Description
It is a dangerous potato pest capable of causing extensive damage if not properly controlled. Although the natural ability of this pathogen to spread is limited, long distance dissemination can occur through resistance spores present both in infected tubers and in soil or plant residues adhering to them.
In Italy, the first reports date back to the early 1970s in a circumscribed outbreak in Valtellina, from which it fortunately did not spread, as further phytosanitary inspections conducted in the following years have shown that it has been eradicated.
However, in August 2025, during a phytosanitary investigation activated on a report from a hobbyist potato farmer, the pathogen was found in Val dei Mocheni.
DISTRIBUTION. Synchytrium endobioticum has a rather fragmented global distribution. The pathogen is native to the Andean region of South America and was introduced to England around 1880 from where it rapidly spread throughout the European range and into North America around the early 1900s.
Currently, S. endobioticum is present in 15 EU Member States (Bulgaria, Czech Republic, Denmark, Estonia, Finland, Germany, Greece, Ireland, Italy, Luxembourg, the Netherlands, Poland, Romania, Slovakia and Sweden). However, the reports concern a distribution of the pathogen confined to distinct areas.
In 2020, in Germany, S. endobioticum was found in ware potatoes in Lower Saxony during a phytosanitary inspection but the origin of the infestation could not be identified.
The current distribution on a global scale can be found at this link: https://gd.eppo.int/taxon/SYNCEN/distribution
BIOLOGY. Winter spores have thick walls and can remain dormant and infective for at least 40 years even in the absence of host plants.
Several studies have shown that, in addition to the long survival period, disease recurrence under favourable conditions can also occur from a single winter sporangium of S. endobioticum after more than 40 years.
Soil temperatures of at least 8°C as well as soils with an adequate water content are required for the germination of winter and summer sporangies and the dispersal of zoospores.
Therefore, potato black scab is favoured by cool summers with average temperatures of 18°C or lower and moist soils, especially during the tuber's development phase.
HOST PLANTS Potato(Solanum tuberosum) is the main natural host of Synchytrium endobioticum. In Mexico, unconfirmed reports suggest that the pest affects wild species of the genus Solanum L.
DIFFUSION ROUTES.
NATURAL SPREAD. In fields infested with S. endobioticum winter sporangia, natural dispersal by wind or water is rather limited. However, winter sporangia can be moved within the same field or between adjacent fields by water runoff (rain or irrigation water), wind and the movement of soil particles.
ANTHROPOGENIC SPREAD. Human-assisted spread of S. endobioticum can occur through the transport and subsequent sowing of seed potatoes. In addition, soil adhering to potato tubers or the roots of even non-susceptible plants intended for planting can cause long-distance movement of the pathogen.
Equipment, vehicles, machinery or footwear may transfer the pest's sporangia from the infected field to other adjacent and non-infected fields.
Sowing potatoes of non-resistant varieties may accelerate the multiplication of the pathogen and the accumulation of the inoculum due to the lack of symptomaticity, and is therefore not recommended.
Potato tubers intended for consumption or processing may pose a risk, particularly those that are asymptomatic or have inconspicuous warts, as they may be planted (especially on small farms and in private gardens), discarded (whole potatoes or skins) or used for livestock feed.
Manure and its annual distribution can further spread the disease, as winter sporangia survive the transit of the animal digestive tract of cattle fed on infected tubers or even grazed in pathogen-infested fields.
Soil and plant material used as fertiliser (even after composting) from potato processing industries, as well as process water reused for irrigation and/or cleaning potatoes or equipment used in cultivation operations, may also contribute to the spread of the pest.
SYMPTOMATOLOGY. Symptoms of Synchytrium endobioticum generally occur on the stolons and tubers of infected potato plants. It should be noted that only occasionally do infected plants show general symptoms of reduced plant vigour. However, in the case of a severe attack, warts may be present on the upper part of the stem, on the leaves and also on the flowers. In severely affected plants or very susceptible potato varieties, warts are also found on the lower leaves, on the aerial buds at the base of the stem and on emerging shoots, blocking their development and taking on an irregular cauliflower-like appearance. Particularly susceptible to infection are the young parts of the shoot, including the lower leaves, lateral buds and dormant and growing apical buds. These symptoms are rather difficult to recognise and often the disease is not noticed until the tuber harvest stage.
The typical symptom of S. endobioticum is the proliferation of warts on the tubers that can be seen throughout the growing season. The warts vary greatly in shape, but tend to be spherical. The infection of tubers originates in the eye tissue, but the excrescences can reach extremely variable diameters (even larger than 8 cm) and even encompass the entire tuber. The surface warts are greenish in colour due to their exposure to sunlight, while the underground warts are white-brown in colour. At maturity, all warts turn dark brown-black and eventually rot and disintegrate, sometimes before harvesting. Usually, the disease is not noticed until the tubers are uprooted.
Early infection on young developing tubers causes distortions and sponginess, making them unrecognisable, while characteristic cauliflower-like protuberances develop in older tubers. Similar warts may also appear on the stolons, but never on the roots. In fact, the root system of the potato does not get infected (unlike the roots of tomato plants). Finally, it must be considered that under conditions particularly favourable to the fungus, the tubers may not even develop.
The plant disease may continue to develop even after the tubers have been harvested and small warts, barely visible at the time of weeding, may grow larger during the storage phase of the tubers (especially if prolonged).
PHYTOSANITARY CONTROL. In the event of new findings, the Provincial Plant Protection Service implements immediate phytosanitary measures to contain the possible spread of the pathogen, including
- a ban on the movement out of the infected area of plants and plant products (even of non-host species), plant residues and cultivation soil/substrates that are, or are suspected to be, contaminated
- soil or debris originating from an infested zone may only be moved, used and/or stored outside the infested zone under conditions which ensure that there is no identifiable risk of spreading the harmful organism;
- prohibition of planting/producing/harvesting/storing seed and ware potatoes;
- prohibition of growing plants for replanting outside the infested area;
- plants other than host plants may only be moved outside the infested area without root systems and/or residues that may have been in contact with the soil. Alternatively, the soil must be removed by appropriate methods to ensure that there is no risk of the harmful organism spreading;
- thorough disinfection with a 20% sodium hypochlorite solution of footwear and equipment used within the infected field;
- machinery is cleaned of soil and plant debris before or immediately after being moved out of the infested area and before entering any production site located in the buffer zone or outside the demarcated area;
- destruction of all plants, tubers and plant debris (including those of non-host species) in the infested zone;
- disposal as special waste (to be disposed of in closed containers) of consumables used in the infected area such as gloves, sacks, sock covers, etc.; and
- fencing, where possible, the infected area to prevent the occasional transit of animals and wildlife;
- prohibiting grazing, especially of transhumant flocks.
REFERENCE LEGISLATION: Regulation (EU) 2016/ 2031; Implementing Regulation (EU) 2019/2072; Legislative Decree No. 19/2021; Commission Implementing Regulation (EU) 2022/1195 of 11 July 2022 laying down measures for the eradication and prevention of the spread of the harmful organism Synchytrium endobioticum (Schilbersky) Percival