A collection of highbush blueberry (Vaccinium corymbosum L.) cultivars planted in the field for propagation in South Bohemia was surveyed in May and July of 2009 for the occurrence of detrimental viruses. A total of 67 plants of 10 cultivars (Berkeley, Burlington, Blue Crop, Bluetta, Darrow, Duke, Gila, Jersey, Late Blue, and Northland), were observed for typical Blueberry red ringspot virus (BRRV) symptoms that appear as reddish ring spots and blotches on stems and fruits, exclusively on the upper surface of the older leaves but not the underside. Samples of leaves were collected and maintained at --20°C until used for DNA extraction, then assayed for BRRV infection using PCR. Controls originated from the same blueberry cultivars in vitro. DNA was extracted from leaf tissue with a NucleoSpin Plant II kit for isolating genomic DNA according to the manufacturer's instructions (Macherey-Nagel, Düren, Germany). Primer pair BRRV15/16, which amplified fragments of the reverse transcriptase gene (1), was used in PCR for BRRV detection. The program used for PCR amplification was 94°C for 2 min, followed by 35 cycles at 94°C for 30 s, 49°C for 30 s, and 70°C for 45 s, followed by a final extension at 70°C for 5 min. The total PCR volume of 25 μl contained 20 ng of DNA, 200 μmol liter--1 dNTPs, 0.5 μl of each primer BRRV15 and BRRV16 (20 pmol μl--1), 75 mM Tris-HCl pH 8.8, 20 mM (NH4)2SO4, 0.01% Tween 20, 2.5 mM MgCl2, 2.5 U of Taq Purple DNA polymerase, and stabilizers (Top-Bio Ltd., Prague, Czech Republic). Amplifications were conducted in an MJ Research (Waltham, MA) thermocycler. Aliquots (4 μl) of each PCR product were analyzed by electrophoresis in tris-acetate-EDTA buffer. No BRRV symptoms were observed on the plants in early spring, yet BRRV was detected in one symptom-free bush of cv. Darrow by PCR. In July, typical symptoms developed on that and another cv. Darrow bush that was also positive by PCR. DNA fragments of the expected sizes were amplified from total nucleic acid samples of both infected blueberry bushes using primers BRRV15/16, while no amplification products were detected in plants without symptoms. The amplicons obtained with primers BRRV15/BRRV16 were sequenced and revealed 97.5%-nt identity to the BRRV putative reverse transcriptase gene (GenBank Accession No. AF404509). The 845 nt of the amplicon has been deposited at GenBank under Accession No. HM107773. The disease was likely introduced in infected planting material, since no highbush blueberry plantations exist in the vicinity and V. corymbosum is not native to the Czech Republic. In conclusion, to our knowledge, this is the first report of Blueberry red ringspot virus (genus Soymovirus, family Caulimoviridae) in V. corymbosum L. in the Czech Republic. Symptom observation and PCR testing for BRRV should therefore, be incorporated into the certification scheme for highbush blueberry in the Czech Republic.
Reference: (1) J. J. Polashock et al. Plant Dis. 93:727, 2009.
