Dahlia mosaic virus and Tobacco streak virus in Dahlia (Dahlia variabilis) in New Zealand.

Dahlia (Dahlia variabilis Hort.) is a significant ornamental plant in New Zealand. Symptoms such as mosaic, ring spots, mottling, and veinal chlorosis, suggestive of a viral infection, are often seen in various dahlia collections. To better understand the incidence of viruses in dahlia in New Zealand, several popularly grown cultivars were evaluated for viruses that are known to infect dahlia. Viruses that were tested included Cucumber mosaic virus (CMV), Dahlia mosaic virus (DMV), Impatiens necrotic spot virus (INSV), Tobacco streak virus (TSV), and Tomato spotted wilt virus (TSWV). At least one symptomatic plant was tested from each of the following cultivars: Akito Dawn, Cincinnati Dancer, Hamari Accord, Hamari Rose, LeBatts Prime, LeVonne Splinter, Riverlea Tropicana, Spartacus, Tartan, Tui Connie, and Wandas Antartica. Except for DMV, initial testing was done by ELISA with commercially available kits for the above viruses. In the case of dahlia mosaic, samples were tested for DMV that was described previously (4) and two additional and distinct caulimoviruses (DMV-D10 and DMV-Holland) that were found to be associated with dahlia (1,2). Primer pairs, ORF6st: ATG GAA GAA ATT AAG GCG T and ORF6end: TTG TCT TCA TCC ATA AAG CAG; DenF1: CAG CAA GAA ACA GGA ATT GA and DenR: TTA CAG TCG AAG CTG CTA AA; and Kapht-F: ATG AGT AAT GCT TCA GCA A and Kapht-R: TGA CCA TGG CTT CTA ACT GT were used for the specific detection of DMV-D10, DMV-Holland, and DMV, respectively (1). None of the samples tested were ELISA positive for CMV, INSV, or TSWV. To verify the TSV infection, TSV-specific primers (5'-GTC CAG ACC ATC CAT CCA AC-3' and 5'-TTG ATT CAC CAG GAA ATC TT-3'), designed based on sequences available in GenBank, were used in reverse transcription (RT)-PCR. For DMV, the diagnostic tests used were electron microscopy and PCR followed by amplicon cloning and sequencing. Electron microscopic observation of leaf-dip preparations showed near isometric virions, approximately 50 to 60 nm in all samples tested. PCR showed that all samples tested were positive for DMV-Holland and DMV-D10. While DMV-Holland is a typical caulimovirus, DMV-D10 was found to exist as an endogenous plant pararetroviral sequence in dahlia (3). One sample each from two cultivars, Spartacus and Tui Connie, were positive for TSV by ELISA, RT-PCR, followed by the sequence analysis of the cloned amplicon. The impact of TSV-infected dahlias as a potential source of inoculum remains to be seen. Our results suggested the prevalence of dahlia mosaic-associated caulimoviruses in several dahlia cultivars and the presence of TSV in New Zealand dahlias. Dahlia mosaic continues to be prevalent in several parts of the world (1), and with the current findings in New Zealand, testing for these viruses should be conducted to ensure virus-free status of the propagating material. References: (1) V. Pahalawatta et al. Plant Dis. 91:1194, 2007. (2) V. Pahalawatta et al. Arch. Virol.153:733, 2008. (3) V. Pahalawatta et al. Virology 376:253, 2008. (4) R. D. Richins and R. J. Shepherd. Virology 124:208, 1983.