated management strategies.