Start out with virus-free dahlia tubers and plants. Although virus disease symptoms can take time to develop, always inspect plants early in the season for symptoms. If a plant looks suspicious, isolate from others until a decision can be made. Remember, some of the viruses that infect dahlias can come from other hosts besides dahlias. For example, Zinnia is a host of dahlia mosaic virus and tomato spotted wilt virus has many ornimental hosts.
Eliminate infected plants and tubers so they do not act as a source of inoculum for the rest of the dahlias. This may not completely control a virus problem because some virus-cultivar combinations may be nearly symptomless but the same virus in another cultivar can be serious. Also, infections initially occurring late in the season may not cause recognizable symptoms until the following year. If infected plants can not be removed try to isolate infected from healthy plants, control airborne vectors and avoid physical contact with the infected plants. Handling infected plants often breaks leaf hair cells which results in contamination of hands and tools. Soap and water is effective for removal of virus from hands and tools. Bleach is also useful for disinfecting tools.