PI-005: Chapter 5 - Seed Potato Crop Inspection
7.0 Guidelines for Crop Assessment

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Once a seed potato crop has been determined to be eligible for crop inspection, it will be inspected to confirm variety identity and determine compliance with the regulatory standards for the various classes of seed potatoes under the Seeds Regulations Part II, as well as to assess the overall health status of the crop. Appendix 8 lists tolerances for defects, diseases and varietal mixtures upon first, subsequent, and final crop inspection for all classes of seed potatoes, with the exception of Nuclear Stock class. Diseases scored under the tolerances for field grown seed potatoes, seed potato crop entered for certification, may consist of bacteria, fungi, viruses, or other biological entities affecting seed potato quality. The damage that they cause varies from year to year depending on a number of biotic and abiotic factors.

During crop inspection the inspector attempts to confirm variety identity and accurately describe the status of a crop at the time of inspection based on random visual examinations of growing plants performed by walking through the fields. Any abnormal and/or unusual cultural conditions observed in the field are also recorded. To perform crop inspection an inspector must be familiar with symptom recognition as well as some of the terminology used to describe disease symptoms or physiological disorders.

This section is intended to be used as general guide in assisting inspection staff in assessing potato plants and in completing the Inspector's Field Notes (CFIA/ACIA 1298). Inspectors should refer to the variety descriptions and other relevant documentation to confirm variety integrity.

The following seven images are some of the most common terms used in describing symptoms on potato plants:

image - Chlorosis. Description follows.
Chlorosis - Abnormal light green or yellow coloration of leaves due to insufficient production of chlorophyll, which may be caused by lack of light, mineral deficiency, infection (particular with viruses), or genetic factors.

image - a and b: Lesion. Description follows.
a and b: Lesion - A distinct area of diseased plant tissue. A canker on the stem (a) or a confined area of necrosis on the leaf (b).

image - Mottling. Description follows.
Mottling - A variegated pattern marked with yellowish spots or patches of many colours or shades.

image - Necrosis. Description follows.
Necrosis - Death of plant cells or plant tissues, usually accompanied by darkening or discoloration, occurring as a disease symptom.

image - Rotting. Description follows.
Rotting - Disintegration and decomposition of plant tissue, and may be of two general types, dry rot or soft rot. A dry rot is a firm dry decay; and a soft rot is soft, usually watery, and often odoriferous.

image - Rugose Mosaic. Description follows.
Rugose Mosaic - Severe mosaic accompanied by deformation such as crinkling, curling or ruffling of the leaf surface, usually caused by a combination of multiple viruses.

image - Variegation. Description follows.
Variegation - Uneven green colour patches, spots, or streaks on plant leaves with sharply defined borders of colour changes. This feature differs from the viral infection complex called mosaic. Mosaic is not as pronounced, and the light and dark areas have more diffuse boundaries (mottling).

7.1 Assessing and Scoring Diseases Requiring Counts

Plant counts are used to determine disease infection and varietal mixtures levels in the field. They are taken at regular intervals in a randomized pattern throughout the crop/field to ensure the inspection results are representative of disease levels and varietal mixtures in the field. One count consists of 100 consecutive plants in a row. They can be counted individually or by a calculated field pace as indicated below.

The actual number of diseased plants or varietal mixtures per count is recorded and used to determine the percentage within the following categories in the Inspector's Field Notes (CFIA/ACIA 1298). Additionally, any plant parts including tubers not properly rogued should be included in the above inspection count.

  • Number of plants showing mosaic
  • Number of plants showing Potato Leafroll Virus (PLRV)
  • Number of plants showing blackleg
  • Number of plants showing wilts
  • Number of foreign plants (plant of different variety)
  • % misses

Here are some basic calculations that inspection staff should be aware of:

  • There are 43,560 sq ft per acre.
  • There are 2.471 acres/hectare (ac/ha) or .405 hectares/ac.
  • E.g. at 12 in. (30 cm) plant spacing within row and a 36 in. (90 cm) row spacing, there is 1 plant per 3 sq ft (0.27 m2.).

