PI-005: Chapter 9 – Seed Potato Tuber Inspection Standard

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Table of Contents

Seed Potato Inspection Manual

Chapter 9 - Seed Potato Tuber Inspection Standard

9.0 Seed Potato Tuber Inspection Standard

9.1 Objective

The objective of this chapter is to assist the inspector during the inspection of graded seed potato tubers. It explains the process for scoring defects and diseases, and recording these findings into the Tuber Inspection Report (CFIA 3076). The inspection results are used to determine if the seed potato tubers are meeting the tolerances of the Seeds Regulations Part II (Section 48.1). Note that these tolerances are also highlighted in Table 9.6 which should be referred to throughout this chapter.

9.2 Definitions

The import requirements of a foreigncountry or an arrangement between a grower, supplier and purchaser for a quantity of graded seed potatoes, which specifies the minimum and maximum size of the graded product;
Bacterial Ring Rot; the disease caused by the bacterial pathogen Clavibacter michiganensis subsp. sepedonicus;
Seed potatoes established in Section 47 of the Seeds Regulations Part II;
A variety of potatoes;
External and/or internal tuber injury or damage caused by insects, animals, chemicals, adverse environmental conditions and/or handling activities;
Externally damaged:
Any puncture, cut insect or animal damage that affects the viability of more than one eye or provides an infection site for disease;
The potato is compact, solid, not shriveled, flabby or pliable and is unyielding to moderate pressure;
The potato is soft, limp, pliable and yields to moderate pressure;
Tubers that are not of the variety being inspected;
Leaf roll necrosis:
A network of brown strands (necrosis) starting at the stem-end of the tuber, progressing into the vascular ring and extending into the internal tissues (cortex), caused by the PLRV;
Tubers not exhibiting the characteristic shape for the variety due to physiological or environmental factors (e.g. growth cracks, dumbells, bottle-necked shapes);
Potato Leaf Roll Virus;
Potato Mop Top Virus;
Pressure bruise:
Damage resulting from external pressure on the tuber resulting in dark discoloration of the flesh and a depression affecting more than one eye or >10% of surface;
Seed potato:
A tuber, or any part of a tuber, that is certified pursuant to the Seeds Regulations Part II for seed reproduction purposes;
Soft rot or wet breakdown:
Any soft, mushy or leaky condition of the tissue caused by active disease organism or physiological conditions (e.g. blackleg, bacterial soft rot, active fusarium, frost, pythium, late blight, etc.);
Stem-end discolouration:
Usually has an external expression such as darkening around the stem-end that is normally limited in its penetrations and is typically found beyond the vascular ring;
The short fleshy underground stem bearing buds or eyes;
Tobacco rattle virus; also known as corky ring spot or spraing;
Seed potatoes that:
a. are distinguished by common morphological, physiological, cytological, chemical or other common characteristics, and
b. retain their distinguishing characteristics when reproduced;
Zero tolerance:
a. In respect of a disease, the requirement of the absence of a disease in a plant, or in any part thereof (including a tuber), and
b. In respect of a varietal mixture, the requirement of the absence of the mixture of two or more varieties.

9.3 General Requirements

This chapter outlines the requirements and tolerance levels of seed potatoes observed during inspections. Topics that are covered include: general requirements; tolerance levels for size, maturity, colour, malformations and other defects; and special inspection procedures.

9.3.1 Tuber Standard

All classes of seed potatoes, with the exception of nuclear stock class, must be graded to meet the regulatory requirements. Shipments to foreign markets may be required to meet a more restrictive standard. Tolerance levels for tuber standards are found in Table 9.6.

9.3.2 Similar Varietal Characteristics

All seed potato tubers are required to be true to type and variety. This means that potatoes within a container must be of the same cultivar and similar in general shape, skin colour, flesh colour or russetting of the skin. Therefore, long type white skinned potatoes could not be mixed with round type potatoes, red skinned potatoes nor any other type of potatoes. Any tuber found during the inspection of selected samples which is of another variety must be scored as foreign. The percentage of foreign tubers in a lot shall not exceed the percentage permitted under Sections 47.11 - 47.8 of the Seeds Regulations Part II for that class (refer to Table 9.1).

Note: Due to varietal characteristics, fluctuations in growing conditions and growing seasons may affect tuber shape (round, oval, long, oblong, elliptical, etc.). Tuber shape may also vary if the potatoes are harvested early. For example, small long types are often round when dug before full maturity.

Table 9.1: Percentage of Foreign Varieties Allowed in a Shipment by Class
Table description

The following table demonstrates the percentages of foreign varieties allowed in a shipment, categorized by seed class.

Seed Class

Percentage %

Nuclear - Elite 2


Elite 3


Elite 4






9.3.3 Properly Packed

Seed potatoes must be properly packed. This means that the potatoes are packed in a manner that is not likely to result in damage to the potatoes during handling or transport.

9.3.4 Properly Marked

The general requirements on labeling are prescribed under Section 54 of the Seeds Regulations Part II. All markings are to be provided either directly on the package or on an attached tag. The tags should contain the following information:

  • The class, variety and size of the seed potatoes;
  • The crop certificate number;
  • The date of issuance of the tag; and
  • The declarations from Section 53.5(d).

The markings on the packages shall not make references to non-certified potatoes.

9.3.5 Packages

Containers of seed potatoes shall be new and free from stains and shall not be so soiled, warped, broken, wet or otherwise damaged as to affect the shipping quality or saleability of the potatoes packed therein. The containers shall be securely closed in a manner appropriate for the type of container.

Potatoes may be packed in various types of containers such as jute sacks, crates, barrels, and cartons. There are no standard container sizes for seed potatoes but shipments are generally made in the following units:

  • 50 lb (22.7 kg),
  • 55 lb (25.0 kg),
  • 100 lb (45.4 kg),
  • 110 lb (50.0 kg), and
  • tote bags.

