These symptoms look similar to two things: a viral disease, and herbicide damage. In either case, there is nothing you can do to correct the problem other than try to keep the plant healthy by providing optimal water, light and temperature requirements. Herbicides can come from unlikely sources like commercial compost from animal sources. They can also drift on wind currents, especially during warm weather. Viruses are spread most often by insects like aphids. If you have other dahlias not affected, it is possible for the virus to spread.