Symptoms on tubers

The symptoms vary greatly:

- Common scab characterised by shallow or deep-pitted corky lesions (pustules) which depend on various factors, including the type of strain, the cultivar and the climatic conditions.

On susceptible potato cultivars, the lesions form craters (photo 1) in the tubers (photo 2 to 4) and necrosis may be observed on the stem base (photo 5) and on the stolons. On other varieties, attacks may be superficial corky lesions on the tuber surface (photo 6) or star-like corky lesions (photo 7).

- Netted (or flat or surface) scab is characterised by corky spots that usually coalesce to form a network of polygonal corky lesions on the tuber surface (photos 8 and 9). Necrosis on the stolons and on roots and a reduction of root system (photo 10) may also be observed on highly susceptible cultivars.

In the case of an early attack, the yield may be affected.