Symptoms on tubers

Whilst attacks by wart disease occur mainly below soil level (photo 1), in very damp conditions symptoms can be observed in the aerial parts of the plant.

Tumours caused by cell proliferation can be of variable size from a few mm to the size of a fist. They develop on the stolons, tubers (through the lenticels) and on the underground parts of the stem.

Unlike powdery scab or root-knot nematode, the wart tumours never form on roots. The tumours are first whitish and then blacken on contact with the air (photo 2) and they release a black spore powder. They can turn green when exposed to the light (photo 3).

The symptoms may evolve during storage.