Potatoes
8. Internal Defects

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Internal Defects are defects which cannot be detected without cutting the potato.

8.1 Black Heart (C)

Black heart is a non-parasitic disease and it is caused by high temperatures accompanied by poor aeration. It is mainly a storage problem. The centre portions of tubers are usually affected most severely. The tissue turns slate gray, then dark and finally black. In extreme cases, the potato breaks down completely, and in those cases, it should be scored as decay.

Canada No. 1 and Canada No. 2

Any amount of black heart should be scored for either grade.

Visual Aids Refer to photos 27 & 28 (USDA).

8.2 Black Leg (C)

This is a bacterial disease. Its name is derived from the inky-black colour of the stem just above the soil line. Typically, the rot caused by black leg starts at the stem end and appears as slightly sunken, black tissue which may eventually extend through the centre of the tuber. Black leg rot may be common in the field and continue to develop in storage. Secondary infection may develop.

Black leg is scored as follows:

A) Canada No. 1

  1. If the tissues are soft and watery, score black leg against the decay tolerance.
  2. If the tissues are dry and removal of dry rot causes a loss of more than 5% of the total weight of the potato, score as black leg.

B) Canada No. 2

  1. If the tissues are soft and watery, score black leg against the decay tolerance.
  2. If the tissues are dry and removal of dry rot causes a loss of more than 10% of the total weight of the potato, score as black leg.

8.3 Hollow Heart (P)

Hollow heart is a condition brought about by too rapid or irregular growth. It often occurs during wet seasons in potatoes grown in very fertile or heavily irrigated soils. Hollow heart consists of more or less irregular cavities of varying size within the potato and is usually lined with light-brown to brown dead tissue. This defect is usually found, but not always, in large, rough, misshapen potatoes. The potato should be cut lengthwise, parallel to the flat side, to determine the defect. If the potato is not cut lengthwise, the hollow heart may not be detected.

Hollow heart is scored when:

A) Canada No. 1

  1. the area affected exceeds that of a circle 13 mm (½ inch) in diameter on a potato that has a diameter of 63 mm (2 ½ inches) or a weight of 170 g (6 oz) *(TM).

B) Canada No. 2

  1. the area affected exceeds that of a circle 19 mm (¾ inch) in diameter on a potato that has a diameter of 63 mm (2 ½ inches) or a weight of 170 g (6 oz) *(TM).

For areas allowed on correspondingly larger or smaller potatoes, for both Canada No. 1 and Canada No. 2, please see Table 4.

Note: When hollow heart with discolouration is found, and the discolouration falls within the allowed area, the defect is not to be scored. If the discolouration falls outside the allowed area, even if the hollow heart does not, then it is to be scored as hollow heart.

Visual Aids Refer to photos 121-122e (USDA).

Table 4

Guide for Maximum Area Allowed for Hollow Heart, or Light Brown Discolouration (Brown Center) Canada No. 1 and Canada No. 2
Potato Diameter Potato Weight Area Allowed Diameter of Circle
Canada No. 1
Area Allowed Diameter of Circle
Canada No. 2
inches mm oz g inches mm inches mm
2 51 4 113 9 16
2 ½ 63 6 170 ½ 13 ¾ 19
2 ¾ 70 7 198 16 22
3 76 8 227 ¾ 19 1 25
3 ¼ 83 10 283 22 1 ¼ 32
3 ½ 89 14 397 1 25 1 ½ 38
3 ¾ 95 16 454 1 ⅛ 29 1 ⅝ 41
4 102 20 567 1 ¼ 32 1 ⅞ 47
4 ¼ 108 24 680 1 ⅜ 35 2 50
4 ½ 114 28 794 1 ½ 38 2 ⅛ 54

1. Note: Correspondingly lesser or greater areas in smaller or larger potatoes.

2. Note: These dimensions are based on area affected not aggregate area.

3. Note: When determining maximum area allowed use the size column which allows the greatest area affected (i.e. In the Canada No. 1 grade a potato 76 mm (3 inches) in diameter, which weighs 283 g (10 oz) is allowed 22 mm (⅞ inch) in diameter area affected.)

8.4 Internal Discoloration (C) and (P)

This section has been modified as a result of the Canada/U.S. potato grade harmonization talks. Both Canada and the U.S. will score internal discoloration the same way, i.e., 5% / 10% cut-off or 3/6 spots.

For the purposes of scoring this defect in Canada, only the term internal discoloration will be used, even though the discoloration may be caused by various factors such as heat necrosis, net necrosis and internal brown spot, etc., with the exception of black heart, which has a tolerance established in the Regulations (Subsection 88 (f)).

U.S. inspectors will use the names of the following factors causing internal discoloration when describing this defect: net necrosis, stem-end browning, vascular discoloration, internal potato necrosis, internal brown spot, heat or drought necrosis, internal black spot, internal mahogany browning and internal pink to purple discoloration.

Internal discolouration can be caused by various disorders. They are mainly vascular discolourations, net necrosis, heat necrosis, internal brown spot or black heart. Some internal discolouration is associated with chilling or freezing injuries (see Section 7.9 for more information). In addition, it has been claimed that certain non-parasitic factors are sometimes involved. For example, chemical vine killing may produce internal discolouration similar to vascular discolouration, but usually a light brown, narrow ring results. Since inspectors are not pathologists, it will not be their duty to name the specific discolouration.

