I used a garden blend in mine . It was recommened that I not use a compost or aged manure mix for dahlias because both are too rich in nitrogen.

I have always used a combination of aged cow-manure (mostly), and some peat-moss added to my very sandy soil, with bone meal added at planting time. I have great results, my plants are taller than normal,but I think that is due more to the fact that I only have 6 hours of sun , than to much nitrogen. Tons of blooms this year!

Pdshop,
Depending on how large your beds are, they sell Garden soil in the big garden centers that you could mix with COMPOSTED cow manure.
If the manure is composted, it means it has broken down and is ready for the plants to utilize.
Fresh manure would burn nitrogen, and they usually don't sell that. Good composted manure won't have a smell.
Dahlias are heavy feeders, so the manure will really help create healthy plants and beautiful blooms for you.

I would worry about the herbicides that are contaminating both compost and manure products. If you can go totally organic and get compost from an organic farm like a dairy. Herbicides are showing up in compost made from grass clippings and in manure from animals that eat hay from fields sprayed with herbicides. Even grass clippings from a yard where herbicides were sprayed months ago can contaminate a garden for dahlias. They are especially sensitive to the chemicals. Check out web site at Washington State University:
http://king.wsu.edu/foodandfarms/documents/Clopyralid.pdf

Thanks for all the input. What if I just covered and sterilzed the soil? Add something to the soil to get rid of the virus?

I don't know that you need to redo your entire soil bed if you suspect a virus. Don't they travel in the plant tissue itself, not the soil? It'd be a lot easier to toss the suspect plants and start over with new or healthy ones next season. I'm not sure, I've just never read anything on viral soil contamination.

One person told me to put clorox in every single hole that a tuber was in?
I would hope that the soil isn't contaminated. I would have to get it tested somewhere.

The viruses that affect dahlias are not soil borne. Plants become infected by insects that graze on infected plants and spread the virus to other plants. Once a dahlia catches a virus, it is nearly impossible to cure it. You may have other pathogens in the soil such as fungus or nematodes. Fungus can be controlled with fungicides. Nematodes are more difficult to control and commercial growers fumigate their soil to control nematodes.

How long have you been growing in that same location year after year? If a long time you probably have soil fungus as Ted suggested.

I would keep the soil and add horse manure.

I have been using the site for 5 years. Is sthat too long? I don't know where I could go? How old for the horse manure? So you don't think I should replace the soil altogether?

I have horses so any manure used around here comes from them. Try asking at your local stable or doing a walk and nock at a farm and ask if they have any aged manure they would like to sell. Most people will probly just give it to you. I put my fresh manure in a pile all winter until about march when I start a new pile. I stop adding to the first pile in feb. and start using it in april. So about three months from being fresh manure to being usable. If you ammended your beds even with fresh in the fall it would be fine for spring planting. But again I was told by several dahlia farms to not use manure on dahlias aged or not. Of course I didn,t have that info. years ago when I grew dahlias and I don,t seem to remember any problems.