For containers holding more than 50.0 kg (e.g., bulk boxes and mini totes), a bulk movement document is used to cover the shipment.

For the purposes of repackaging, the Permit Relating to Packing of Seed Potatoes (CFIA 1347) is required for packages less than 20 kg. Several containers are used for sale for home garden use. These include:

  • 2.2 lb (1.0 kg),
  • 5 lb (2.27 kg),
  • 10 lb (4.54 kg), and
  • 20 lb (9.07 kg).

9.4 Size

9.4.1 Requirements

All documentation for any quantity of graded seed potatoes whether in containers or in bulk must specify the minimum and maximum size of the graded tubers in millimeters. The minimum acceptable size must not be less than 30mm, and not more than 70mm for long type varieties or 80mm for round type varieties. For any quantity of graded seed potatoes, at least 95% of the tubers by weight must be within the minimum and maximum sizes. Nuclear stock and Breeders Selection seed potatoes are exempt from these size requirements.

Inspections honour contractual agreements between a grower and purchaser on size. Where a grower and purchaser have in place an agreement for graded seed potatoes the agreement must state the minimum and maximum size for the graded tubers. The grower or purchaser must produce when requested to, a copy of the agreement clearly stating the minimum and maximum size. The term agreement can be used on all tags and record of bulk movement to denote the size for the graded tubers.

Note: The percentage of undersize or oversize specimens should be noted in the identified area on the Seed Potato Tuber Inspection Report (CFIA 3076).

9.4.2 Measuring Size

The measurement for minimum and maximum diameter shall be the greatest distance of a potato taken at right angles to the longitudinal axis, disregarding the position of the stem.

To determine over- and undersize specimens, inspectors shall use a calibrated sizer. Suspected specimens are placed on the sizer. If the potato supports its own weight without falling through the sizer, it will be scored as oversize when checking the upper size range. If the potato passes through the sizer of the lower size range it will be scored as undersize. Refer to Table 9.2 for a general guide on determining the number of tubers within a size category.

Table 9.2: Potato Size Reference Table
Table description

The following table is used as a reference to determine potato size; it provides examples using the diameter measured in inches, the diameter measured in millimetres, the approximate weight in ounces, the approximate weight in grams, and the count per cwt.

Diameter in inches

Diameter in Millimetres (mm)

Approximate Weight in Ounces

Approximate Weight in Grams (g)


per cwt






1 1/2





1 5/8





1 3/4





1 7/8










2 1/4





2 1/2





2 3/4










3 1/2





Note: The above table is to be used for reference purposes only.

9.5 Maturity and Firmness

9.5.1 Terminology and Requirements

Firmness is a term used in the Seeds Regulations Part II to indicate tuber quality for germination. Unlike table stock requirements there is no reference to the amount of skin missing or feathering on the surface of the tuber, nor is it a factor used in the Seeds Regulations Part II to determine maturity. It is, however, an indication of the length of time the tubers were left in the ground to achieve skin set. Uniform terminology can be achieved through adherence to the following terms:

  • Firm means that the potato is compact, solid, not shriveled, flabby or pliable, and is unyielding to moderate pressure.
  • Flabby means that the potato is soft, limp, pliable and yields to moderate pressure.

When a lot of potatoes fail to meet the standard on account of lacking firmness, the exact percentage of specimens failing the specific requirement is to be reported. For example, if the lot fails due to a high percentage of tubers lacking firmness (i.e. flabby), the percentage of specimens should be reported as: average 22% flabby, the remainder firm. All findings are reported into the Seed Potato Tuber Inspection Report (CFIA 3076).

Graded seed potatoes should be mature and firm. It should be remembered that as tubers age in storage they begin to lose moisture and turgidity. In some instances when shipments are made late in the shipping season (i.e. late April or May), tubers may have difficulty meeting the firmness requirements.

In any lot, 98% of the tubers should be firm and not shriveled or flabby, otherwise they could be shipped under the defect conditions as set out in Section 57 of the Seeds Regulations Part II.

Note: Seed potato tubers have no skinning requirements.

9.6 Colour and Cleanliness

There is no colour requirement for seed potatoes, such as what is normally seen in a commodity like apples. However, it does not exclude defects that affect the colour of the skin such as scald or sunburn, nor does it exclude the requirement of not permitting artificial colour.

Presence of soil can interfere with performing a good inspection. Therefore, it is necessary for the tubers to be reasonably clean to perform a visual inspection. Tubers which are not reasonably cleaned should be refused documentation until the dirt is removed to the inspector's satisfaction.

However, tuber cleanliness and freedom from soil is a very serious issue domestically and for importing countries, since soil can vector many disease pathogens that may be of quarantine significance. Many countries have strict tolerances for adhering soil. In the Seeds Regulations Part II there are no tolerances on soil cleanliness, but there is a prohibition on washing of seed tubers. Under the Seeds Regulations Part II no seed tubers shall be washed. This is intended to prevent the establishment of a suitable environment for the development of pathogenic organisms such as bacteria and fungi.

Reasonably clean means:

  • The potatoes are reasonably free from soil, and
  • Where the potatoes are in a container, there is not more than a slight amount of loose soil or foreign material in the container.

If prior to being packed the potatoes have been washed they will then be ineligible for seed. Brushing the tubers is allowed and when performed properly does not provide for an environment suitable for disease development.

Agreements specifying soil limitations may be required by various importing countries. Such conditions are governed by phytosanitary certification and should be expressed as g/kg. If the soil exceeds the permitted quantity, then the grower should be given the opportunity to re-grade the seed stock to comply with the requirements.

9.7 Malformed

The shape of potatoes should be considered from the standpoint of the shape characteristic of a particular variety. Some varieties have peculiar shape characteristics that make them distinct.

