Causal agent(s) and transmission
Potato wart is a serious disease, caused by a soil-borne fungus, Synchytrium endobioticum. It is an obligate parasite, which does not produce mycelium but an abundance of dissemination sporangia which are responsible for tumour formation on underground potato organs. These sporangia produce zoospores, organs of dissemination and infection.
This fungus can survive for many years (up to 30 years) in the soil in the form of a cyst (survival spores).
The primary infection may be caused by seed tubers, infected soil or by any plant for planting (flower bulb or nursery stocks) carrying infected soil.
Although potentially pathogenic to several Solanaceae (tomatoes, aubergines and weeds), it is mainly found in potatoes. In the case of seed production, strict quarantine measures must be taken.
Potato wart disease may be a serious concern for potato production and is very difficult to eradicate once introduced in a field. Severe legislations are enforced to avoid its dissemination; in Europe, for example, this pathogen is a regulated pest and is classified as a quarantine organism (A2 list).
The variability of this severe pathogen is a major concern and takes the form of numerous pathotypes (over 20), characterised by their virulence towards various potato cultivars. Pathotype 1, identified in Germany and in the Netherlands, is predominant. The disease is especially present in cold, humid climates, e.g. in Northern and Central Europe (Poland and the Czech Republic).
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.
Control
The disease is found in areas with a cool, humid climate: average temperatures in summer below 18°C and annual rainfall greater than 700 mm. The disease is favoured by soils that retain moisture.
As there is no effective chemical control against potato wart, which can remain infective in a soil for more than 20 years, preventive measures and strict inspections and surveys are essential:
- the use of healthy certified seed tubers;
- avoiding the introduction of soil or planting material from infected areas and careful examination of introduced potato tubers;
- the use of resistant cultivars, particularly in the case of potato cultivation in potentially infested areas.
In Europe, national lists of resistant cultivars are available.