Survey for Possible Reservoirs for Leafroll Viruses in North CoastSurvey for Possible Reservoirs for Leafroll Viruses in North Coast
This project addresses one specific concern: are there reservoirs of virus in plants other than cultivated grapevines that might be an important factor in the epidemiology of grapevine leafroll disease? To answer this question, we collected samples of 83 wild grapes (Vitis spp.) and 85 non-Vitis weed, cover crop and native species found in and near nine vineyards symptomatic for leafroll disease, and tested them for grapevine leafroll associated viruses (GLRaV) 1-5, 7, and 9, and vitiviruses GVA, GVB, and GVD. We also collected wild Vitis samples from two habitats distant from vineyards to assess virus incidence in a natural environment. The wild Vitis spp. genotypes were determined by DNA fingerprinting, and the genotypes were then analyzed and grouped into three categories: V californica, V. vinifera hybrids, and wild V. vinifera plus rootstock cuttings. We detected GLRaV-2, GLRaV-3, GVA, and GVB in all three groups of wild Vitis. Overall virus incidence was 27 percent, but percent infection differed depending on the genotype. At least one of the four viruses was detected in: 18%of the V. californica samples, 48 percent of the V. vinifera hybrids; and 100 percent of the wild V. vinifera plus rootstock cuttings. We also detected GLRaV- 3 in one out of six willows (Salix spp.). No viruses were detected in any of the other 84 host species we sampled. Viruses were detected in wild Vitis spp. in habitats both near to and distant from vineyards. Preliminary evidence indicates that there is no correlation between virus infection in vineyards and in nearby wild Vitis spp. Our sample size is small, however, and further sampling is needed. An ornamental grapevine, ?Roger?s Red?, was included in our samples. It is grown for its red fall foliage and was previously thought to be V. californica. Roger?s Red tested positive for GLRaV-2 and GVB. DNA fingerprinting indicated that it is a hybrid of V. californica x V. vinifera ?Alicante bouschet?. Alicante bouschet is a tintureiro variety known to have red leaves in the fall. Both the virus infection and genetic background could account for its red leaves in the fall. This survey establishes that wild Vitis spp. are hosts for two of the most important leafroll-associated viruses?GLRaV-2 and -3? in addition to GVA and GVB. This is the first evidence of a leafroll virus in a woody host besides Vitis spp. and has important implications for leafroll disease management.