First Report of the Root-Knot Nematode Meloidogyne incognita on Bletilla striata Rchb.f. (Orchidaceae) in China.

Bletilla striata Rchb.f., is a perennial herbaceous bulbous plant known as the Chinese ground or hyacinth orchid classified in the Orchidaceae. It is native to southeast Asia and mainly distributed in China, Japan and northern Myanmar (He et al. 2017). It has the functions of astringent hemostasis and analgesia, and can also be used to treat traumatic bleeding, ulcers, swelling and chapped skin. Therefore, it occupies an important position in traditional Chinese medicine (Xu et al. 2019). In June 2023, three farmers in Mengzi (103.39°N, 23.21°E), Yunnan Province, China, observed that some Bletilla striata Rchb.f. plants grew poorly with small and chlorotic leaves (Figure 1 A). We suspected that these symptoms were caused by root-knot nematode infection, but the galls on the roots were small and inconspicuous (Figure 1 A). The presence of nematode females in both the galled regions and the normal roots (Figure 1 B), revealed by fuchsin staining (Byrd et al. 1983), indicated that the symptoms were probably caused by root-knot nematode infection. To estimate the incidence rates, we randomly selected 100 B. striata Rchb.f. plants from each of five fields representing a total area of 3000 m2. In these fields, the occurrence of stained root-knot nematodes were 19.3%, 17%, 18.3%, 15%, and 13%, respectively. The gall rating of the infected plants in the B striata Rchb.f. samples collected from the five fields was 2 (rating scale of 0 to 5). Females (n=20), second-stage juveniles (J2s, n=20) and egg masses (n=20) were extracted and collected from roots and soil for morphological and molecular identification. The females had a white, pyriform body and their perineal patterns exhibited a high and square dorsal arch, lacking distinct lateral line (Figure 1. C & D). Measurements of females (n = 20) were: body length (BL) = 708.64±89.6 µm (554.36 to 844.51 µm); maximum body width (BW) = 461.73±47.44 µm (365.25 to 561.49 µm); stylet length (ST) = 15.49±3.15 µm (10.55 to 19.78 µm); and distance from dorsal esophageal gland opening to the stylet knobs (DGO) = 3.33±0.27 µm (2.77 to 3.93 µm). Measurements of J2s (n=20) were BL = 417.7±47.67 µm (342.16 to 499.68 µm); BW = 15.74±2.66 µm (11.05 to 25.63 µm); ST = 12.49±1.12 µm (10.19 to 15.02 µm); DGO = 2.64±0.59 µm (40.17 to 68.74 µm); tail length = 51.93±8.55 µm (10.43 to 27.22 µm); hyaline tail terminus = 18.23±3.99 µm (1.48 to 3.98 µm). These morphological features match the description of Meloidogyne incognita (Eisenback et al. 1981). To further confirm the species, we selected three infected plants from each field for molecular identification, the ITS region amplified using the primers 18S/26S (5'-TTGATTACGTCCCTGCCCTTT-3',5'-TTTCACTCGCCGTTACTAAGG-3') (Vrain et al. 1992) . A 729 bp PCR product of ITS region (accession nos. OR463907) was obtained from all infected plants. The amplicons from 18S/26S primer pair were sequenced and the sequences showed 95.29% homology with sequences of M. incognita (accession nos. MT209948.1). Moreover, a 835 bp DNA fragment (accession nos. OR469000) was obtained using the specific primers Mi-F/Mi-R (5'-GTGAGGATTCAGCTCCCCAG-3',5'-ACGAGGAACATACTTCTCCGTCC-3') for M. incognita (Meng et al. 2004), the sequence showed 99.28% homology with sequences of M. incognita (accession nos. ON416569). The morphological features and molecular data confirmed the identification of the root-knot nematode on B. striata Rchb.f. as M. incognita. To confirm the pathogenicity, ten healthy B. striata Rchb.f. seedlings were each inoculated with 500 freshly hatched J2s isolated from field Bletilla striata Rchb.f.. Five healthy seedlings without J2 inoculation were used as controls. At 60 days after inoculation, most of the inoculated plants exhibited similar symptoms to those initially observed by farmers in the field. On average, 1532 J2s were recovered from each inoculated plant, yielding a reproductive factor of 2.1. The gall rating for these inoculated plants was 2. Fuchsin staining revealed the presence of root-knot nematode females within the roots, with an average of 17 females detected per inoculated plant. No symptoms were observed in the control plants. This is the first report of M. incognita infecting B. striata Rchb.f. in China. M. incognita can cause severe infection and damage to some crops, resulting in serious economic losses (Eisenback, 2022). The growers need to take measures to prevent the spread of this nematode.