Vascular Discolouration: This disease is caused by a fungus and the symptoms are a slight discolouration of tissue below the stem end. Discolouration of the vascular ring appears as a slight netting or in some cases a severe brown coloured streaking in part or all of the vascular ring, sometimes extending nearly to the bud end of the tuber.

Net Necrosis: This discolouration is usually a result of current season infection with the leafroll virus. Tuber shape and external appearance may be perfectly normal, with net necrosis being seen only when the tuber is cut. The phloem tissue is more or less filled with a network of fine lines, usually brown in colour. This netting may extend only a short distance or throughout the entire potato. Discolouration increases in amount and severity during storage. The finer netting and presence of phloem discolouration differentiates it from the vascular type.

Heat Necrosis: The cause of this non-parasitic disease is the result of high temperatures, especially when vines die early on light sandy soil. Affected tubers show slate-grey to brown patches in tissue near and perhaps associated with the vascular system. There are no external symptoms and diagnosis depends on cutting the tubers.

Internal Brown Spot: This disease is probably due to a lack of adequate soil moisture during the latter part of the growing season. no external symptoms are evident. Groups of dead cells which are free from fungi and bacteria appear as irregular, dry, brown or rust-coloured spots scattered through the central portion of the tuber.

Mahogany Browning: This appears as reddish-brown areas or blotches in the flesh. They occur in irregular patches anywhere in the flesh. The margins are not definite, and no sharp lines exist between discoloured and normal tissue. The colour of affected tissue varies in intensity from light to reddish brown. The affected tissues are of normal texture. Varieties vary in susceptibility, but under sufficient exposure to chilling, temperatures (0°C to 1°C), most will develop some degree of mahogany browning.

Internal discolouration is scored when:

A) Canada No. 1

  1. any discolouration which cannot be removed without wasting more than 5% by weight of the potato; or
  2. there are more than the equivalent of three (3) scattered spots 3 mm (⅛ inch) in diameter on a potato that has a diameter of 63 mm (2 ½ inches) or a weight of 170 g (6 oz).

Correspondingly lesser or greater affected areas are allowed on smaller or larger potatoes.

B) Canada No. 2

  1. any discolouration which cannot be removed without wasting more than 10% by weight of the potato; or
  2. there are more than the equivalent of six (6) scattered spots 3 mm (⅛ inch) in diameter on a potato that has a diameter of 63 mm (2 ½ inches) or a weight of 170 g (6 oz).

Correspondingly lesser or greater affected areas are allowed on smaller or larger potatoes.

8.5 Light Brown Discolouration (Brown Center) (P)

This is a condition that may develop while tubers are very small. The center of the potato shows areas of dead, brown cells. During the growing season these cells can split apart and form cavities (i.e. Hollow Heart).

Light brown discolouration shall be scored (based on a potato 63 mm (2-½ inches) in diameter or 170 g (6 oz) in weight) when:

A) Canada No. 1

  1. the area affected exceeds that of a circle 13 mm (½ inch) in diameter. (correspondingly lesser or greater areas in smaller or larger potatoes)

B) Canada No. 2

  1. the area affected exceeds that of a circle over 18 mm (¾ inch) (correspondingly lesser or greater areas in smaller or larger potatoes)

Note: See Table 4 for a guide for area of maximum allowed on various potato sizes.

Visual Aids Refer to photos 123 & 124 (USDA)

8.6 Presence of Insects (C) and (P)

The presence of any live or dead worms or insects within the flesh of the potato will be scored against both grades.

If the worms or insects are "live", treat as a condition defect as they may not have been present at shipping point.

If the worms or insects are "dead", treat as a permanent defect, since they were probably present at shipping point.

In cases where there are both "live" and "dead" worms or insects present, treat as a condition defect.

8.7 Watery Translucent Flesh (C)

This defect generally affects white fleshed potatoes of the Superior variety and is believed to be related to drought conditions. The flesh will be firm, with a watery translucent appearance accompanied by yellow greyish discolouration within the vascular ring.

When scoring this defect, the potato will be cut in half lengthwise through the greatest width. The random composite sampling procedure for hidden defects will be used when a preliminary sampling reveals that the defect is present.

Watery translucent flesh is scored when:

A) Canada No. 1

  1. the colour of the flesh within the vascular ring is materially affected when contrasted to the colour of the unaffected flesh outside the vascular ring; and
  2. the area affected is greater than 18 mm (¾ inch) in the aggregate on a 64 mm (2 ½ inches) potato and is watery and darker than pale yellow.

B) Canada No. 2

  1. the colour of the flesh within the vascular ring is seriously affected when contrasted to the colour of the unaffected flesh outside the vascular ring; and
  2. the area affected is greater than 39 mm (1 ½ inches) in the aggregate on a 64 mm (2 ½ inches) potato and is watery and darker than light yellow.

8.8 Other Condition Defects

Other internal defects which are not described in Section 8.1 to 8.6 are scored as free from any injury or defect or a combination there of that:

A) Canada No. 1

  1. affects the flesh of the potato and cannot be removed without the loss of more than 5% of the weight of the potato; or
  2. materially affects the appearance, edibility or shipping quality of the potatoes.

B) Canada No. 2

  1. affects the flesh of the potato and cannot be removed without the loss of more than 10% of the weight of the potato; or
  2. seriously affects the appearance, edibility or shipping quality of the potatoes.
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