First Report of Cactus Virus X Infecting <i>Selenicereus polyrhizus</i> in Argentina

Dragon fruit, also known as pitahaya (Selenicereus spp., Cactaceae), is native to the Americas and is now cultivated worldwide due to its economic value. Its ease of propagation and high productivity have driven the expansion of pitahaya cultivation in Argentina’s subtropical region; however, vegetative propagation also poses a high risk for the introduction and spread of systemic viral diseases. In October 2018, sampling was conducted in a 1-hectare plot within red-fleshed pitahaya (S. polyrhizus) plantations in Yuto, Jujuy, Argentina. Approximately 10% of the plants in the plot were symptomatic. Symptomatic cladodes were collected from five plants showing virus-like symptoms, such as chlorotic spots and pale greenish-yellow mosaics, and transplanted under greenhouse conditions. Potexvirus-like particles of approximately 600 nm in length were observed in sap dip preparations from the symptomatic samples using a JEM 1200 EXII transmission electron microscope (JEOL, Akishima, Tokyo, Japan). Total RNA was extracted using the RNeasy Plant Mini Kit (Qiagen, Hilden, Germany), and first-strand cDNA was synthesized with Moloney murine leukemia virus reverse transcriptase (Promega, Madison, WI, USA) and random primers. PCR amplification was performed using the potexvirus-generic primers Potex 2RC and Potex 5 targeting the RdRp gene. Amplicons of the expected size (∼580 bp) were obtained from all symptomatic samples, whereas no amplification occurred in an asymptomatic cladode sample maintained under greenhouse conditions. Two RT-PCR products were purified and sequenced in both directions, yielding 100% identical nucleotide (nt) sequences. The consensus sequence corresponding to partial RdRp of cactus virus X (CVX) was deposited in GenBank database under the accession number PP895196. Whole-genome sequencing of CVX was performed on one pitahaya sample using Oxford Nanopore Technology. Basecalling was performed using Dorado v8 (https://github.com/nanoporetech/dorado), and raw FASTQ reads were filtered for quality and length using NanoFilt v2.6.0 (https://github.com/wdecoster/nanofilt). A total of 61,070 high-quality reads (modal length 339 nt) were mapped to the reference genome (GenBank accession AF308158) using minimap2 v2.17-r941 (https://github.com/lh3/minimap2), achieving an average coverage depth of 2,453×. De novo assembly of the mapped reads was performed with minimap/miniasm (https://github.com/lh3/miniasm), and the resulting draft assemblies were polished with Medaka (https://github.com/nanoporetech/medaka) to improve consensus accuracy. This process produced a final consensus sequence of 6,614 nt (excluding the poly(A) tail; GenBank accession PX516790), sharing 96‒98% nt identity with available CVX isolates from China, Taiwan, South Korea, and the United States, with 100% query coverage. No contigs corresponding to other plant viruses were detected. Phylogenetic analysis using the maximum likelihood method in IQ-TREE version 1.6.12 showed that the CVX isolate from Argentina clustered with isolates from China, Taiwan, South Korea, and the United States, sharing 97‒99% nt identity. Given the increasing importance of pitahaya in northern Argentina, its asexual reproduction, and the phytosanitary restrictions associated with CVX, further epidemiological studies are necessary to assess its impact and to prevent its potential spread.