First report of Apple hammerhead viroid infecting apple trees in South Korea.
In August and September 2018, apple trees of 'Fuji' and 'Hongro' cultivars looked abnormal with poor growth. Suspecting infection by viruses, leaf samples were collected from a total of 22 Fuji and Hongro apple trees in Uiseong, Yeongcheon, and Jangsu in Korea Republic. To perform comparative analysis of virus infections, leaves from a healthy and a diseased tree of Fuji and Hongro apple cultivars were chosen. Total RNA was extracted from each of the 4 selected samples with a HiYield Total RNA Mini Kit, and paired-end RNA sequencing was conducted using an Illumina HiSeq4000 system at Macrogen. Analysis of the de novo assembled contigs showed that the apple trees were infected with several viruses including Apple green crinkle associated virus, Apple stem grooving virus, Apple stem pitting virus and Apricot latent virus. In addition, the samples were also infected with Apple hammerhead viroid (AHVd), recently identified in apple trees. AHVd was found in 3 samples but not in the abnormal-looking Hongro sample. RT-PCR tests confirmed that 17 out of the 22 samples were infected with AHVd: 5 out of 6 healthy-looking Fuji samples, 5 out of 6 abnormal-looking Fuji samples, 4 out of 5 healthy-looking Hongro samples, and 3 out of 5 abnormal-looking Hongro samples. Thus, there was no correlation between presence of AHVd and poor growth. This is thought to be the first report on the presence of AHVd infecting apple trees in Korea Republic.