Causal agent(s) and transmission
Silver scurf is caused by the fungus Helminthosporium solani. There is no visible symptom on foliage during the growing season. However, at the end of the growing period, the fungus causes blemishes and silvery lesions on the tubers.
The pathogen is transmitted by infected seed tubers or, in rare cases, by the soil where the fungus is able to survive on dead and decaying organic matter, potentially subjecting a subsequent potato crop to a new cycle of infection.
During storage, the disease can develop if the temperature and humidity are favourable.
The optimum temperature lies around 20- 25°C but the fungus is capable of developing from 5°C onward.
Silver scurf is a common disease of potato tubers but its direct effect on yield can be counterbalanced by adequate fertilisers and good irrigation management. It can cause tubers to shrivel during storage, and thus reduce their marketability.
Moreover, when fresh ware tubers are washed before selling, silver scurf, a surface-blemishing disease, may downgrade the tuber aspect, and thus the marketable yield and induce economic loss.
Symptoms on tubers
Light silvery lesions (photos 1 and 2) are clearly visible on the tuber surface of red-skinned cultivars (photo 3).
These patches spread and eventually are covered with very fine black dots; these are the fructifications of the fungus (conidiophores that carry spores) (photo 4).
At harvest, these symptoms are barely visible and appear mainly during storage when the temperature and humidity are favourable.
The symptoms of silver scurf and black dot are often confused with each other, even though some criteria make it possible to distinguish them. For H. solani, the surface lesions are better defined and more silvery with finer black punctuations than for black dot (photo 5: silver scurf, on the left – black dot, on the right). In favourable moist conditions, sporulation of H. solani occurs at the edge of the silver scurf lesions and this differentiates it from black dot.
When the fungus has covered a large portion of the tuber, the superficial layer of the skin becomes detached; the tuber shrivels and shrinks due to dehydration (photo 6).
Silver scurf on potato tubers is usually visible after harvesting although the primary infection has occurred earlier, in the field.
During the growing cycle of the potato, the seed tuber has usually been considered as the main source of inoculum. But today, there is sufficient evidence to show that soil residues of numerous plants, crops and weeds can also play a role in the dissemination process by hosting and maintaining the infectivity of the fungus during its winter resting phase.
At harvest, a prolonged delay (more than 4 weeks) between haulm destruction and harvest increases the risk of tuber infection.
During storage, temperatures below 5°C inhibit fungal growth but this is not compatible with tuber quality for the fresh market. Ventilation and relative humidity in storage are the other factors necessary to control fungal growth. Efficient ventilation can reduce excessive moisture but this may encourage uncontrolled sporulation and dispersion of the spores present in the storage areas and/or on the tubers which can act as primary inoculum on healthy tubers. No chemical control is possible at this stage.
Various measures can restrict the spread of silver scurf without entirely preventing its development:
Prior to planting and during the growing season:
- destruction of susceptible crop residues and weeds before planting;
- use of healthy seed tubers;
- seed treatment: certain seed fungicide treatments on seed tubers may have a positive effect on the skin quality of daughter tubers;
- crop rotation and avoidance of cultivars that are too susceptible (thin-skinned cultivars); o reduction of the delay between haulm destruction and harvest (which should be done as soon as the skin is set).
At harvest and during storage:
- thorough disinfection of storage areas before use;
- dry tubers thoroughly to encourage rapid healing of the wounds and to limit disease development;
- store in suitable conditions (low temperature and limited air moisture);
- limit the effects of condensation during storage and when taking tubers out of storage as the disease may develop rapidly after cold storage if the conditions are favourable (saturation point humidity and temperature over 6-8°C).