If that is just the bottom leaf - then don't worry about it. They will lose those bottom leaves pretty quickly. You will have additional bottom leaves go yellow as the plant gets taller, just snip them off.
If the whole plant is going yellow, then thats another story. It could be to much water if you have been having the rain we have had, but not from a single watering. Dahlias love water once the have established their roots. I hear some aficionados water lightly, twice a day.
