Mechanical Transmission and Rootstock Reservoirs as Factors in the Widespread Distribution of Viroids in Grapevines

Abstract

With the recent evidence of the almost ubiquitous occurrence of grapevine viroids in California, intriguing questions have arisen concerning the significance of viroids to vine growth and productivity. Of fundamental importance in addressing these considerations is an understanding of the manner in which such a general distribution of grapevine viroids has been accomplished. GV₁, GV₂, and GV₃ were demonstrated to be transmissible agents in Emperor seedlings and viroid-free, shoot tip cultured Cabernet Sauvignon grapevines. Analyses of multiple selections of these varieties have never detected the presence of GV₂. Therefore, the transmission of GV₂ in these tests suggests that these varieties are not resistant to GV₂, but probably have never been exposed to this viroid. A survey of samples taken from the rootstock collection at the University of California, Davis, detected at least one grapevine viroid in all varieties tested. Two of the newly released rootstock varieties tested contained an unusually high concentration of the less commonly found GV₂. Therefore, two possible sources for the spread of grapevine viroids have been identified: <i>(1)</i> mechanical inoculation among vines, which might be accomplished during routine cultural practices; and <i>(2)</i> systemic transmission between grafted rootstocks and scion varieties containing grapevine viroids.