We produced transgenic chrysanthemum (Dendranthema grandiflora) plants expressing double-stranded RNA-specific ribonuclease gene (pac1) derived from Schizosaccharomyces pombe using an Agrobacterium-mediated transformation method. Three transgenic lines stably expressing Pac1 protein were selected. After inoculation with chrysanthemum stunt viroid (CSVd), these lines showed a decreased frequency of infection, less accumulation of viroid, and attenuation of growth retardance compared with control plants. They also showed a significantly lower infection frequency against tomato spotted wilt virus (TSWV) than in control plants. Hybrid plants between one transgenic line and a wild chrysanthemum species, D. pacificum, were produced to test their susceptibility against TSWV infection. TSWV infected all the plants of the wild species resulting in a high mortality, whereas the hybrid plants expressing Pac1 protein showed either complete resistance or high susceptibility. These results suggest the presence of an endogenous tolerance gene against TSWV in the cultivar used for transformation. The virus infection was suppressed in transgenic plants, indicating that the commercial use of this transgene would limit the spread of viruses to wild populations of Dendranthema. This is the first study to demonstrate that a single tolerance gene in transgenic plants can attenuate both viroid and virus diseases.