Involvement of Viroids in Grapevine Diseases and the Impact on Vine Performance

The role of viroids as agents of plant disease is well established. Control experiments have been initiated to evaluate the role of grapevine viroids as the causal agent of the Yellow Speckle (YS) disease and in association with grapevine fanleaf virus in the expression of Vein Banding (VB) symptoms. YS symptoms which are difficult to evaluate under climatic conditions in California were induced under growth chamber conditions with California GYSVd-1 isolates from Mission and Cabernet franc but not from Zinfandel. Symptom expression of YS is affected by vine physiology and development. These observations confirm the presence of the Yellow Speckle disease agent in California vines. However, the disease expression as well as the viroid-disease relationship appears to be more complex than initially presumed. The first experimental demonstration of synergism between a viroid and a virus has been accomplished with the induction of VB symptoms. Both grapevine viroids and grapevine fanleaf virus (GFLV) were required. Thus, VB can no longer be considered as simply a late season expression of fanleaf and the importance of viroids to vine performance has been demonstrated. Performance trials at Oakville of the first available viroid-free vines in the world have been concluded due to the Phyloxera pressure on the own-rooted Cabernet Sauvignon. Data on vine growth, yield, fruit maturity and wine quality suggest a difference in vine performance especially in the growth parameters. These differences point to a potential for viroids to affect and reduce growth. The first viroid-free rootstocks can now be utilized to expanded these studies. The continued development of additional viroid-free commercial varietals, rootstocks, as well as rootstock germplasm sources is essential to our investigation of the impact of viroids on grapevines. Propagation and maintenance of viroid-free materials in a Foundation Vineyard at UC-Davis will provide greater availability to FPMS for general distribution as well as for indexing purposes in the absence of a viroid background. Monitoring for the incidence of transmission to viroid-free vines interplanted in a commercial planting of viroid-containing vines indicated that no field spread had occurred after six growing seasons.