SLUGS - Hollowed out tubers are a good indication of slug attack. Sprinkle slug pellets around the growing plants. Less likely causes are soil living caterpillars. Rake in phoxim or diazonon if root eating grubs are know to be a problem.
SCLEROTINIA ROT - The bases of the stems are attacked, and show fluffy, white mould. Black cyst-like bodies occur in the rotten stems. Lift and burn the affected plants WITHOUT DELAY as it can spread to healthy plants, rendering them useless.