Dry rot is the term used to describe dry decomposition, usually starting in cuts, bruising or other injuries. Dry rot is most often caused by Fusariumspp. (7,8).Other conditions or pathogens may result in dry rot. White knot or Starch spot (9) is a bruising which appears as a hard white lump in the tuber flesh a few millimeters below the skin; this is similar to Black spot except the coloration is lacking. Both White knot and Black spot (10) start out as a bruising injury but may develop into a dry rot in severe cases. Additional pathogens resulting in dry rot include, Late blight - Phytophthora infestans (11), Early blight - Alternaria solani (12), Blackleg - Pectobacterium carotovorum subsp. carotovorum, as well as severe Skin Spot - Polyscytalum pustulans, among others. As well, mechanical and physiological damage which has dried, such as pressure bruising, or dried frost and freezing injuries often result in dry rot. In storage below 4°C, dry rot fungi in small lesions may remain dormant, however upon planting may be quickly attacked by soft rot bacteria which rot the tuber very quickly. Certain pathogens may be causative agents of both dry rot and soft rot.
As in the case of soft rot or wet breakdown, it is not always necessary to identify the specific pathogen affecting the tuber. Dry rot is scored under the dry rot tolerance when it affects the flesh of the potato and penetrates more than 6 mm in depth or is less than 6 mm in depth but covers more than 10% of the surface area. It may be slightly moist but not wet, and thus is not considered a "wet" rot. Refer to Seeds Regulations Part II for tolerances.