Evidence for a Stress-related Viroid Complex
Observations of field symptoms suggest that the "
" 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.