Vascular discoloration refers to a darkening of the ring of tissue that runs around the tuber just below (about a quarter of an inch) the skin. This ring contains the vascular system, phloem an xylem, of the tuber. This appears dark in the raw tuber and darker upon frying (picture of potato chip with discoloration). The discoloration can be all around the tuber or just in part of the ring usually around the stem end where the tuber was attached to the plant, via the stolon. There are several causes for this discoloration, primarily wilts, zebra chip, vine desiccation, rapid vine death, and physiological stress during the season while tubers are bulking or growing. See Physiological Disorders / Internal Disorders / Vascular Discoloration.
Wilting caused by pathogens within the vascular system of the vine can move into the tuber vascular system causing discoloration. The most notable of the wilts is Verticillium wilt or early dying. The discoloration begins at the stem-end and progresses further around the tuber. See Diseases / Wilts / Early Dying. Other wilts can also cause a vascular discoloration as well as rots (picture of Fusarium infected tuber). See Diseases / Wilts / Black Leg, Fusarium wilt, and Stem Canker as well as Diseases / Tuber Rots / Soft Rot and Dry Rot.
Zebra chip, besides net necrosis, may have a vascular discoloration of the tuber associated with it. See Diseases / Tuber Blemishes / Zebra Chip.
When vines die too quickly at the end of the season, the tubers may develop a vascular discoloration specifically at the stem end. It does not spread around the tuber. The common causes for too rapid vine death are mechanical and chemical vine desiccation. See Chemical Effects / Vine Desiccation / Mechanical and Chemical.
A major problem for the french fry industry is another form of vascular discoloration that occurs at the stem end called Stem-End Discoloration or SED. This discoloration refers to the appearance of a brown arc during frying at one end of a french fry (picture of fry with SED). The cause is physiological and not pathogenic, and is related to environmental stress occurring during the season at the time the tubers are actively growing (log phase of growth). See Physiological Disorders / Internal Disorders / Jelly End.
French fry with stem-end discoloration.
Potato chip with vascular discoloration.
Vascular Discoloration caused by Fusarium
Tuber Blemishes
- Black Dot
- Silver Scurf
- Common Scab
- Early Blight
- Black Pit
- Late Blight
- Zebra Chip
- Net Necrosis (PLRV)
- Vascular Discoloration