First Report of Root-Knot Nematode <i>Meloidogyne incognita</i> on <i>Olea europaea</i> in the Republic of Korea

Olive (<i>Olea europaea</i> L.), a member of the Oleaceae family, is a perennial evergreen tree valued for its fruits and leaves, both widely used for nutritional and medicinal purposes. In response to climate adaptation strategies and alternative income sources, South Korea introduced olive cultivars in 2009. Since then, olive cultivation expanded from 0.2 ha in 2017 to 20.9 ha in 2021 (Choi et al. 2023). Among the most destructive plant-parasitic nematodes (PPNs) affecting olives are root-knot nematodes (RKNs, <i>Meloidogyne</i> spp.) and root-lesion nematodes (<i>Pratylenchus</i> spp.). To date, eight <i>Meloidogyne</i> species have been reported in olive rhizospheres, contributing to reduced vigor, root galling, and yield losses of 5-10% (Sasanelli et al. 2002; Archidona-Yuste et al. 2018). During a recent survey of PPNs in olive orchards, root galls were observed on trees growing in a commercial plantation located in Jeju (33°29'01.6"N, 126°41'07.6"E; altitude 207 m). Approximately 1 kg of rhizosphere soil, including galled root fragments, was collected from affected trees for analysis. Morphological traits of females and second-stage juveniles (J2s) were consistent with <i>Meloidogyne incognita</i>. The perineal patterns of females were oval to rounded with high dorsal arch, flattened at the top, striae usually wavy, weakly demarcated by forked striae. Second-stage juveniles (J2s) were characterized by a slender, elongated, filamentous body; a stylet with a distinct, rounded knob at its terminus; a median bulb with a clearly defined valve in the esophageal region; and a tapering tail. Mean measurements (N=10) of J2s were a body length of 368 ± 13.5 μm (337.2-387.8 μm), body diameter of 13.7 ± 0.9 μm (12.5-15.6 μm), stylet length of 13.6 ± 0.6 μm (13.0-14.9 μm), tail length of 44.7 ± 1.6 μm (41.1-47.0 μm), tail diameter of 9.4 ± 1.2 μm (7.9-11.8 μm), and hyaline tail terminus length of 9.6 ± 1.7 μm (7.9-12.4 μm). To further confirm the species, mitochondrial DNA from cytochrome c oxidase subunit II to 16S rRNA were amplified and sequenced using the C2F3/1108 primer set (Powers and Harris 1993). Resulting sequences (PV607132 and PV607133) were at least 99% identical with 100% query coverage to other sequences of <i>M. incognita</i> in Genbank (LC547506 and JX100438). A species-specific <i>HinfI</i> site, distinguishing <i>M. incognita</i>, was identified, and phylogenetic analysis further supported its classification as <i>M. incognita</i>. In addition, the nuclear ribosomal DNA regions, including the D2-D3 expansion segments of the 28S rRNA gene (PV603641-PV603642) and the internal transcribed spacer (ITS) region (PV602317-PV602318), were amplified and sequenced (Oliveira et al. 2023). The resulting 28S rRNA and ITS sequences showed 100% identity with <i>M. incognita</i> sequences in GenBank (ON118556 and MT490915, respectively). To confirm pathogenicity, a modified Koch's postulates test was conducted under greenhouse conditions, inoculating 300 eggs from a single egg mass onto each of three <i>O. europaea</i> cv. <i>Leccino</i> plants. After three months, root galls containing females and egg masses were observed on inoculated plants, and the reproductive factor was 2.91 with a standard deviation of 0.359. To the best of our knowledge, this study is the first report of <i>M. incognita</i> infecting <i>O. europaea</i> in South Korea. As olive cultivation in Korea is in its early stages, regular monitoring, early detection, and integrated control are essential to prevent RKNs spread and ensure sustainable olive production.