First Report of Meloidogyne incognita on Coral dealbatus in Shaanxi, China.

Coral dealbatus belonging to Crassulaceae, is a new kind of health care vegetable as both medicine and food (Qin et al., 2022). Because of its obvious health care function, C. dealbatus was widely cultivated in China and market demand increased quickly. In August of 2022, a large number of C. dealbatus showed the symptoms of stunting and leaf yellowing in Dali county, Weinan, Shaanxi province, China (109°43'E, 34°36'N). Many galls were observed on the roots of infected plants, and females were observed under the plant epidermis. Infected roots and soil samples were collected, the females, males and second-stage juveniles (J2s) were isolated. The female had a spherical body with a protruding neck, the stylet of females was slender and curved toward the back slightly. The perineal pattern of female (n=20) was round or elliptical, with high and squared dorsal arch, without obvious lateral lines. Morphological measurements of females (n=20): body length (L)=782.09±54.54 ( 518.52 to 1137.76) μm, body width (W)=439.51±19.23 (336.51 to 551.74 ) μm, stylet length (ST)=15.39±0.67 (12.55 to 18.80) μm, stylet knob height (STKH)=2.02±0.09 (1.88 to 2.46) μm, stylet knob width (STKW)=3.69±0.15 (2.91to 4.58) μm, distance from dorsal esophageal gland orifice to base of stylet (DGO)=2.32±0.17 (1.77 to 3.48) μm, vulval slit length (V)=23.99±0.75 (20.71 to 28.83) μm, and vulval slit to anus distance (V') = 18.62±0.55 (14.95 to 21.20) μm. The males showed a trapezoidal labial region, with a high head cap and concaved at the center of the top end in lateral view; and had blunt tail that bended slightly towards the abdomen, stylet knobs were prominent, speculum were in pairs and acicular. Measurements of females (n=10) were: L=1377.82±198.09 (1040.66 to 1726.59) μm, W=37.32±4.49 (28.35 to 41.90) μm, ST=21.48±1.23 (19.69 to 23.51) μm, STKH=2.99±0.12 (2.82 to 3.23) μm, STKW=5.34±0.41 (4.64 to 6.06) μm, DGO=2.54±0.13 (2.31 to 2.77) μm. J2s had the following characteristics: L=435.57±40.75 (414.92 to 462.14) μm, W=16.73±2.62 (12.76 to 21.95) μm, ST=12.66±1.02 (10.68 to 14.76) μm, STKH=1.58±0.29 (1.07 to 1.98) μm, STKW=2.22±0.38 (1.63 to 2.70) μm, DGO=2.26±0.18 (2.03 to 2.70) μm, tail length(T)= 87.97±9.71 (72.98 to 92.53) μm, hyaline tail terminus (HT) = 12.44±2.21 (9.59 to 13.90) μm. The nematode had uniform morphological characteristics with Meloidogyne incognita (Orton Williams, 1973). DNA was extracted from ten single females, and the species-specific primers Mi2F4/Mi1R1 (ATGAAGCTAAGACTTTGGGCT/TCCCGCTACACCCTCAACTTC) were used for identification of M. incognita (Kiewnick et al., 2013), and a 300bp fragment was amplified by this pair of primers, confirming the nematode was M. incognita. 18S rDNA gene was amplified using the primer pair 18S/26S (TTTCACTCGCCGTTACTAAGG/TTGATTACGTCCCTGCCCTTT) (Vrain et al.,1992), and the sequence was submitted to GenBank (GenBank Accession No. OR477177). Sequence aligment was conducted and showed 100% identical with the known sequence of M. incognita (GenBank Accession Nos. MH113856 and OQ269709). The result of identification was also confirmed by amplifying the sequence of NADH dehydrogenase subunit 5 (nad5) from mitochondrial DNA region using primers: NAD5-F/R(TATTTTTTGTTTGAGATATATTAG/TCGTGAATCTTGATTTTCCATTTTT) (Janssen et al. 2016). A611bp fragment was amplified and the sequence (GenBank Accession No. OR520436) showed 100% identical with other M. incognita sequences (GenBank Accession Nos. OP753345 and MT683461). In order to determine the pathogenicity of the nematode, infestation test was conducted in greenhouse. Ten 20-day-old healthy plants were cultured in pots with sterilized soil respectively and 2000 J2 hatched from egg masses of M. incognita were inoculated to the root of the plant. Five non-inoculated healthy C. dealbatus were used as negative control. After cultured at 25℃ for 60 days, roots were galled as observed in the field, and the symptoms of the root inoculated artificially with M. incognita were the same as those in the field. The nematodes were collected from inoculated roots, and identified as M. incognita with the species-specific primers Mi2F4/Mi1R1. An average of 7362 J2 was recovered and the reproduction factor value was 3.68. No galls were observed in control plants. These results suggested that C. dealbatus is a host for M. incognita. To our knowledge, this is the first report of M. incognita parasitizing C. dealbatus. This finding may be important to C. dealbatus industry and appropriate strategies should be taken to deal with the spreading of M. incognita.