Submitted to: Korean Journal of Agricultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/2/2016
Publication Date: 4/14/2016
Citation: Han, J., Park, C., Seo, E., Kim, J., Hammond, J., Lim, H. 2016. Occurrence of Apple stem grooving virus in commercial apple seedlings and analysis of its coat protein sequence. CNU Journal of Agricultural Science. 43:21-27.

Interpretive Summary: Virus infections are responsible for reduced yield and quality in many crops, and are especially problematic in vegetatively-propagated crops such as apple. Three major viruses (Apple stem grooving virus (ASGV), Apple chlorotic leaf spot virus and Apple stem pitting virus) affect apple trees in Korea, and no insect or other vector has been shown to transmit these viruses between trees in nurseries and orchards; nor have these viruses previously been demonstrated to be transmitted through seed. The primary means of transmission has been presumed to be mechanical transmission through grafting of infected shoots onto rootstocks, or of shoots onto vegetatively-propagated infected rootstocks. Probable seed transmission of ASGV through apple seeds was demonstrated, and the coat protein sequence of the putative seed-transmitted isolates was found to differ from all previously-characterized isolates of ASGV by a single nucleotide in the coat protein gene, resulting in a single amino acid difference. This suggests a possible association between this variant and seed-transmissibility. This information provides the basis for further research to confirm seed transmissibility as a potential means of spread of ASGV, which will be useful for scientists and nursery producers seeking to improve apple productivity by reducing virus prevalence.

Technical Abstract: Virus infections are responsible for reduced yield and quality in many crops, and are especially problematic in vegetatively-propagated crops such as apple. Three major viruses (Apple stem grooving virus (ASGV), Apple chlorotic leaf spot virus and Apple stem pitting virus) affect apple trees in Korea and pome fruit trees worldwide. The transmission of the three viruses was reported by grafting, with no report of transmission through mechanical contact, insect vector or seed, except some herbaceous hosts of ASGV. As RNA extraction methods for fruit trees, Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) and multiplex RT-PCR techniques have been improved for reliability and stability, low titer viruses that could not be detected in the past have become detectable. We studied seed transmission of three apple viruses through apple seedling diagnosis using RT-PCR. Nineteen seeds obtained from commercially grown apples were germinated and two of the resulting plants were ASGV positive. Seven clones of the amplified ASGV coat protein (CP) genes of these isolates were sequenced. Overall sequence identities were 99.84% (nucleotide) and 99.76% (amino acid). Presence of a previously unreported single nucleotide and amino acid variation conserved in all of these clones suggests a possible association with seed transmission of these ‘S’ isolates. A phylogenetic tree constructed using ASGV CP nucleotide sequences showed that isolate S sequences were grouped with Korean, Chinese, and Indian isolates from apple and Indian isolates from kiwi.