Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2006
Publication Date: 6/1/2006
Citation: Weiland, J.J., Larson, R.L., Freeman, T.P., Edwards, M.C. 2006. First report of Beet black scorch virus in the United States. Plant Disease. 90(6):828. Interpretive Summary: Sugarbeet production in the U.S. continues to be plagued by known and some unknown pathogenic agents. Recently, sugarbeet roots in Colorado were obtained that appeared to have the disease Rhizomania, which causes root stunting and a massive production of root hairs. Rhizomania is caused by a virus called beet necrotic yellow vein virus (BNYVV). When tested for the present of BNYVV, these samples were found to be devoid of this virus. Upon more careful testing, a virus of sugarbeet previously documented only in China was found to be harbored by these infected roots. The virus, beet black scorch virus (BBSV), normally causes black scorching the leaves of sugarbeet plants that are infected with the virus. Yields can be impacted dramatically by BBSV infection. Although the results from the present report confirm that BBSV was found in the sugarbeet roots from Colorado, the symptoms differed from those seen in China. Thus, root vein swelling and high root hair production appear to be associated with this virus in these samples. The result points to the need to test carefully for infectious diseases that may be missed by standard disease screening programs in diagnostic labs.
Technical Abstract: n October of 2005, sugar beet (Beta vulgaris L.) plants exhibiting symptoms of 'Rhizomania' disease, caused by beet necrotic yellow vein virus (BNYVV; 3),were observed in a production field near Greeley, CO USA. Of seven roots exhibiting moderate to severe symptoms characteristic of this disease, only the two roots with the mildest symptoms tested positive for BNYVV by ELISA (all roots tested negative for the presence of the related beet soilborne mosaic virus (BSBMV). Root hairs characteristic of the disease were combined from the remaining five roots, ground in phosphate buffer, and the supernatant from the suspension was mechanically applied to leaves of Chenopodium quinoa in an effort to isolate an infectious agent. Five days post inoculation yellow lesions with necrotic centers were visible on inoculated leaves, well in advance of those typically observed for BNYVV or BSBMV. Lesions exhibiting a similar rate of development on C. quinoa subsequently were induced from inocula comprised of root vein extracts from 4 of 7 additional beet roots tested from this location. Transfer of the infection from the C. quinoa lesions to 32 healthy C. quinoa and 10 sugar beet plants (hybrid ACH9369; American Crystal Sugar Co., Moorhead MN) resulted in 100% infection. Inoculated leaves of C. quinoa exhibited a high density of necrotic local lesions within 3 dpi, whereas inoculated leaves of sugar beet exhibited pin-point, necrotic to diffuse, chlorotic local lesions evident by 5 dpi. Examination of symptomatic leaf tissue of C. quinoa by transmission electron microscopy revealed aggregates of virus-like particles of icosahedral symmetry within the cell cytoplasm. Following a virus minipreparation procedure, nucleic acid extracted from the partially-purified virus was found to be single-stranded RNA by ribonuclease digestion and alone was infectious when inoculated to C. quinoa leaves. The size of the apparently single RNA genome was estimated to be ~3.5 kilobases in length and a candidate for the single coat protein (CP) had an estimated mass of ~25 kilodaltons. The sole reference set found in the literature for a virus naturally occurring on sugar beet with similar characteristics was that for beet black scorch virus (BBSV), a virus recently accepted by the International Committee on the Taxonomy of Viruses (ICTV) into the genus Necrovirus within the family Tombusviridae (2). Prior to this communication, BBSV has only been reported in China, where it was first documented affecting sugar beet in the late 1980's (1). Using the published sequence of BBSV (Genbank Accession #AY626780), DNA primers directed to the 3’-half of the BBSV genome were used in reverse transcription-polymerase chain reaction (RT-PCR) to produce an amplicon from the unknown virus. Sequencing the amplicon revealed 88.8% nucleotide sequence identity to the BBSV CP gene and 97% amino acid sequence identity to the predicted CP gene product. Combined, the nucleotide sequence and physical characteristics confirm the presence of BBSV in a U.S. sugarbeet field for the first time. To our knowledge, this constitutes the first report of the occurrence of BBSV outside of China.