Below ground part of stem turns brown. Aerial tubers are sometimes formed. Use certified seed. Treat seed as for scab. Foliage is mottled or leaflets tend to roll up.
Tubers are often small, and in case of spindle tuber, elongated.
Several viruses can infect potato without causing noticeable symptoms. However, yields can be reduced drastically. Use of seed certified to be true to variety and free of disease agents is the best way to control tuber-borne viruses. This virus is vectored by thrips insects. Early symptoms are dead spots on upper leaves and death of the upper plant parts.