Evidence for a Stress-related Viroid Complex
Observations of field symptoms suggest that the "Mystery disease" expressed in the Napa Valley is similar to yellow speckle disease common to Australia.
Two viroids, grapevine yellow speckle-1 and-2 (GYSVd-1 & GYSVd-2), have been reported as agents inducing yellow speckle. Analyses of samples collected in Napa Valley detected GYSVd-1 in every vine from field sources of the "mystery disease".
Studies indicate that the causal agent of yellow speckle can be present in non-symptomatic field grown vines. Yellow speckle and vein-banding disease expression therefore, appear to be related to a stress complex of biotic, climatic, and cultural factors.