If you have been experiencing hot weather, than I think you have Smut on your dahlias, but hopefully, not on your mind. This fungal disease can be controlled by picking off all infected leaves and then spraying the plants with a wettable sulphur fungicide. Try not to overwater and give good air circulation. Dahlias are also susceptible to viruses which could be from bad bulbs and there is no cure for this. Dig and destroy the plants and tubers so the virus does not spread. A virus infection will show light green bands on the leaves or yellowish blotches and spots. If it is Smut you can tell because the leaves have spots that turn brown, dry out and then fall out leaving some leaves with holes in them. Sending a photo would help in this mystery diagnosis. Keep growing, Marianne Binetti