First report of peach latent mosaic viroid in peach trees in Korea

Peaches, belonging to the genus Prunus, are among the most popular stone fruits and the sixth most popular fruit produced in Korea. Among the known viruses that infect peaches, Plum pox virus (PPV), the causal agent of Sharka disease, is the most serious worldwide. In addition, two viroids, Hop stunt viroid (HSVd) and Peach latent mosaic viroid (PLMVd), infect a wide range of peach trees. To evaluate the presence of viruses and viroids infecting stone fruit trees in Korea, the leaves of various stone fruit trees were sampled from diverse peach orchards in May 2013 in Korea. We extracted total RNAs using Fruit-mate for RNA Purification (Takara, Shiga, Japan) and the RNeasy Plant Mini Kit (Qiagen, Hilden, Germany), and prepared several libraries for stone fruit transcriptomes using the TruSeq RNA Library Preparation Kit v2 (Illumina, CA, USA) according to the manufacturer’s instructions. The libraries were paired-end-sequenced using Illumina’s HiSEquation 2000 (Macrogen, Korea). The obtained raw data were de novo assembled using the Trinity program (Haas et al. 2013). The obtained transcriptomes were BLAST-searched against viral reference genomes to identify viruses and viroids infecting the stone fruit trees. Interestingly, we found sequences of PLMVd only in peach transcriptomes derived from the five commercial cultivars Baekchunhwangdo, Cheonjungdo, Jangtaek, Mibae’, and Sumi, indicating PLMVd infection in peach trees. However, we did not examine disease symptoms caused by PLMVd in the infected peach trees. In May 2015, we examined disease symptoms caused by PLMVd in five PLMVd-infected peach cultivars. Out of five cultivars, we observed several foliar symptoms, such as mosaics, blotching, and chlorotic creamy calico, caused by PLMVd in the peach cultivar Sumi grown in an orchard in Icheon, Korea. We did not observe any viral disease symptoms in other four peach cultivars, indicating that PLMVd in the four cultivars might be asymptomatic. Leaves showing PLMVd disease symptoms from Sumi were sampled and subjected to total RNA extraction using Fruit-mate for RNA Purification and the RNeasy Plant Mini Kit. Using several primer pairs for the viruses and viroids infecting peaches, we performed reverse-transcription (RT)-PCR. The RT-PCR showed that Sumi was only infected with PLMVd. Using PLMVd-specific primers, forward 5′-GACGTCTACCCGGGATTCAAA-3′ (position 233-253) and reverse 5′-GTAATCCAGTTTCTACGGCGGTA-3′ (position 232-210), which were designed based on the PLMVd reference genome (Accession No. M83545.1), we amplified the full-length genome of PLMVd. The amplified PCR products were cloned into the pGem-T-Easy Vector (Promega, Fitchburg, WI, USA) and sequenced. As a result, we obtained a complete PLMVd genome referred to as PLMVd isolate Sumi, based on the isolated peach cultivar and deposited it in GenBank as KT005802. The PLMVd isolate Sumi was 337 nt in length. A BLAST search revealed that the pathogenic PLMVd isolate Sumi was closely related, with 98% identity (330/337 nt), to the known S2 isolate (EF591867.1), which originated in peach plants displaying yellowish symptoms in China (Xu et al. 2008). In general, PLMVd infection does not induce observable disease symptoms in infected peach trees. Of the examined peach cultivars infected with PLMVd, only PLMVd isolate Sumi displayed PLMVd disease symptoms. Taken together, this is the first report of PLMVd-infected peach trees in Korea, and our results suggest that many peach cultivars seem to be infected with PLMVd. However, only a few PLMVd isolates, such as PLMVd isolate Sumi, might be symptomatic.