Misshapen specimens could take different forms. The potato may have been exposed to adverse environmental conditions or have a physiological defect and be curved, pointed or creased, have the form of a dumbbell, a bottle neck or have knobs on the tuber. In general, irregularities in soil moisture and nutrients are the common environmental factors responsible for misshapen potatoes (see Appendix 9-2). Note that blind or stitch end tubers (i.e. potatoes which lack eyes on the bud end) have reduced viability as seed.

Misshapen specimens should be scored as malformed when the potatoes are adversely different from the normal varietal characteristics.

9.8 Special Inspection Procedures

9.8.1 Method of Cutting to Determine Depth of Penetration

In determining the depth of penetration in removing a defect, the recommended procedure is to first make a straight cut about 1/4 inch (6 mm) deep across the potato to remove all or most of the damage. If the damage is still present, then another cut should be made to determine the full extent of the damage. The depth of penetration is a general indication of the severity of the defect.

9.8.2 Procedure for Internal (Hidden) Defects Suspect (Biased) Sampling

Outward Appearance Gives Some Indication of Hidden Defects

When the outward appearance gives some indication of a defect, the inspector should select the suspected specimens in the total sample and cut to verify the selection. Those specimens found to be defective must be scored against the total sample.

For example, if an inspector was inspecting a 100 lb (45.4 kg) (approximately 400 tubers) bag of potatoes and observed that the larger, rough potatoes had stem-end discolouration, the inspector would proceed by opening the bag and picking out all the larger, rough potatoes. If there were 10 tubers of this type, and as a result of cutting every specimen to reveal defects, the inspector found five tubers showing stem-end discolouration penetrating in excess of 13 mm, the five tubers of stem-end discolouration would be scored against the total sample of 400 tubers:

(5 ÷ 400) x 100 = 1.25%.

This sample would be within the established standard of 4% for internal discoloration. Random SamplingOutward Appearance Gives No Indication of Hidden Defects

When the outward appearance of the tuber gives no indication of a defect, the inspector should collect a series of subsamples from each package to create a composite sample which is representative of the entire lot being inspected. The inspector should determine an appropriate quantity of tubers for the subsample. For example, an inspector could inspect five large sacks of potatoes (i.e. 100 lb sacks, containing approximately 400 tubers each), and could discover black heart was present, even though the outward appearance of the potatoes gave no indication of the defect. In this case, the inspector might decide to take 200 tubers completely at random from the 5 sample sacks. He/she would cut every specimen from the composite sample 200 tubers and, for the purpose of this example, the inspector found two tubers showing black heart. The two tubers of black heart would be scored against the 200 tuber sample:

(2 ÷ 200) x 100 = 1%.

This count would be applied to the defects under damage with a tolerance of 2%. If those tubers had begun to breakdown as a result of the oxygen deficiency then this lot would not meet the 0.1% tolerance for wet break down. Black heart is a physiological defect and is not pathogenic in nature.

Note: When using the composite sample, the sample must be selected completely at random and the size of the sample must be accurate and consistent.

Never score the same specimen twice.

9.9 Permanent Defects

Permanent defects are ones that do not progress over time and are consistent from shipping point to destination. In contrast, condition defects can progress over time and under adverse conditions if there is the opportunity for a disease to develop in storage or during transit to destination.

The subsequent sections will discuss the following permanent defects: insect injury, grass root injury, growth cracks, hollow heart, internal discolouration, mechanical damage and digger cuts, rhizoctonia, scab, russetting, skin spot, and sunburn.

9.9.1 Insect Injury Flea Beetle Injury

Flea beetle, Epitrix tuberis, can cause surface injury, internal injury or both. Surface injury consists of pimple-like eruptions or rough, winding trails about 1/16 inch (1.6 mm) wide and of varying length. These trails are produced by larvae feeding just beneath the epidermis of new tubers. Internal injury consists of single or groups of narrow, brown slivers or feeding tunnels that extend into the tuber for 1/4 to 3/4 inch (6 to 19 mm). Cracks occur where these tunnels come together.

Flea beetle larvae injury is scored under the damage tolerance when it:

  • Affects more than 10% of the surface area; or
  • Materially affects two or more eyes of the tuber. Grub Damage

Grub damage is caused by the larvae of June beetles or cutworms Polyphylla spp. The earlier the feeding damage occurred during tuber development the larger the affected area will be. Injury consists of small to large feeding cavities 1/4 to 1 inch (6 to 25 mm) or more in diameter. The irregular-shaped cavities usually are wider than deep, with characteristic rough ridges inside. Half or more of the tuber can sometimes be consumed.

Grub damage is scored under the damage tolerance when it:

  • Affects more than 10% of the surface area in the aggregate; or
  • Materially affects two or more eyes on the tuber. Wireworm

Wireworm (click beetle) larvae, Agirotes obscurus, have overlapping life cycles, so that tuber damage can occur at any stage during tuber development. Wireworm severely damages the crop by preventing plant emergence. In the field, the inspector can remove the affected seed piece and observe characteristic damage.

Early injury occurs when the tubers are small and the larva causes deep, funnel-shaped cavities to be formed as the potatoes mature. Mid-season injury results in the formation of pits 1/4 to 2 inch (6 to 13 mm) deep and 1/16 to 1/8 inch (1.6 to 3 mm) in diameter, sometimes lined with discoloured scar tissue. Late injury consists of clean cut round holes and scar tissue is not prominent.

Wireworm injury is scored under damage tolerance when:

  • Two or more eyes per potato regardless of the depth are damaged by holes; or
  • Affects more than 10% of the potatoes in the lot regardless of the number and depth of the hole.

9.9.2 Grass Root Injury

Usually grass root injury is caused by quack grass. The sharp growing points of the rhizomes penetrate into or completely through the tubers. There can be evidence of the rhizome attached to the potato.

Grass root injury is scored under damage tolerance when:

  • Two or more eyes of the tuber are materially affected regardless of the depth; or
  • Affects more than 10% of the potatoes in the lot regardless of the number and depth of the hole(s).

