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First Report of Meloidogyne incognita on Cardamine Violifohia in China.

Cardamine violifohia is an economically-important medicinal plant, and also a valuable plant for strong ability to accumulate selenium (Se) (Ebba et al. 2020). It is not only be used to extract selenium protein and selenium polysaccharide, but also widely used to develop selenium-supplement reagent. In September 2020, root-knot nematodes (RKN; Meloidogyne spp.) infection experiments showed that galls and egg masses were observed on the roots of numerous C. violifolia plants in Enshi (30°32'25.67″ N; 109°48'48.46″ E), Hubei Province, China. Meanwhile, the overground plants of C. violifohia were stunted and leaves were yellow. Almost 5% C. violifohia plants were affected by the disease. The roots with galls were collected, and nematodes were dissected and extracted (Fig. S1). Based on phytopathological clinic, the number of galls on each plant was 91.87 ± 19.01, and egg masses was 15.27 ± 5.36 (n = 15). Nematodes and galls were collected from soil and infected roots (Barker 1985). The morphological diagnostic of the nematode species was measured as follows. Measurements of adult females (n=20), body length = 628.15 ± 73.69 μm, width = 356.77 ± 36.72 μm, stylet length = 15.58 ±0.93 μm. Meanwhile, a high and trapezoidal dorsal arch with thick striations was observed in the perineal region of females. Second-stage juveniles (J2s) (n=20): body length = 377.09 ± 18.19 μm, body width = 15.64 ± 1.24 μm, stylet length = 13.31 ± 1.04 μm, tail length = 42.49 ± 4.64 μm, hyaline tail terminus = 12.35 ± 2.02 μm and presented well developed esophageal glands. Eggs (n=20): length = 80.81 ± 3.47 μm, and width = 37.09 ± 2.98 μm. All the morphological characteristics of the identified species were consistent with the descriptions of Meloidogyne incognita (Kofoid & White, 1919) Chitwood, 1949 (Whitehead, 1968). Molecular identification was carried out by PCR with the M. incognita-specific primers Mi-F/Mi-R (Meng et al. 2004) and 28S rDNA D2/D3 region primers MF/MR (Hu et al. 2011). The target fragments of 955 bp and 478 bp amplified by of the primer pairs Mi-F/Mi-R and MF/MR were observed under a UV light, which confirmed that these nematodes collected from C. violifohia were M. incognita (Fig. S2). Fragments were, sequenced (MZ596342 and MZ566843, respectively) and aligned with available sequences on NCBI, which were 100% identical to the MK410954, MN728679, and MK410953, MF177882 M. incognita sequences, respectively. Pathogenicity testing was conducted to perform Koch's postulates in a greenhouse by inoculation of 500 J2s from the original population into C. violifohia seedlings (n = 30, 5-6 leaves stage). After 7 weeks, all inoculated plants exhibited the same symptoms that observed in the field initially. Different life stages of M. incognita were observed in dissected galled tissues. The average reproductive factor was 37.30 ± 6.13, which is considered as the pathogenicity of M. incognita to C. violifohia. Therefore, C. violifohia is a suitable host for M. incognita in China. The growers should be informed of the current findings to avoid serious economic losses that might be caused by this pathogenic nematode, and prepare for proper management action. To our knowledge, this is the first report of M. incognita infecting C. violifohia in China.