Meloidogyne mali was originally described in Japan on roots of an apple rootstock (Malus prunifolia) (Itoh et al. 1969) and found on elm trees in Italy, Netherlands, Belgium, France and United Kingdom, and euonymus in the USA (EPPO 2018; Prior et al. 2019). In Italy, the nematode was initially described as a new species, Meloidogyne ulmi, but was later synonymized with M. mali (Ahmad et al., 2013). During the study of Meloidogyne species in Republic of Korea, galled roots were found on Acer palmatum collected in Naejangsan National Park, Republic of Korea located at 35°29'29.1"N, 126°55'42.7"E, altitude 147.8 m. Morphologically, the perineal patterns of the females was very similar to M. mali due to rounded dorsal arch and smooth, finely-spaced, indistinct striae. Lateral field shallow, narrow, and faint. Phasmids large, very distinct. Head region of second–stage juveniles flattened anteriorly to hemispherical, slightly set-off from body, without annulations, low head cap. Stylet slender, sharply pointed cone, cylindrical shaft with rounded knob sloping posteriorly. Tail conoid with irregular, and rounded end. Rectum undilated. Several micrographs were made from 25 J2s and females for mean, standard deviation and range. J2s were measured with a body length: 408.2 ± 25.1 (366-449) µm, maximum body width: 15.9 ± 1.0 (14.1-17.9) µm, stylet length: 14.1 ± 0.5 (13.1-15.3) µm, hyaline tail terminus: 10.0 ± 0.9 (8.3-11.0) µm and tail length: 31.7 ± 3.0 (26.0-36.1) µm. Females (n=25) were characterized by a body length: 656.7 ± 102.7 (516-947) µm, a stylet length: 16.4 ± 2.2 (13.9-19.0) µm, a vulval slit length: 22.2 ± 1.8 (19.8-25.7) µm, and a vulva-anal distance: 20.2 ± 2.4 (17.1-25.4) µm. Morphological measurements and configuration of perineal patterns (Fig. 1S) were comparable to M. mali (Itoh et al. 1969; Ahmed et al. 2013; Gu et al. 2020). To confirm pathogenicity, a modified version of Koch’s postulates was conducted in the greenhouse by inoculating 300 eggs from a single egg mass onto each of three, two-year-old A. palmatum plants, grown in sterilized sandy soil. After about one year, symptoms developed on the maple tree roots, with numerous galls containing females and egg masses by visual inspection. In addition, PCR was performed for the 28S rDNA D2-D3 segment and ITS region using the primers D2A, D3B, TW81 and AB28. The resulting sequences (MW522548, MW522549, MW523004 and MW523005) were at least 99% identical to other 28S rDNA D2-D3 segment and ITS region sequences on Genbank (MT406757 and JX978229). The molecular phylogenetic relationships of this species strongly supports M. mali (Fig. 2S). To the best of our knowledge, this is the first report of M. mali in Republic of Korea. The host range of M. mali includes many species which are of economic importance in fruit trees (e.g. apple, chestnut, fig, mulberry), forestry trees (e.g. elm, maple, oak, Yew), and vegetable crops (e.g. cabbage, carrot, cucumber, eggplant, soybean, watermelon). The potential danger to these economically important plants caused M. mali to be added the EPPO Alert List and also the Quarantine List of the Korean Animal and Plant Quarantine Agency. Additionally, in our survey around the Naejangsan National Park, M. mali was not found on other economically important host crops, such as grapes. Although this nematode was not detected other crops, it requires regular monitoring because it poses a serious threat to the future production of these crops.

Meloidogyne incognita is a devastating plant parasitic nematode that causes root knot disease in a wide range of plants. In the present study, we investigated host-induced RNA interference (RNAi) gene silencing of chitin biosynthesis pathway genes (chitin synthase, glucose-6-phosphate isomerase, and trehalase) in transgenic tobacco plants. To develop an RNAi vector, ubiquitin (UBQ1) promoter was directly cloned, and to generate an RNAi construct, expression of three genes was suppressed using the GATEWAY system. Further, transgenic Nicotiana benthamiana lines expressing dsRNA for chitin synthase (CS), glucose-6-phosphate isomerase (GPI), and trehalase 1 (TH1) were generated. Quantitative PCR analysis confirmed endogenous mRNA expression of root knot nematode (RKN) and revealed that all three genes were more highly expressed in the female stage than in eggs and in the parasitic stage. In vivo, transformed roots were challenged with M. incognita. The number of eggs and root knots were significantly decreased by 60–90% in RNAi transgenic lines. As evident, root galls obtained from transgenic RNAi lines exhibited 0.01- to 0.70-fold downregulation of transcript levels of targeted genes compared with galls isolated from control plants. Furthermore, phenotypic characteristics such as female size and width were also marginally altered, while effect of egg mass per egg number in RNAi transgenic lines was reduced. These results indicate the relevance and significance of targeting chitin biosynthesis genes during the nematode lifespan. Overall, our results suggest that further developments in RNAi efficiency in commercially valued crops can be applied to employ RNAi against other plant parasitic nematodes.

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To evaluate the efficacy of natural nematicides for the control of root-knot nematode in strawberry greenhouses, commercial essential oils were examined by 24-well culture plate bioassay for their nematicidal activities against second-stage juveniles and eggs of Meloidogyne hapla. Based on the mortality of M. hapla juveniles at a concentration of 125 μg/mL, the most active essential oil was Alpinia galanga (100%), followed by Carum carbi (22.3%), Eugenia caryophyllata (9.4%), Cinnamonum zeylanicum (7.2%), Mentha pulegium (2.4%), and Foeniculum vulgare (2.1%). Moreover, A. galanga significantly reduced hatching at 7, 14, and 21 days after treatment. The volatile constituents identified in the A. galangal oil were methyl cinnamate (87.4%), 1,8-cineole (4.4%), β-pinene (2.5%), α-pinene (2.2%), and p-cymene (1.1%), as major constituents. Results of this study show that A. galangal essential oil and its major constituents may serve as an environmental friendly agent of a promising natural nematicide to control Meloidogyne spp.