9.9.3 Growth Cracks

Growth cracks may be caused by very rapid growth, usually when a rainy period follows a long dry spell. Growth cracks most often occur towards the bud end of the tuber and usually extends lengthwise. The cracks vary in size but usually heal over with no rot following the injury.

Growth cracks are assessed on their damage to the growing points of the tuber and are scored under off-type (malformed) tolerance when:

It affects two or more growing eyes of the tuber; or

Affects more than 10% of the tubers in the lot.

9.9.4 Hollow Heart

Hollow heart is a condition brought about by too rapid or too irregular growth. It often occurs during wet seasons 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, but not always found, 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 may be associated with other malformations of tubers and shall be culled appropriately. Where tubers appear normal and sound and no breakdown occurs, hollow heart is not scored. However, when hollow heart is associated with decay, it is scored under the appropriate category.

9.9.5 Internal Discolouration

Internal discolouration can be caused by various disorders. They are mainly vascular discolourations, heat necrosis, internal brown spot or black heart. Some internal discolouration is associated with chilling or freezing injuries (see Section 9.10.10 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 cause of the discolouration. Vascular Discolouration

This defect can be caused by a number of factors including viral (e.g. net necrosis), fungal, bacterial and chemical injuries. 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 part or all of the vascular ring, sometimes extending nearly to the bud end of the tuber. Heat Necrosis

The cause of this defect is the result of high temperatures, especially when vines die early in 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 (IBS)

This defect is probably due to a lack of adequate soil moisture during the latter part of the growing season or during the harvesting process. 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 flesh of the tuber.

Internal discolouration caused by the sources listed above (i.e. in Subsections - is scored under the tolerance for internal discoloration when:

  • The discolouration is found near the surface area or at stem-end and penetrates in excess of 13 mm; or
  • The discolouration is found toward the centre portion of the tuber; or
  • IBS may be symptomatic of other diseases such as Tobacco Rattle Virus. Black Heart

Black heart is a non-parasitic disease and it is caused by high temperatures accompanied by poor aeration (oxygen deficiency). It is mainly a storage problem. The centre portion of the tuber is usually affected most severely. The tissue turns slate grey, then dark and finally black and may dry and split towards the centre portion of the tuber. In extreme cases, the potatoes break down.

Score black heart under damage.

9.9.6 Mechanical Damage and Digger Cuts

One of the most common causes of loss in seed potatoes is mechanical injury in the form of cuts, punctures and bruises. Cuts on tubers are a result of mechanical injuries suffered during digging and/or sorting. Cuts can be deep into the tuber or may be lengthwise or crosswise slab cuts exposing large areas.

Bruised or shattered potatoes are characterized by slight to severe breaks in the epidermis. This injury may take different forms such as punctures, scratches etc. Characteristically, bruised or shattered potatoes are associated with pocket rot or Fusarium dry rot.

Mechanical damage and digger cuts are scored as damage when:

Cuts which are not well healed, rough and exceed an aggregate area of more than 10% of the surface; or

Any kind of injury which materially affects two or more eyes of the tuber.

9.9.7 Rhizoctonia

Rhizozoctonia solani or black scurf is a fungus on the tuber that is identified by the presence of hard, black or dark brown bodies called sclerotia resting on the surface of the tubers. Frequently it is called the dirt that will not wash off. The sclerotia will vary in size from small specks to large masses nearly an inch (25 mm) across. Frequently, the fungus is defined as mild to severe netting or scurf, and in extreme cases a portion of the tuber is affected.lkjgtboigbo Rhizoctonia is rated in the same manner as scab (see Section 9.9.8 and Table 9.5) and is scored either singly (if only one disease is present) or in combination with scab when both are present on the tubers being inspected.

9.9.8 Scab

This disease can be caused by two different organisms. Powdery scab is caused by the fungus Spongospora subterranea, while the common scab, surface and Russet scab is caused by the bacterium Streptomyces scabies. In general, these diseases are treated equally for domestic and some export destinations. However, powdery scab may have a zero tolerance for some buyers in export markets or be of quarantine significance, in which case the shipment is required to be free of that pathogen. Powdery Scab

The symptoms of powdery scab on young tubers are faintly brown raised areas which occur in patches or singularly. In approximately a week, under ideal conditions these areas enlarge to about 1/4 inch (6 mm). At harvest, and in storage, the pustules dry and break down leaving circular to oval pits which are filled with brownish spore balls. When mature, these pustules become yellow brown to black. Sometimes these pits enlarge to form cankers in the tuber. A dry sunken rot may develop in the tuber. Common Scab (Surface Scab)

The first symptoms of common scab are minute reddish-brown lesions around the breathing pores (lenticels) of young tubers. They increase in size, turn dark and form circular scabbed areas that are either isolated or in large corky masses. They may be superficial spots or greatly roughened blotches. Russet Scab

Russet scab may occur in isolated or in large areas on the skin of a potato. The scab will appear as smooth, rough or may be cracked.

Smooth russetting means net-like, streaked, patchy or solid, readily apparent but, smooth to touch.

Rough russetting means bark-like, pebbly or thick, not blending. It has more of a coarse sand paper feel when lightly brushed over the affected area.

Scab is treated in the same manner and under the same tolerance as rhizoctonia. The percentage of scab is recorded separately but the total amounts are combined under the same tolerance when both diseases are present. The levels and percentages of scab and rhizoctonia are as follows:

Table 9.5: Rating Levels for Scab and Rhizoctonia Infections
Table description

The following table outlines the rating levels for scab and Rhizoctonia infections from trace scab or rhizoctonia to severe scab or rhizoctonia; it provides the percentage of the surface area affected and the percentage of the tolerance for the two diseases.

