A number of fungi and bacteria are capable of causing decay in potato tubers. Symptoms are often present at lifting, but some of the problems will also spread through stored tubers, and a few will only develop after prolonged storage.
What are potato tuber rots?
Potato tuber rots are a frequent cause of losses prior to, or after, lifting. Significant problems often follow a wet growing season, particularly if the tubers are then lifted from wet soil. Some of the pathogens causing tuber rots also produce symptoms on the aerial parts of the plant.
Blackleg develops when bacteria spread along the stolons from decaying stems and enter the daughter tubers.
Contaminated seed tubers are the most important source of the blackleg bacterium (Pectobacterium atrosepticum). Both this and the soil-borne soft rot bacterium (P. carotovorum) can also infect tubers via damage, or act as secondary colonisers following other diseases. Wet soil during growth and lifting of the crop make bacterial problems much more likely.
Blight affects the tubers when spores are washed down into the soil from lesions on the foliage. Blight does not spread to any extent during storage, but secondary bacterial rots do, and can cause extensive losses. The fungus-like blight pathogen also affects tomato crops.
Pink rot, dry rot and gangrene are caused by soil-borne fungi. Any damage suffered by the tubers at lifting will make them much more prone to infection. It may take weeks or even months of storage for symptoms of dry rot or gangrene to develop.
In addition to the diseases mentioned above, there are several other bacterial and fungal diseases causing decay of tubers, e.g. rubbery rot, violet root rot, watery wound rot. The bacterial diseases brown rot and ring rot, whilst unlikely to develop in garden crops, are caused by notifiable organisms. Where significant problems develop it may be worth having a sample examined by experts to determine the cause.