First report of Papaya leaf curl virus in papayas in Korea and recovery of its symptoms

Papaya (Carica papaya) is one of the most common tropical fruits throughout the world. In July 2014, symptoms of curling, deformation, and yellowing of young leaves, which are known typical symptoms of Papaya leaf curl virus (PaLCuV), were observed in papayas cultivated in greenhouses of Gokseong and Haenam located in the southern part of the Korean peninsula (Chang et al. 2003). Total DNA from six plants was extracted using a Viral Gene-spin viral DNA/RNA extraction kit (iNtRON Biotechnology, Seongnam, Korea) and used as templates in PCR amplifications with PaLCuV-F (5′-GCATTATGTATAGCCCACGA-3′) and PaLCuV-R (5′-GTCAGCCAATGATGCTTACG-3′) primers. The primer set was newly constructed based on the existing PaLCuV sequences reported in Taiwan (GenBank Accession Nos. JN703795 and KC161184). The amplicon of every reaction had the same predicted size, 250 bp. To obtain the full sequence of PaLCuV, sample DNA was amplified by rolling circle amplification and digested with the restriction enzyme BamHI, which cuts PaLCuV only once (Kil et al. 2014). The amplified DNA, of size 2.8 kb, was cloned into the pGEM-3zf(+) vector (Promega, Madison, WI) and sequenced by a commercial sequencing service (Macrogen Inc., Seoul, Korea). NCBI BLAST results revealed that PaLCuV isolate Korea (KT266873) has 99.5% sequence identity to PaLCuV isolate PF-1 (KC161184). A phylogenetic tree constructed with the full sequences of PaLCuV isolates also showed that PaLCuV isolate Korea has the closest relationship to the PF-1 isolate. Southern blot hybridization using the PCR product as a probe showed the typical double-stranded and single-stranded appearance of PaLCuV DNA. Interestingly, symptoms of PaLCuV appeared in spring, and the affected leaves did not grow and expand well until August. However, in September, newly produced leaves were asymptomatic. Dense scars caused by severe stunting persisted on the middle of trunks of infected plants. PCR analysis showed that the fully recovered leaves of PaLCuV-infected papaya contained PaLCuV DNA, but in lesser amounts than papaya leaves with severe symptoms. The phenomenon of recovery of symptoms from geminivirus infection has been reported, but to our knowledge, this is the first report of PaLCuV infecting papayas in Korea, and of recovery of its symptoms (Góngora-Castillo et al. 2012). As the average atmospheric temperature has risen, various tropical fruits have begun to be grown in Korea. But most seeds and seedlings of tropical crops cultivated in Korea have been imported from Southeast Asia, which is a repository of plant virus. Because of the risk from new and emerging viruses in imported materials and their unknown infectivity to new hosts in Korea, an effective quarantine system is important to manage these risks.