Causal agent(s) and transmission
Powdery scab is caused by a soil-borne obligate fungus, Spongospora subterranea, which forms superficial lesions on the tuber surface and galls on roots and stolons of susceptible potato cultivars.
This pathogen is also the vector of the Potato mop-top virus (PMTV) which causes external and internal necrosis of the tuber, known as spraing.
It can survive for a very long time in the soil. It constitutes a major source of inoculum.
Infected tubers also contribute to the spread of the disease into healthy areas. Moreover, due to the capacity of the contaminating spores to be dispersed in the wind like powder, any farm equipment or organic substrates (manure, soil substrate etc), as well as soil or dust storms can carry contaminating spores and transmit both the fungal vector and its virus.
Until now, tomato and potato crops are the only known hosts of this fungus.
This disease may occur worldwide wherever the potato is grown. It thrives best in cool climates. When significantly established in a field, and when the climatic conditions have been optimal (cold and humid on emergence), powdery scab attack can lead to a complete yield loss in the case of the crop of a susceptible cultivar.
Moreover, being the vector of the Potato mop top virus makes powdery scab a very serious threat to any potato production.
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 (photo 8).
After harvest, these gall-bearing roots remain in the soil and represent a significant source of inoculum, as do all the contaminated groundkeepers.
Risk factors
During the potato growing cycle, and more specifically on emergence and at tuber initiation, low temperatures (9 to 17°C) and the presence of water are the most decisive factors in the development of the fungus. Environmental conditions that enhance the opening of lenticels are also be favourable to contamination.
During storage, infected tubers may contaminate the environment through air- dispersal of spores (photo 9) by the ventilation system. Due to their high potential to survive in unfavourable conditions, these spores can be carried by any substrate that then can become a source of contamination.
Control
When the disease is absent, the most efficient control measure is to use healthy seed tubers and to avoid the introduction of the disease through infested soil or any other substrate that might have been in contact with an environment contaminated by spores.
When the fungus is already present, the best control measures are:
- draining the wet areas;
- avoiding over irrigation at tuber initiation;
- using resistant or tolerant cultivars (avoiding tuber resistance linked to root susceptibility);
- maintaining long crop rotations (at least 5 years between two potato crops).
When authorised, fungicides can be used to treat the seed tubers and/or the infected soil but their efficiency is very limited in the case of severe infestation.
Exploiting cultivar resistance can be the most efficient control measure. However cultivar assessment is difficult due to the complexity of the host-parasite relationship.