Causal agent(s) and transmission
Gangrene is a dry rot of tubers mainly caused by the fungus Phoma exigua var. foveata (the most aggressive type) or by the species P. exigua var. exigua.
These fungi survive in the soil as saprophytes. Infected soil adhering to the tuber acts as a propagator of the fungus, which, at harvest, contaminates the tuber surface through wounds.
Storage areas and equipment can also be sources of contamination.
Gangrene, as well as dry rot due to Fusarium spp., has a significant economic impact in cold or temperate climates such as in Northern Europe. Once potato production becomes mechanised and relies on machinery for harvesting, handling and grading, the incidence of the disease may increase.
Inadequate storage conditions will increase the incidence and severity of the disease. For seed potato production, this fungal group may be a significant cause of quality degradation.
Symptoms on foliage
Attacks on growing crops often go unnoticed.
Black pycnidia may appear at the base of the stems at the end of the growing season or even later, after haulm destruction.
Symptoms on tubers
Dark depressed areas may be observed on tubers, characterised by thumb mark spots (photo 1) which are frequently found around the wounds, the eyes, the lenticels or at the heel-end.
When the tuber is cut across the lesion, the flesh is shown to be infected by a dark-brown dry rot (photo 2), and pycnidia develop in the lining of the cavities (photo 3).
Control
Tuber rot caused by fungi of the genus Phoma thrives in the wounds created during harvesting and packing operations and by moderate storage temperatures of around 8 to 10°C.
Low temperatures, immediately after harvesting or packing, encourage the development of Phoma spp. as cold slows the healing of tuber wounds.
The following preventive control measures can be implemented to limit gangrene:
- good farm hygiene (cleaning and disinfection of buildings and equipment, destruction of waste, etc.);
- choose, if available, tolerant cultivars;
- harvest within 3 to 4 weeks of haulm destruction when tuber skin is sufficiently mature;
- minimise tuber wounds at harvest and during handling operations (grading etc.);
- eliminate infected tubers;
- prior to storage, allow time and adequate conditions (temperature and ventilation) to favour the healing of tuber surface and wounds caused by handling;
- use appropriate long term storage conditions;
- in the case of seed production only, chemical fungicides can be applied after harvesting on cleaned (‘soil-free’) tubers.