These symptoms are not those of a disease, but rather look like feeding damage from voles or an insect or slug. Among insects, likely culprits would be beetle larvae, primarily wireworms (the larvae of click beetles), white grubs (the larvae of scarab beetles), and flea beetles. The Sweet Potato Flea Beetle is our primary guess for this pattern of damage (see linked page), though without the culprit present, it's hard to be certain.

To discourage this from happening next year, make sure all plant residues (pieces of root, dead foliage and stems, etc.) are removed from the bed for winter, so they don't harbor any pests or diseases. As noted on the N.C. State page above, cultivars 'Centennial' and 'Jewel' are among those reported to have some resistance to flea beetle feeding.

Miri