Dinner Plate Dahlias What Would Cause Dinner Plate Dahlias To Dry Up?

What would cause dinner plate Dahlias to dry up? - dinner plate dahlias

They have several floors of a dahlia plate, is the second year bulbs. Many of the leaves of some of them seem to be just dry. We were terribly hot and humid weather this year - but perhaps not mold or one of the dahlias are known viruses. They are planted in an area where the clay soil - about 5 feet high. Could it be that the roots are in the clay? Any ideas would be appreciated.