Soft rot or wet breakdown is the general term used to describe tuber decomposition which may involve one or more fungi and/or bacterial agents. Soft rot may also be induced by mechanical or physical damage to the tuber such as by frost injury (1), freezing and chilling injuries (2) and bruising, and subsequent infection. Freezing injury most often shows a complete break down and watery rot of affected tubers. Tubers which have been exposed to freezing temperatures and allowed to warm up for 1 week to 10 days usually express classic symptoms of soft rot. Pathogenic agents of soft rot may include: Fusarium spp. (3), Blackleg or bacterial soft rot - Pectobacterium carotovorum subsp. carotovorum (4, 5), Early blight - Alternaria solani, Late Blight - Phytophthora infestans, Leak fungus - Pythium ultimum var. ultimum (6), among many others. Certain pathogens may be causative agents of both soft rot and dry rot.
In scoring soft rot or wet breakdown it is not always necessary to identify the specific pathogen affecting the tuber. Images below are not representative of all symptoms which may be encountered; any soft, mushy breakdown, soft leaking specimens or soft and watery tissue must be scored under the tolerance for soft rot or wet breakdown. Refer to Seeds Regulations Part II for tolerances. Quarantine pests may exhibit similar symptoms to soft rot, consult additional resources for information.