Disease Level

% Surface area affected

% Tolerance

Trace scab or rhizoctonia

0 -1

not scoreable

light scab or rhizoctonia

1- 5

10 %

moderate scab or rhizoctonia

5 - 10


severe scab or rhizoctonia

> 10

0 %

combined total

1 - 10 %

not more than 10%

9.9.9 Russetting

The exact cause of russetting on non russet type potatoes is unknown. Other than affecting the outward appearance and marketability of the affected potato, it does not lead to decay in storage or in transit.

There are two types of russetting that may occur, either smooth or rough. Smooth russetting may appear as streaked, patchy or solid and is smooth to touch. Rough russetting will be considered as rough when the potatoes show rough skin with minor shallow splitting. Russetting that is not characteristic of the variety is scored under damage when more than 50% of the surface area in the aggregate is affected.

9.9.10 Skin Spot

Skin spot, caused by Polyscytalum pustulans, is characterized by purplish-black, slightly raised spots up to 2 mm in diameter, singly or in groups over the surface of the tuber. They may appear either at random over the surface or aggregated around the eyes. Sometimes larger necrotic (dead) areas form over the tuber surface. These necrotic areas can be picked out, leaving circular pits of healthy flesh. Skin spot is scored under damage or dry rot when:

  • Purplish-black, slightly raised areas which do not penetrate the flesh, affect more than 10% of the surface area in the aggregate (damage); or
  • It penetrates the flesh and affects more than one eye of the tuber (dry rot).

9.9.11 Sunburn

Sunburned tubers are caused when tubers are exposed to the sun during the growing season. The skin of the tuber turns green with chlorophyll, similar to the above ground parts of the plant. Typically, the bud end is affected and may involve only one area of the tuber and can be accompanied by yellow or cream-coloured discolouration of the inner flesh.

Sunburn is caused by exposure to sunlight as opposed to greening, which is caused by exposure to artificial light in storage.

Although sunburn does not always affect the germination quality of the affected portion of the tuber (specifically eyes), it is usually possible for experienced inspectors to determine, without cutting, whether individual specimens are scoreable by judging the size, location of the affected area and the intensity of green colour.

Sunburn is scored under damage when the green colouration is extreme to the extent where more than one eye is damaged and tuber vigor is affected.

9.10 Condition Defects

Condition defects are the ones that can progress during transit and reflect the perishable nature of seed potatoes. At shipping point, these defects may not be present or may not be evident, but as the potatoes are handled and during shipment they may be subjected to conditions which will allow symptom expression in excess of permitted tolerances. This is why at destination, some of these defects are provided increased tolerances to compensate for the potential of disease development in transit.

The condition defects discussed in the following sections include: BRR, blackleg, blight, air cracks, bruises, soft and dry rot, enlarged lenticels, external discolouration, freezing and chilling injuries, greening, silver scurf, skin checks, spraing, sprouts, and insects.

9.10.1 Bacterial Ring Rot

BRR is one of the most serious diseases of potatoes in Canada, caused by the pathogen, Clavibacter michiganensis subsp. sepedonicus. It is highly infectious and is readily spread by potato cutters, planters, harvesters and even containers, and therefore, is treated as a zero tolerance disease. It is one of the condition defects where a zero tolerance is required at all times.

Tubers expressing typical symptoms are described as having reddish areas near the eyes or the skin may be cracked or swollen. However, depending on the timing of the infection, detection, and the cultivar, symptomless 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 cheesy 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 and infections would be identified in the field, but there are occasions when infected tubers are found in storage. If storage conditions and temperatures are proper, little change takes place in the amount of disease development, however, the rot can advance rapidly under ideal disease developmental conditions.

Any amount of bacterial ring rot is scoreable and will lead to a rejection of all the seed lots produced on the farm unit and a full investigation. Suspect samples must be submitted to the Centre of Expertise for Potato Diseases, Charlottetown, PEI for confirmation. All lots on the farm unit should be held under detention until results from the lab confirm or deny the presence of the pathogen causing BRR.

9.10.2 Blackleg or Bacterial Soft Rot

Blackleg or bacterial soft rot are caused by the bacteria, Erwinia carotovora subsp atroseptica and E. carotovora subsp. carotovora. The name blackleg is derived from the field symptom expressed as an inky-black discolouration of the stem just above the soil line. Typically, tuber rot caused by blackleg starts at the point of attachment between the tuber and the stolon. Tuber symptoms appear as slightly sunken, black tissue at the stem-end and can eventually extend through to the centre of the tuber. The rot may be soft and watery or rather dry, leaving a shell rot in certain stages. Secondary infection may develop in some cases.Lori Welbourne Bacterial soft rot is typically a storage disease which gives tuber symptoms similar to blackleg and is accompanied by a distinctive odour when the tissue begins to break down.Lori Welbourne If the tissues are soft and watery, score against the soft rot and wet breakdown tolerance. If the tissues are dry, score this disease against the dry rot tolerance.

9.10.3 Blight

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

caused by the pathogen, Alternaria solani, is expressed as circular to oblong dark brown to black lesions that are about 1/4 - 2 inches (6 - 51 mm) in diameter. Often they are slightly sunken and have raised purplish borders. The number of lesions on a tuber may vary. 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, l/8 - 2 inch (3 - 13 mm) deep beneath the lesions, is black and often surrounded by a yellowish zone. blight,

caused by the pathogen, Phytophthora infestans,may develop into 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 1/4 inch (6 mm). However, secondary organisms may result in a partial or complete breakdown of the affected tuber. This pathogen may have specific quarantine considerations for some markets.

Both early and late blight are scored as follows:

  • If wet, score against the soft rot or wet breakdown tolerance; or
  • If dry and sunken and penetrates in excess of 1/8 inch (3 mm) into the flesh of the potato, score as dry rot.

9.10.4 Air Cracks

Air cracks sometimes occur during harvest or packing, and after packing if the packages are too tight or handled roughly.

