The bacterium, Erwinia carotovora var. carotovora causes bacterial soft rot. It often occurs in conjunction with other diseases such as late blight, leak and blackleg. Serious losses in stored potatoes generally occur when the potatoes have been frozen, injured or harvested under excessively wet conditions. Tubers may be infected via the breathing pores (lenticels) and tissue will collapse around the pore to form brown sunken lesions. If tubers become badly bruised during handling, the whole tuber may become infected. Initially infected areas are cream coloured, later becoming brown, slimy and foul smelling. A distinct line in the tuber will delineate diseased from healthy tissue. Excessively wet growing conditions may result in tubers rotting in the ground.
The soft rot bacteria live freely in the soil and usually invade potatoes following mechanical injury or infection by other disease organisms. Infected tubers can spread the disease to healthy tubers particularly under conditions of high humidity and poor ventilation. Tuber wounding during the washing process may allow soft rot bacteria present in the wash water to enter the tuber and cause damage rendering the tubers unmarketable.