
Certified GKH Gardening Expert
It could be botrytis blight. These articles should help:
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/dahlia-pests-and-diseases.htm
https://www.gardeningknowhow.com/plant-problems/disease/treating-botrytis-blight.htm
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/bugs-on-dahlia-plants.htm
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/different-varieties-of-dahlia.htm
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/dahlias-annual-or-perennial.htm