Nobody seems to have an answer for you, sorry. My first thought was hard water scale, but unless you're misting them, that doesn't make sense. Powdery mildew was another thought, but that is finely powdered and not very flaky.

How are the plants doing otherwise? Perhaps it's nothing much to worry you.

Hi Poochella,

Thank you for your reply. Plants are doing ok. One variety better than the other. The not so good variety now seems to have black on the a couple of leaves and the leaves tend to curl up? The white stuff is very flaky. Comes off very easy. I water at the base of the plants. I hope I am successful this year. I LOVE DAHLIA'S!!!