Grapevine red blotch: Molecular biology of the virus and management of the disease

Abstract

Red blotch is a recently recognized disease of grapevine for which the graft-transmissible grapevine red blotch-associated virus (GRBaV), a proposed member of a new genus within the family Geminiviridae, is the causal agent. The virus affects fruit quality, delays ripening, and probably reduces yield and vigor. Estimated economic losses range from $2,213 to $68,548 per hectare over a 25-year productive life span of a vineyard. The genome of GRBaV is circular and consists of a single molecule of single-stranded DNA with seven predicted open reading frames. Foliar symptoms consist of red blotches that expand and coalesce in late summer and fall and irregular chlorotic areas that become necrotic later in the season in redberried and white-berried Vitis vinifera cultivars, respectively. Visual diagnosis is often unreliable due to several confounding abiotic and biotic factors, including similarities with leafroll disease symptoms; therefore, PCR-based assays are recommended for an accurate diagnosis. Although red blotch disease was only recognized in 2008, GRBaV was detected in archival grapevine leaves sampled in 1940 in California and kept in a herbarium collection, suggesting the virus was present in vineyards more than 70 years prior to its identification. Surveys of vineyards revealed the occurrence of GRBaV in some of the major grape-growing regions in the USA and Canada. Outside of North America, the virus was found so far in Switzerland in material introduced from the USA and in South Korea. An early account of the Virginia creeper leafhopper as a vector of GRBaV was not confirmed; instead, the three-cornered alfalfa treehopper was shown to be a likely vector of epidemiological significance. Disease management strategies almost exclusively rely on roguing and replacing vineyards using planting material derived from clean, virus-tested stocks. Advancing our understanding of disease epidemiology and viral gene expression are important future research topics for red blotch disease and GRBaV.