Poor soil conditions. Amend your new bed big time. Dig it deeper & put in lots of organic material. Fertilize the plants with an all purpose 10-20-20 or something like that twice- once when planting & once in July. Make sure it's draining properly- where's all the rain going in that hardpan? You might consider raising the bed- sometimes easier than digging up hardpan.
Keep the mother tuber- if she's in good shape & has an eye she'll grow again next year- keep her separate from the others so if she rots she doesn't rot the new ones - & vice versa.

thanks -- you're right about the water, where does it go? It puddles up and sits for awhile and doesn't drain down thru', 'cause of the hardpan - I'm surprised actually I didn't have a whole bed of rotten ones!! Will have to start amending the bed when I get the raised bed built -- hated to move, my last dahlia bed had wonderful soil, built up from compost over 10 years, wish I could have brought it with me. Where about's your farm - do you open it to the public in summer? Kathy

We're east of Ferndale, WA - about 13 miles south of the Canadian border. Yes, we do let people come & look at our gardens in summer as long as they call ahead & let us know they're coming so someone can be here to show them around.