They do not seem to be growing as quickly as last year & leaves are turning yellow. I believe last year we had spider mites. West Pittston Master Gardeners helped us get rid of them quickly.
Luzerne County Pennsylvania
The problem could be pests or it could be environmental. Yellow leaves can be a sign of over watering or not enough sun. It might mean a nutrient deficiency. You can find spider mites by taking a sample and looking with a magnifying glass for little black dots and indications of insect activity. Here is a link to a fact sheet on spider mites on Dahlias.
I recommend you take a sample to your local Penn State Extension and have them send it to the plant clinic for analysis. Also, pick up a soil test and send a sample of the soil in your dahlia bed. They will tell you if there is a nutrient deficiency.