Sounds to me like you are overwatering. Do the pots have good drainage? How often are you watering them?

I have many in pots but I do not water them until they are very dry (dry 2"to 3" down). Its been in the 90's here and I still only water them once a week ( 10 inch pots) My potted dahlis's are 4 ft tall and blooming.

Hope this helps.
Willow

I don't think that it's a watering issue - but I'll definitely give this a try, less watering that is - but the leaves often look dull and droopy, so I was afraid that I was inconsistently watering and as a result this left my dahlias open to diseases. I have a bad feeling the tubers are infected or rotting or something, which means I don't have much of a chance for remedying them.

Thanks very much for your response.

You didn't say how big they where but try digging one up and checking the tuber. If the tuber has some rot , cut the rot off and spray the cut with some lysol and replant.
Hope this helps.

Willow

Are your pots lifed up a bit so they can drain properly?? Sounds like they are water logged