I'd spray them with bleach and put them somewhere warm to allow the bleach to dry for a few hours. You could do a test with just one and put it in a plastic bag, add peat moss/sawdust/perlite or vermiculite, and see how it looks 24 hours later. If it looks good, without any mold growth, repeat the process with the others. Please give each one a separate bag so any problem you face won't transfer to the other dahlias.
My reason for not using paper bags is that you still want to give them a spritz of water and I'd hate to see the paper bags deteriorate.
Use new, dry, plastic grocery bags, not the old ones where they had been, which could contain mold spores - dispose of them.
