Virus does not live in the soil. It is spread by insects that feed on infected plants and then transmit the virus to your plant. Once a dahlia gets a virus, it will not recover. However, the virus will probably never kill the plant either. Most dahlias have some virus infection(s). Some varieties are more resistant than others. If your plants did not thrive because of the virus, you need to replace your stock.

I believe that spider mites over winter in bushes and plants in your garden or your neighbors garden.

Thanks Teddahlia,
Now how do I get rid of the spider mites? Should I spray the bushes around where I put the dahlias as well as the plants? I use Merit for spider mite and than change to something else so the spider mite don't form an immunity.
I think I would rather have spider mite than the virus.
Where are all the other dahlia people that used to be here? Thanks again.

PD, there are many soil borne virus'. None affect dahlias. The virus' that infect dahlias are carried by insect vectors.
Here is how I got rid of spider mites. Add 3 oz superfine spray oil to 1 gal water. At first appearance of mites spray plant to runoff stage and the soil surrounding it. Repeat this every 5 days for three times. Spray oil is also ovicide so you get the mies and their eggs at same time. This breaks the breeding cycle. Be sure to spray underside of foliage.

PD, if it turns out you have virus in a plant here are some tips. If you smoke make sure you wash your hands with soap and water before handling plants. Carry a little squeeze bottle of alcohol.. Disinfect knives, shears etc, after working on virus plants.

I have not had to personally deal much with spider mites. Have only had two minor infections over the 20 years of growing dahlias. In both cases the infected plants did not die and the mites did not spread to other plants. From what I have read, people who spray for them alternate between at least three miticides. These chemicals are very expensive. Once you spray, you are committed to chemical control of both mites and insects. If you do not spray or instead use organic methods, the insect predators that eat mites and insects will increase in numbers and reduce infections naturally.

You all make sense. If I had the years left, I would grow veggies in there for a season and let the soil have arest. BUT I don't. I will just do the best I can next spring. Thanks for all your help. we can't let the bugs win.

AMEN TO THAT, STEVE

"If you smoke make sure you wash your hands with soap and water before handling plants."
Tobacco is contaminated with plant virus that can be transmitted by smoker's hands to susceptible plants.

Dahlias are not very susceptible however.