Potatoes
9. Decay and Wet Breakdown

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Decay (C)

The term decay is used in describing a deterioration or decline involving decomposition, which is induced by a fungi and/or a bacteria, and which is of a complete and progressive nature. Inspectors are not pathologists, so it is not their duty to name the specific decay affecting the tuber.

Any amount of decay is scoreable against both grades.

The following are scored under the tolerance for decay:

  1. Any soft, mushy breakdown (soft rot);
  2. Any soft leaking specimens (freezing injury described in Section 7.9);
  3. Bacterial ring rot (described below);
  4. Soft and watery tissue of black leg (described below);
  5. Wet or soft blight (described below).

Bacterial Ring Rot (C)

Bacterial ring rot is one of the most serious diseases of potatoes in Canada. It is highly infectious and is readily spread by potato cutters, planters, harvesters and even containers.

Infested tubers may have reddish areas near the eyes or the skin may be cracked or swollen. Lightly infected tubers may appear healthy. When the tuber is cut across the stem end, a creamy-yellow to light-brown rot shows in the vascular ring; the rot is crumbly to cheesy and odourless. If you squeeze a cut tuber between the thumb and fingers, a substance oozes from the affected part of the ring. Often infected tubers are invaded by soft rot organisms and may disintegrate, leaving only the outer shells. Such hollow tubers are often found in the field.

Most of the severely affected tubers do not reach storage but are occasionally found there. If storage conditions and temperatures are proper, probably little change takes place in the amount of decay. Otherwise, the rot advances and is classed as soft rot.

Suspected bacterial ring rot should be scored as decay on the worksheet.

Bacterial ring rot is scored as follows:

A) Canada No. 1 and Canada No. 2

  1. Any amount of bacterial ring rot is scoreable against the decay tolerance in both grades.

Note: Suspect samples should be submitted to the Regional Plant Health Lab for verification.

Blight (C)

There are two kinds of blight: early blight and late blight. These diseases are caused by two different types of fungus.

Early blight lesions on tubers are dark brown to black, of several shapes including circular to oblong and about 6 to 51 mm (¼ to 2 inches) in diameter. Often they are slightly sunken and have raised purplish borders. The lesions on a tuber vary from one to many. The affected areas may be similar to those caused by the late blight organism, but early blight areas are shallower and sharply set off from the healthy tissue by a layer of cork. The decay does not spread irregularly into healthy tissue as does late blight. The flesh of the tubers, 3 to 13 mm (⅛ to ½ inch) deep beneath the lesions, is black and often surrounded by a yellowish zone.

Late blight may develop in a dry or wet rot either before or after harvest, depending on environmental conditions and the presence or absence of secondary organisms. At first, a brown or purplish-black metallic discolouration of the skin and a reddish-brown discolouration of the tissue just below the skin develops, usually not penetrating more than 6 mm (¼ inch). However, secondary organisms may result in a partial or complete breakdown of the affected tuber.

Both early and late blight are scored as follows:

A) Canada No. 1
Free from blight.

  1. If wet or soft, score against the decay tolerance;
  2. If dry, score against the external defect tolerance

B) Canada No. 2

  1. If wet or soft, score any amount against the decay tolerance; or
  2. If dry, and cannot be removed without the loss of more than 5% of the weight of a potato, score against the external defect tolerance.
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