The message from Noydb contains these words:
That looks (to the untrained eye) like a fungal disease, quite possibley
> septoria. If so, give it a shot of a copper or sulfur based fungicide every
> 8-10 days and destroy (burn or remove from the property) all dead plant
> material / deep plow the area in the fall.
> Bill
> --
> Zone 5b (Detroit, MI)
> I do not post my address to news groups.
Could be the dreaded potato blight which is what you could be talking about with septoria.
I hope the potatos are being moved around rather than in the same spot as thats just fatal for em.