Why Are My Potatoes Rotting in the Ground?

You dig up your potato crop expecting firm, healthy tubers and instead find soft, brown, collapsed potatoes — sometimes with a foul smell, sometimes with a distinctive musty odour, sometimes completely disintegrated into a wet mush. Tuber rot in the ground is one of the most demoralising things to discover at harvest. The cause determines how serious the problem is and what you can do to prevent it in future seasons, so identifying which type of rot you are dealing with is the first step.

Late blight tuber rot

Late blight (Phytophthora infestans) primarily attacks the foliage but the spores wash down through the soil with rain to infect tubers. Blight-infected tubers have firm, reddish-brown rot beneath the skin that spreads inward — it does not collapse the whole tuber at once but the firm discolouration is unmistakeable when you cut through. The smell is musty but not overpowering. Blight rot in tubers progresses slowly at first but the infected tubers will break down completely in storage. After a blight outbreak on the foliage, the recommended practice is to cut all haulm, wait two weeks for spores to die in the soil surface before the skin hardens, then harvest promptly. Any tuber with blight rot should be discarded, not stored.

Bacterial soft rot

Bacterial soft rot caused by Pectobacterium species produces a very different result: a wet, slimy, foul-smelling collapse that turns the entire inside of the tuber to liquid mush while leaving the skin externally intact until pressed. If you pick up what looks like a whole potato and it squashes in your hand, or cuts open to reveal liquid brown mush with a strong rotting smell, this is bacterial soft rot. It enters through wounds — harvest damage, slug holes, cracking — and thrives in warm, wet, oxygen-deficient soil. Waterlogged conditions in late summer and autumn are the most common trigger.

Pink rot and other fungal rots

Phytophthora erythroseptica causes pink rot, where the cut surface of a tuber turns from white to pink within 30 minutes of exposure to air. Pythium species cause a similar wet rot. These are all favoured by waterlogging. Improving drainage — raised ridges, better soil structure, earlier harvest in wet seasons — is the most important preventive measure. Rotating potato ground every three to four years reduces the build-up of all soil-borne pathogens significantly.

Salvaging the crop

When you find rot in the ground, harvest everything immediately rather than leaving more tubers exposed to spreading infection. Sort carefully: any tuber with soft spots, discolouration, or smell should be discarded or composted far from the vegetable garden. Store only firm, undamaged, dry-skinned potatoes — do not store any that have wounds or any sign of rot, however small. Even a slightly soft area on one stored potato can infect a whole box within a week.