They may appear as fresh, longitudinal cracks and are scored if they materially or seriously affect the germination quality of the potato.

Air cracks are scored under damage when:

  • The damage cannot be removed without the loss of more than 10% of the surface area of the tuber; or
  • The surface crack(s) affect(s) more than one eye of the tuber.

9.10.5 Bruises (Shattering)

The term bruises or shattering is used to describe recent injury to a potato from rough handling. Bruises or shattering are characterized by slight to severe breaks in the potato skin and nearly always result in damage to the underlying tissue. Punctures, scratches or breaks may take place in many forms. In some lots, almost invisible bruising or shattering is followed by extensive rot.

Damage by bruising or shattering is scored when it:

  • Affects more than 10% of the surface area in the aggregate; or
  • Seriously affects the germination or shipping quality of the potato.

9.10.6 Soft Rot or Wet Breakdown

Soft rot or wet breakdown is used to describe a deterioration or decline involving decomposition, which is induced by one or several fungi and/or bacterial pathogens, and which is of a complete and progressive nature. Inspectors are not pathologists, so it is not their duty to name the specific pathogen affecting the tuber.

However, any amount of breakdown is score able. The following diseases are scored under the tolerance for soft rot or wet breakdown:

  • Any soft, mushy breakdown (soft rot);
  • Any soft leaking specimens (freezing injury described in Subsection 9.10.10);
  • Soft and watery tissue of blackleg (described in Subsection 9.10.2);
  • Wet or soft blight (described in Subsection 9.10.3).

9.10.7 Dry Rot

This term is used to describe the dry decomposition usually starting in cuts, bruising or other injuries. Dry rot is caused by Fusarium spp. and/or other fungi. It may be moist but not wet. If the dry rot is active, affected lots over tolerance should be held over 48 hours prior to regrading. Inspectors should follow the same rule as decay regarding the identification of this disease.

Dry rot is scored under the dry rot tolerance when it affects the flesh of the potato and penetrates more than 1/4 inch (6 mm) in depth or is less than a 1/4 inch depth but covers more than 10% of the surface area.

9.10.8 Enlarged Lenticels

Excessive moisture before harvest is the main cause of this disorder. Normally lenticels are inconspicuous on tubers but under high moisture conditions they rise about 1/16 inch (1.6 mm) and form numerous whitish protuberances over the tuber surface. If tubers are held in moist atmospheres, these elevated areas remain whitish, but when the tuber dries, they become skin coloured, open and somewhat depressed. They make excellent openings for micro-organisms, particularly soft rot bacteria.

Enlarged lenticels are not scored as a defect in seed potatoes unless accompanied by other pathogenic organisms.

9.10.9 External Discolouration

External discolouration is broken into two parts: healed over flesh and scald. Healed Over Flesh

Occasionally, potatoes are harvested without allowing sufficient time for the skins to set. As a result, the skin of affected potatoes may be damaged and through time will heal and a second skin will develop. This is sometimes called healed over flesh.

Affected potatoes will have areas that are either darker or lighter brown than the normal skin colour. This could materially or seriously affect the potato's appearance. This type of discolouration usually affects only the skin area and not the flesh portion of the potato.

Healed over flesh is not scoreable for seed potatoes. Scald

Scald usually results from a rapid loss of moisture through skinned areas and oxidation of immature potatoes. Frequently the tissue exposed by feathering that has not healed over is scalded and turns dark. In many cases, the affected area becomes sunken and is usually followed by a rot; however, occasionally it may develop into a wet rot. Affected tubers turn green or yellow more readily and may badly shrivel. Frequently rot develops even where no further change in the surface is apparent.

Scald is not scored on its own, but can be classified either as soft rot, dry rot, or scored as soft and flabby when applicable.

9.10.10 Freezing and Chilling Injuries

The freezing point of potatoes varies from -2°C to -0.5°C, depending primarily on the total solids content. Some varieties are consistently higher in total solids than others, so this is responsible for some of the variation, as are maturity and the area of production. The extent of the injuries caused by exposure to sub-freezing temperature is directly related to the lowest temperature exposure and the period of time during which the tubers were exposed. Freeze damage of tubers will inhibit germination. Chilling injury will reduce the vigor of the tuber and/or may contribute to no-top disease where the mother tuber will directly produce progeny tubers without producing a plant. Mahogany browning -

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 - 1 °C), most will develop some degree of mahogany browning. Frost injury or frost necrosis -

appears as a net necrosis of tuber tissue. Fine, dark brown to grey streaks or strands occur in the vascular ring or generally in the tuber in all directions, giving the impression of a network. If the netting is due to frost, there is usually various stages of breakdown. When an inspector believes frost injury has occurred, the potatoes should be scored under soft rot or wet breakdown, otherwise, they should be scored under internal discolouration. Freezing injury -

shows a complete break down and watery rot of affected tubers. In some cases, such as field frost, where only portions of the tubers are frozen, the damaged tissue later dries up. In this event, the affected portion of the tuber shrivels, becomes dry, grey in colour and extremely hard. In some frost affected tubers, dark grey blotches or areas much like those in black heart develop in the internal tissue. Frozen tubers usually break down completely, resulting in a wide variation of symptoms, principally wet rot.

It should be mentioned that when an inspector suspects that tubers have been exposed to freezing or chilling temperatures, if possible, the tubers should be held for 24-48 hours. Tubers that have been chilled and allowed to warm up for 24-48 hours usually express classic symptoms of the injury.

Freezing and chilling injuries are scored when:

  • Any amount of freezing injury is evident (i.e. wet or leaking). Score against soft rot or wet breakdown tolerance;
  • Any type of chilling injury internal discoloration penetrates more than 2 inch (13 mm) into the flesh. Score against internal discoloration tolerance.

Note: When complaints are made in reference to freezing it is very important when possible to determine where the freezing injury occurred, within the storage, package, and/or vehicle.

