My Dahlia pinnata Dahlietta® Tessy plants have all developed ugly black coalescing leaves lately. Is this disease or something normal, such as end-of-season senescence? Are these tubers worth saving? Affected areas are indicated in yellow:

It is not typical. I would guess some kind of fungus?

If it is that it wont necessarily effect the tubers, but may overwinter in the soil.

Thank you, Geof. I removed all three plants to the trash. I hope the fungus is specific for dahlials. So much for growing dahlias! That's enough for me.
Carol