First Report of Northern Root-Knot Nematode Meloidogyne hapla on Heracleum moellendorffii in the Republic of Korea

Heracleum moellendorffii Hance is a perennial medicinal herb known by the Korean name "Eosuri". It occurs naturally in Korea, China, and Japan and is traditionally consumed as an edible wild herb and used medicinally in Korea (Kim et al. 2019; Son et al. 2021). However, studies regarding soil-borne pathogens affecting this medicinal crop are still limited, and the occurrence of plant-parasitic nematodes on H. moellendorffii has rarely been documented. During a survey of soil-borne diseases in H. moellendorffii cultivation fields in Gangwon Province, South Korea, root galls were observed at one site located at 37.263537°N, 128.901496°E; 772 m altitude. Diseased plants exhibited reduced leaf size, and gradual chlorosis compared with healthy plants. Approximately 500 g of rhizosphere soil with infected roots were collected from the symptomatic H. moellendorffii plants. The population density of second-stage juveniles (J2s) was recorded at 210 individuals per 100 cm³ of soil. From the galled roots, approximately 30 females were isolated. Based on the morphology and morphometrics of females and J2s, the nematode was identified as Meloidogyne hapla Chitwood, 1949. Female perineal patterns exhibited a low, rounded dorsal arch, distinct punctations around the anus, and well-defined lateral lines. The isolated J2s (n = 20) exhibited a slender, vermiform body and a pointed tail with a distinct hyaline region. Morphometric analysis yielded the following measurements: body length = 390.4 μm (365.0–415.6 μm), tail length = 45.7 μm (39.5–53.3 μm), a = 26.9 (24.8–29.5), c = 8.5 (7.8–9.7), and stylet length = 11.9 μm (10.2–12.9 μm). To further confirm the species, genomic DNA was extracted from single females (n = 10). The internal transcribed spacer (ITS) region, the mitochondrial cytochrome c oxidase subunit I (COI) gene, and the mitochondrial cytochrome c oxidase subunit II (COII) gene were amplified and sequenced using the primer sets TW81/AB28, JB3/JB4.5, and C2F3/1108, respectively (Subbotin et al. 2000; Bowles et al. 1992; Powers and Harris 1993). The sequences were deposited in GenBank under accession numbers PZ626552 for the mitochondrial cytochrome c oxidase subunit I (COI) gene, PZ725265 for the mitochondrial cytochrome c oxidase subunit II (COII) gene, and PZ626509 for the internal transcribed spacer (ITS) region. BLAST analysis showed 99 to 100% identity with M. hapla sequences in GenBank, including PP166667 for COI and AY757889 for COII, MT249016 for ITS. Bayesian analyses of all three regions placed the isolate within a strongly supported M. hapla clade (posterior probability = 1.00). To confirm pathogenicity, a modified Koch’s postulates test was conducted under greenhouse conditions (26 ± 2°C). Healthy H. moellendorffii seedlings grown in a sterile sand–loess mixture (9:1, v/v) were each inoculated with 300 M. hapla eggs. Three replicate pots were prepared for the pathogenicity test. At 12 weeks after inoculation, all inoculated plants developed galls with females and egg masses and showed reduced shoot and root biomass. The final population density reached 3,271 ± 84.1 J2s per 500 ml of soil, corresponding to a reproduction factor (RF) of 10.90. These results confirmed the pathogenicity of M. hapla on H. moellendorffii and indicate that this crop serves as a susceptible host for this nematode. To the best of our knowledge, this is the first report of M. hapla infecting H. moellendorffii in South Korea.