9.10.11 Greening

Greening tubers are found in many warehouses and stores where they are exposed to artificial light. The disorder is serious because it may be an indication of increased solanine levels. Exposed skin surfaces of the tuber turns greenish or green. Usually the inner flesh becomes yellow or cream-coloured. The affected area could be located anywhere on the tuber. Usually the green colour found on greening tubers is lighter than that found on sunburn tubers. Do not mistake greening with sunburn. For a description of sunburn, please refer to Subsection 9.9.11 of this chapter.

Greening is not scoreable.

9.10.12 Pressure Bruises

Pressure bruises develop in storage on some tubers late in the season. There appears to be a relation between maturity, storage conditions and pressure bruises. This defect is most often the result of pressure at points of contact with adjacent potatoes or the floor. In most instances, there will be no discolouration of the underlying flesh at time of packing; however, greyish to black discolouration may develop in the affected tissue. It is impossible to predict whether discolouration will occur, or how extensive it will be, from the appearance of the flattened or depressed areas.

Pressure bruises are scored under special tolerance for pressure bruises (5%) when the sunken and discolored areas are in excess of 10 % of the surface area of the tuber.

9.10.13 Silver Scurf

Silver scurf, caused by Helminthosporium solani, is characterized by a grey, smooth, leathery appearance of the skin and is more noticeable because of the silvery sheen when the tubers are wet. This silvery condition is caused by a fungus which penetrates the tubers through the lenticels and epidermis.

After prolonged storage under warm and moist conditions, the tubers look sooty or smudgy. The disease may spread in storage, especially under high humidity and temperatures. Usually no secondary infection occurs.

Silver scurf is scoreable either under the dry rot or damage tolerance depending on conditions when:

  • The surface is so badly wrinkled, shriveled, or pitted that it seriously affects the germination quality of the potato. Score as damage.
  • The underlying flesh is dry or corky and affects more than 10% of the surface area of the potato. Score as dry rot.

9.10.14 Skin Checks or Thumbnail Marks

This disorder is caused by rough handling, excessively dry atmosphere or is associated with sudden changes in temperature. Skin checks appear similar to the injury caused by pressing a thumbnail directly into the tuber. The surface is covered with many crescent-shaped slits which can be confused with symptoms of bacterial ring rot. The tubers dry out rapidly and shrivel unless stored in moist conditions.

Skin checks or thumbnail marks are not scoreable.

9.10.15 Spraing

Spraing (commonly known as corky ring spot or TRV) is usually caused by two viruses: TRV and/or PMTV. Affected tubers show rings, semi-circles or spots on the surface with similar marks on the interior tissues. The tuber skin over some of the brown, sunken rings often cracks. The affected tissues are moderately firm and corky. Some mild infections may exhibit symptoms similar to IBS (Internal Brown Spot).

Any tubers showing signs of spraing should be submitted to the Centre of Expertise for Potato Diseases, Charlottetown, PEI for confirmatory analysis. If confirmed positive for TRV or PMTV the lot must be decertified.

There is no tolerance in the seed certification program for TRV or PMTV.

9.10.16 Sprouts Outgrown Sprouts

At Shipping Point

Outgrown sprouts are scored when the length exceeds 3/4 inch (20 mm).

At Destination

Outgrown sprouts are scored when the length exceeds 1 inch (25 mm).

Note: A tolerance of 10% by count of the potatoes in the lot is allowed for graded seed. This tolerance should not be included in the general tolerance. This means that 10% of the specimens may have sprouts exceeding the maximum length allowed but the remaining 90% must not exceed the maximum length allowed. Ingrown Sprouts

Ingrown sprouts are a problem when potatoes are stored for long periods at high temperatures (near 16°C) or have been exposed to a sprout inhibitor. The sprouts grow inward and cause bulges and cracks in the tubers. Sometimes small new tubers develop inside the mother tuber. Ingrown sprouts are scored under damage when they affect more than one eye per seed tuber.

9.10.17 Presence of Insects

The presence of any live worms or insects will be recorded and sent to the entomology lab for identification, if necessary. Note if the insects are either live or dead when found.

9.11 Tuber Tolerances

Table 9.6: Tolerance Table for Tuber Diseases/Defects
Table description

The following table outlines the different tolerances of diseases or defects through their percentage by count at their shipping point, and their percentage by count at their destination.


Column I: Disease or Defect

Column II: Percentage by count at Shipping point

Column III: Percentage by count at destination


Soft rot or wet breakdown




Dry rot, including late blight




Scab and rhizoctonia combined

(a) light

(b) moderate








Stem-end discolouration due to top-killing, frost, heat or drought, with penetration from 6 mm to 13 mm




Malformed and damage



Note: The total of combined tolerances for permanent defects and condition defects (not including light scab, light rhizoctonia and stem-end discolouration), shall not exceed 5% of the total number of tubers in the lot. The individual tolerance of any disease or defect must not be exceeded at any time; in addition 5% of tubers in a lot can be affected by pressure bruising.

Here are three examples which explain the destination tolerances for seed potatoes.

Example # 1: The lot has no soft rot or wet breakdown and has 1% dry rot, 2.5% shattering scored as damage. The Seeds Regulations Part II will permit up to 1% dry rot and 3% damage at destination. Therefore, the load meets the destination requirements of the Seeds Regulations Part II, if the inspection was done at shipping point the shipment would not meet the standard.

Example # 2: The lot has 3% permanent defects (2% knobs removed, 0.5% sunscald and 0.5% malformed (off-type)) and 0.3% late blight which is wet. The Seeds Regulations Part II will permit up to 0.5 % for soft rot (active, wet late blight must be scored as soft rot) and 3% malformed and damaged. This lot meets the destination requirements.

