Bacterial soft rot – Erwinia carotovora subsp. carotovora (Ecc) and other bacteria
Soft rot is a very common, complex, and important disease of potato tubers. It is felt that much of the rot that develops in transit and storage is initiated in the field. In addition to E. carotovora subsp. carotovora (Ecc), two other Ercuinias can attack potato and cause soft rot; these include the blackleg organism E. carotovora subsp. atroseptica (Eca) and, in some geographical areas, E. chrysanthemi (Echr). The temperature at which diseased plant material is incubated can influence which soft rot bacteria are isolated. As temperatures increase from 61 ° to 99° F, the dominant populations shift from Eca to Ecc to Echr if all three bacteria are present initially. These pathogens can invade through injuries and enlarged lenticels. The first external symptoms are tan or watersoaked spots on the tuber surface, which eventually become slimy. Lenticel infection can result in raised dark brown lesions up to 1/a inch in diameter. Following entry of the bacteria, the tuber flesh becomes soft and rotten. The cream–colored rotted tissue may be separated from healthy tissue by a brown or black border.
Losses to soft rot can be reduced by avoiding injuries. Under optimum conditions suberization (protective tissue) develops within 24 to 48 hours after an injury. However, at temperatures below 50° F and above 95° F, healing proceeds at too slow a rate to provide much protection against wound pathogens. Because of this, it is best to hold newly harvested potatoes at moderate temperatures (60°–70° F) and at as high a humidity as possible for a period of 7–10 days before storage at low temperature (39° F). The seed should be warmed (at or near room temperature) before planting, and whenever possible, it should be planted in soil with a minimum temperature of 50° F at a 5–6–inch depth.