Why Are My Sweet Potatoes Lumpy and Deformed Underground?

Digging up sweet potato tubers to find them covered in unusual lumps, cracks, protrusions and an overall misshapen appearance — quite unlike the smooth, uniform tubers from a healthy plant — is often the first visible sign of a root knot nematode problem. Root knot nematodes (Meloidogyne spp.) are microscopic roundworms that live in the soil and feed on root tissue, causing abnormal cell growth (galls) that deforms developing tubers and impairs their ability to take up water and nutrients.

Identifying nematode damage

Pull up a plant mid-season and examine the fine feeder roots: small bead-like galls (swellings) on the roots are the classic sign of root knot nematode. On the tubers themselves, you may see irregular surface cracking, internal corking, protruding knobs, and sections of discoloured flesh inside. The above-ground plant may appear stunted or yellowed, particularly in hot, dry weather, because the damaged root system struggles to supply water and nutrients to the vines above.

How nematodes damage the crop

Female nematodes penetrate root tissue and inject secretions that trigger the formation of specialised feeding cells. The plant's normal cell division is disrupted, forming the characteristic galls. A heavy nematode infestation significantly reduces the functional root system, weakening the plant and reducing tuber quality. The tubers themselves become cracked and deformed as the nematodes feed on the developing tuber tissue. Nematode-damaged tubers also have reduced storage life.

Cultural management

Crop rotation is the most important management tool. Do not grow sweet potato or other susceptible crops (tomatoes, peppers, cucumbers) in nematode-infested beds for at least two to three years. Plant a nematode-suppressive green manure crop in the rotation gap — marigolds (Tagetes spp.) are well documented to suppress root knot nematode populations when grown as a solid cover for a full season and incorporated into the soil. Sorghum-sudan grass hybrids also show suppressive effects.

Resistant varieties

Several sweet potato varieties show partial resistance to root knot nematodes. Beauregard and Centennial have been noted as having better field tolerance than older varieties. Choose resistant varieties if nematodes are a known problem in your garden, and source certified nematode-free slips to avoid introducing new nematode populations on the planting material itself.