First Report of Root‐knot Nematode, <i>Meloidogyne javanica</i> , Infecting <i>Acalypha indica</i> in Thailand

Acalypha indica is an annual herb that is commonly found in wet temperate and tropical regions. It is recognised as a medicinal plant with diverse therapeutic uses (Zahidin et al. 2017). Sanseera et al. (2012) reported that the plant contains the bioactive compound L-quebrachitol, which has antioxidant and anticancer activities. Characteristic root galls were observed on A. indica plants growing naturally at Kasetsart University, in the Chatuchak District of Bangkok, Thailand. No visible symptoms of damage caused by root-knot nematodes were detected in the aerial parts of the plants (Figure 1). Plants infested with root-knot nematodes were collected for identification and pathogenicity assessments. To increase the root-knot nematode population, a single egg mass was inoculated onto a 2-week-old okra seedling grown in a 10 cm diameter pot containing 200 g of sterilised soil. At 60 days after inoculation, adult females and second-stage juveniles were extracted and used for morphological analysis. The perineal patterns were characterized by an oval shape, a low to squarish dorsal arch, coarse and smooth striae, and double lateral lines (Figure 2a). The morphometrics of second-stage juveniles (n = 20) were as follows: body length (mean ±standard deviation) 432.3 ±24.8 µm, stylet length 12.3 ±0.8 µm, tail length 54.2 ±3.8 µm, and hyaline tail terminus 15.0 ± 1.3 µm (Figure 2b, d). These morphological features conform to described Meloidogyne javanica populations (Brito et al. 2023; Homkajorn et al. 2023; da Silva et al. 2024). To verify the identification of M. javanica, DNA was extracted from five adult females. PCR was performed using the universal primer set SSU18A/26R (5′-AAAGATTAAGCCATGCATG-3′ / 5′-CATTCTTGGCAAATGCTTTCG-3′) and the species-specific primer set Fjav/Rjav (5′-GGTGCGCGATTGAACTGAGC-3′ / 5′-CAGGCCCTTCAGTGGAACTATAC-3′) (Zijlstra et al. 2000), yielding amplicon sizes of approximately 850 bp and 670 bp, respectively. BLAST searches revealed 100% and 99% identity with M. javanica populations from China (GenBank Accession No. ON123411) and Bangladesh (MN196555). The resulting nucleotide sequences were deposited in GenBank: PX062110 (18S rRNA) and PX088832 (species-specific marker). The root-knot nematode population was used to conduct a pathogenicity test. In a greenhouse, 45-day-old plants were individually grown in 14 cm diameter pots filled with 500 g of autoclaved soil and inoculated with 500 second-stage juveniles per pot (n = 3). Plants without root-knot nematode inoculation served as controls (n = 3). At 30 days post-inoculation, the plants were carefully uprooted, washed, and the numbers of root galls and egg masses recorded using a stereo microscope. The inoculated plants exhibited symptoms consistent with those observed in the field, with mean numbers of galls and egg masses of 25.6 and 13.1 per gram of root, respectively. No symptoms or root-knot nematodes were detected in the control plants (Figure 3). The nematodes were subsequently re-isolated and identified, confirming the presence of M. javanica in the A. indica roots. The authors express their gratitude to Kasetsart University, Thailand.