Causal agent(s) and transmission
Several types of wet rot can be found on potato tubers. They are caused by various Oomycetes, belonging to the family Pythiaceae, such as several species of the genus Pythium, namely P. ultimum, which cause watery rot (also called watery wound rot or leak rot).
This pathogen can survive in the soil for several years as thick-walled oospores and has a wide host range on which it is often the cause of the damping-off of seedlings.
Watery wound rot can be very damaging to potato crops, his occurrence in continental Europe is fairly uncommon. Nevertheless, due to elevated optimal growth temperatures (above 20°C for both), there is a small risk of the disease occurring in southern European countries.
Symptoms on tubers
At first, Pythium spp. cause a grey wet rot, which then turns brown on contact with the air and gives off a slight fishy smell. The texture of the affected tissues is relatively fluid and creamy, and healthy and diseased tissues are clearly separated. During storage, the symptoms may evolve into a watery rot when tubers are not dried properly (photos 1 to 4).
Risk factors
The development of this pathogen is favoured by high soil humidity and moderate to elevated temperatures.
Pythium spp. develops mainly during hot periods and infect the tubers, especially after hot stormy weather at harvest time, by penetrating through wounds or open lenticels. If the harvested tubers are not dried sufficiently, tuber damage may be severe and lead to tuber or lot leaking.
Monoculture or intensive potato production, added to favourable environmental conditions and the presence of primary sources, may increase the severity of the disease.
Control
- Use healthy seed potato tubers (cultivar resistance is not yet available).
- Maintain a lengthy crop rotation and make sure fields are properly drained to avoid water excess in soils.
- Keep harvest damage to a minimum.
- Ensure good skin set prior to harvest and avoid harvesting during hot and humid periods.
- Dry the tubers quickly and thoroughly after harvesting and prior to storage.
- Ensure good ventilation during storage to prevent condensation.