Symptoms

Symptoms on tubers

There is no visible symptom on foliage in the case of a powdery scab attack unless the fungus is also carrying the Potato mop-top virus.

In the case of a light attack, the symptoms on tubers might easily be confused with symptoms of common scab.

The propagative agent of the powdery scab penetrates the skin of the tubers, probably through the lenticels, where it causes brightly-coloured pustules (photo 1); the latter turn dark on maturity (photo 2).

Subsequently, the pustules burst and release a powdery brownish mass containing spore balls (photos 3 and 4). An observation under the microscope of skin fragments taken from the pustules will confirm the diagnosis by the presence of these typical spore balls = cystosori  and prevent any confusion with common scab or other surface blemishes.

In the case of a very severe attack, due to prolonged favourable conditions in heavily contaminated fields, the lesions develop into major tumours either in the form of depressions (photo 5) or they proliferate in teat-shaped forms (photos 6 and 7).

During storage, the final stage in the evolution of symptoms of powdery scab on potato tubers consists of a large number of small empty depressions, surrounded by flakes of torn skin.

Symptoms on roots

The fungus also penetrates the root tissues of susceptible cultivars where it forms root galls (cankers). Immediately after pulling the roots out of the soil, the galls are white, later turning brown (photos 8 and 9).

After harvest, these gall-bearing roots remain in the soil and represent a significant source of inoculum, as do all the contaminated groundkeepers.