Example # 3: The lot has 3% moderate scab, 5% light rhizoctonia, 2% moderate rhizoctonia, 1.5 % off-type, 2% internal discolouration (Blackheart) and 3.5% stem-end discolouration (excessive top-kill). The Seeds Regulations Part II will permit 5% of combined defects and disease, not including light scab and rhizoctonia and stem-end discolouration. So this lot fails to meet the Seeds Regulations Part II due to excess of 5% for diseases and defects listed: 3% moderate scab, 2% moderate rhizoctonia, 2% internal discolouration (damaged) and 1.5 % off-type.

Note: Neither the 3.5% stem-end discolouration, nor the 5% light scab, is used to determine the 5% total.

Record all findings on the Seed Potato Tuber Inspection Report (CFIA 3076).

Appendix 9-1 Assessing the Severity of Disease on Surface Area of Tubers

To establish a uniform scale, certain crop plants have been scanned with sensory equipment to accurately determine the percentage of diseased area of a tuber. Sample keys showing disease severity are shown in Appendices 9-1.1 and 9.1-2.

Procedure:Assess percentage surface area covered by disease on samples of tubers.

Assessing Severity: Calculate the average infection for all the tubers assessed.

All keys are based on a percentage scale. Although only a few degrees of infection are shown (e.g. 1, 5, 10 and 15%), interpolations can be made when assessments are recorded at higher levels. The extent of the interpolation should be dictated by the ability of the observer to detect differences in levels of infection. The percentage of infection noted in the following diagrams, represents the actual area covered by the lesions illustrated.

Appendix 9-1.1 Key for Common Scab of Potatoes (Streptomyces scabies)

The following image is a key for common scab of potatoes, demonstrating different amount of scabs on the visible surface of potatoes labelled with a corresponding percentage.
Image - Appendix 9-1.1 Key for Common Scab of Potatoes
Description for Appendix 9-1.1

This image is of four potatoes divided into two rows and two columns. This image demonstrates different amounts of scabs on the visible surface of potatoes labelled with percentages that correspond with the visible amount of scab.

The first potato at the top left side of the image demonstrates a potato with scabs taking up 1% of its surface area; this potato only has three small black spots.

The potato on the top right side demonstrates a potato with scabs taking up 5% of its surface area; the black spots are more frequent but are still fairly sparse.

The potato on the bottom left side of the image demonstrates a potato with scabs taking up 10% of its surface area; the amount of scabs is less sparse.

Finally, the potato on the bottom right side of the image demonstrates a potato with scabbing that takes up 15% of its visible surface area causing the spots to appear more crowded.

Appendix 9-1.2 Key to Black Scurf of Potatoes (Rhizoctonia solani)

The following image is a key to the black scurf of potatoes, demonstrating different amount of black scurf on the visible surface of potatoes labelled with a corresponding percentage.
Image - Appendix 9-1.2 Key to Black Scurf of Potatoes
Description for Appendix 9-1.2

This image is of four potatoes divided into two rows and two columns. This image demonstrates different amounts of black scurf on the surface of a potato with percentages that label them accordingly.

The first potato at the top left side of the image demonstrates a potato with black scurf taking up 1% of its surface area; this potato only has a few small black spots.

The potato on the top right side demonstrates a potato with black scurf taking up 5% of its surface area; the black spots are more frequent but are still fairly sparse.

The potato on the bottom left side of the image demonstrates a potato with black scurf taking up 10% of its surface area; the amount of spots is less sparse.

Finally, the potato on the bottom right side of the image demonstrates a potato with black scurf that takes up 15% of its visible surface area causing the spots to appear more crowded on the entire surface of the tuber.

Appendix 9-2 Shape of Potatoes

The following image is of silhouettes of potatoes demonstrating that they can have different shapes such as a dumbbell shape, a curved shape, a pointed shape, and a knobby shape.
Image - Appendix 9-2 Shape of Potatoes
Description for Appendix 9-2

This image demonstrates different shapes of potatoes through providing black silhouettes of them.

There are four different kinds of shapes of potatoes in the image;

  • a dumbbell tuber,
  • curved tuber,
  • pointed tuber,
  • and a knobby tuber.

The potato at the top-left side of the image is a dumbbell tuber; its top and bottom are large, and they curve inwards to create a smaller area at the centre.

The potato at the top-right side of the image is a curved tuber; it curves downwards into two points, causing a crease to form in its centre.

The potato at the bottom-left side of the image demonstrates a pointed tuber; it is an odd shape with one of its ends forming into a point.

Finally, the potato on the bottom-right side of the image demonstrates a knobby tuber; there are three lumps protruding outwards from the main tuber.

Appendix 9-3 Seed Potato Tuber Inspection Report (CFIA 3076) - Example

The following image is an example of a Seed Potato Tuber Inspection Report (CFIA/ACIA 3076).
Image - Appendix 9-3 Seed Potato Tuber Inspection Report
Description for Appendix 9-3

This image is of an example of a Seed Potato Tuber Inspection Report. It is intended for inspectors when they inspect seed potato tubers. The report provides a section at the top of the form for information regarding when the inspection occurred, the reason for the inspection (bin, harvest, shipping, dockside, and reinspection), the shippers name, the certification number, and the growers number. In another section at the top of the form it provides check boxes to fill out information regarding the commodity code, inspection, attended premise, sample, analysis, control action, process document, or survey. The report also provides a space to fill out the type of variety, the class, the number and size of packages, and whether it is in a bulk, crate, or bags. The main portion of the form provides multiple smaller boxes to enter information regarding the number of tubers in count, whether there was wet rot, dry rot, internal discolouration, damage, off types, foreign variety, scab, Rhizoctonia, or oversized or undersized potatoes determined by weight. The right portion of the form provides space to enter information regarding the inspection point, the size of the tubers, the estimated production, the temperature of the storage area and the tubers, the vehicle number, the destination and the shipping weight. The bottom portion of the form provides space for observations, and the inspector's signature, number, name, and the date they signed the Report.

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