Yellow pitaya, Selenicereus megalanthus, is a night-blooming, climbing cacti of tropical origin, which has received increasing attention for its potential as a new exotic fruit crop (Lichtenzveig et al. 2000). The crop is grown extensively in Hainan Province, China (3000 ha). In October 2021, a survey was conducted on a farm located in Changjiang (19°21'4″N, 108°47'2″S), Hainan Province, China. Some yellow pitaya plants were found that were stunted and chlorotic, with abnormally thin stems (Fig. 1B), and no symptoms on healthy plants (Fig. 1A). Dead plants were also observed. Many galls and females with egg masses were observed on roots (Figs. 1C & 1D). This is typical of root-knot nematode (RKN) infections, and the incidence of infection was 36.7%. Meloidogyne sp. females and egg masses were dissected from roots of the infected plants. The perineal pattern of females (n= 5) was round to oval-shaped with a high dorsal arch (Figs. 1I & 1J). Second-stage juveniles (J2s) had truncated lips (Figs. 1E & 1F) and long-conical tails with bluntly rounded tips (Figs. 1G & 1H). The J2s body length (n= 24) averaged 416.79 μm (349.21 to 472.76 μm) with a mean width of 15.36 μm (12.47 to 17.52 μm); mean stylet length was 11.16 μm (10.10 to 13.23 μm); tail length averaged 53.73 μm (43.46 to 65.90 μm). The morphological characteristics matched the original description of M. enterolobii (Yang and Eisenback 1983). Males were not found. Genomic DNA was extracted from eight single J2s, and the mitochondrial (mtDNA) region between COII and 16S rRNA gene was amplified with primers C2F3/1108 (Powers and Harris 1993). A 652-bp DNA fragment was obtained, for which the sequence (GenBank accession no. OP122499) was 100% identical to the sequences of M. enterolobii isolates from China（MN269947）and the USA (MN809527). Furthermore, species identification was also confirmed using M. enterolobii specific primers Me-F/Me-R. An amplicon size of ∼230 bp was obtained, which is consistent with those previously reported for M. enterolobii (Fig. 2) (Long et al. 2006). Therefore, this population was identified as M. enterolobii based on morphological and molecular characteristics. Pathogenicity tests were performed in the greenhouse at 26℃ and 80% relative humidity with a 14-h/10-h light/dark photoperiod. Ten RKN-free S. megalanthus seedlings were transplanted into pots containing sterilized soil. After 3 weeks, the roots of 5 plants were inoculated with 3,000 eggs and J2s of M. enterolobii per plant. Five uninoculated plants were used as control plants. After 2 months, no galling or symptoms were observed on the control plants. All inoculated plants had galled roots similar to those observed in the field. Females and egg masses were obtained by dissecting galls. The nematode reproduction factor (RF= final population/initial population) was 1.9. Adult females (n= 5) dissected from inoculated plants were identified as M. enterolobii with sequence-specific primers Me-F/Me-R, thus confirming pathogenicity. The pathogenicity test was carried out twice with similar results. M. enterolobii is one of the most damaging species of RKN, due to its wide host range, high level of pathogenicity, and ability to develop and reproduce on several crops with resistance genes to other RKN (Castagnone-Sereno 2012). To our knowledge, this is the first report of S. megalanthus (yellow pitaya) as a host of M. enterolobii in China. Further studies are needed to develop and evaluate integrated management strategies.
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