Virus infected plants and tubers represent the major source of virus to threaten dahlia cultivation. Infected plants and tubers are a problem because they are sources of inoculum and tubers from infected plants harbor the virus over winter. Rooted cuttings from infected plants will also be infected. If infected plants and their tubers are not removed, the disease problem increases each year.
Some viruses are seedborne. A few percent of seedlings grown from seeds harvested from infected plants will also be infected. The few plants infected from seed can serve as inoculum source for healthy plants and virus spread can occur quickly if vectors are present. In agriculture the combination of a very low level of seed contamination (0.01%) and an active vector can be devastating. In a garden situation, virus contamination of seed of other plants besides dahlias can also provide virus inoculum for the dahlias. Fortunately, only about 15% of known viruses are ever seedborne. Of the viruses that can infect dahlias, cucumber mosaic virus, tobacco streak virus and tobacco ring spot viruses are seedborne in many plant species including some vegetables. Their association with dahlias appears infrequent but they can show up in dahlias. Try to use virus free seed and reject transplants with virus symptoms.