With a fixed row distance of 36 in., Plant density can be estimated for any particular field based on the various plant spacing within row assumptions below:

  • 8 in (20cm) spacing = 21,780 plants/ac or 53,800 plants/ha
  • 10 in (25cm) spacing = 17,424 plants/ac or 43,037 plants/ha
  • 12 in (30cm) spacing = 14,520 plants/ac or 35,880 plants/ha
  • 16 in (40cm) spacing = 10,890 plants/ac or 26,900 plants/ha

7.1.1. Determining the field pace

It is important to precisely establish how many paces constitute a single count of 100 plants. A field pace can be established as follows:

  • Mark a row in the field (at least 10 m away from the edge of the field);
  • Manually count out 100 plants, and then mark the end of the count;
  • The number of paces on average to go from start to finish will constitute the pace/count ratio.
  • Inspectors should pace the 100 plant count using the same pace as when they conduct field inspections. Small variations in this crucial step can make a significant difference on pest incidence calculations once the inspection is completed.

Generally, the number of paces per count of 100 plants will range from 30 - 40 paces, depending on the size of pace, plant spacing, and number of plant misses in the field. As planting densities can vary according to planting date, variety, soil type etc., the process of field pace determination should be repeated every time a new field is inspected.

Note: Consider, as the day goes by, your individual paces may be reduced in length, owing to fatigue. You may need to re-evaluate your number of paces to adjust your plant count.

7.1.2. Establishing the Minimum Number of Counts

It is essential to establish and inspect each crop using a minimum number of counts depending on the class planted and field size as outlined in Table 7-1.

Note: Infected plants which are observed but not part of a count should be noted in the comment section of the Inspector's Field Notes (CFIA/ACIA 1298).

The following table is a guide intended to assist in determining the number of inspection counts based on the class planted, and the field size in hectares.

Table 7-1 Guide to Determine the Number of Inspection Counts
Class Planted Field Size (Ha) Min. # of Counts
Nuclear N.A. All Plants
Pre-Elite N.A. Min of 10% of plants or 10 counts if feasible
E-1 to Foundation < 1.0 Min of 10% of plants or 10 counts if feasible
E-1 to Foundation 1.0 - 1.5 10
E-1 to Foundation > 1.5 - 5.0 15
E-1 to Foundation > 5.0 - 10.0 20
E-1 to Foundation >10.0 - 15.0 25
E-1 to Foundation >15.0 - 20.0 30
E-1 to Foundation >20.0 - 25.0 35
E-1 to Foundation >25.0 - 30.0 40
E-1 to Foundation >30.0 - 35.0 45
E-1 to Foundation >35.0 50

In the case of a Pre-Elite class planted crop, inspect at least 10% of plants or 10 counts. This can be determined as follows:

  • Estimated plant density (based on plant spacing) × # of hectares = total plants
  • Total plants × 10% = Number of plants to examine
  • Number of plants to examine / 100 = Number of counts
  • Inspect a minimum of 10 counts if feasible

Additionally, there are times when inspectors may encounter situations where standard calculations will not give an accurate representation of disease count percentages. For example, an inspector may take 10 counts and find no diseased plants in those counts but the inspector may have seen a number of diseased plants in the field, outside the counted area. The count thus shows zero percentage but it is quite apparent that the field is not totally free from disease or varietal mixture. In such a situation, the inspector should take a few additional counts to enhance their field coverage and inspection results precision. If counts are still clear, the inspector should note the disease or foreign plants in the remarks section of their field report, and bring it to the grower's attention since these finds, if not removed, it could especially affect the class to be assigned in high generation planted crops.

In calculating misses, inspectors must also take random counts, concentrating on the number of plants missing within the count. These random counts are then used to calculate the average percentage of missing plants.

Example – Calculation of misses, foreign plants and disease percentages:

Consider a crop inspection of a field where 50 counts (100 plants per count) are taken; Out of the 50 counts, 7 plants with leafroll and 13 plants with mosaic are found in total.

Disease infection level for leafroll and mosaic would be calculated as follows;

Number of leafroll plants divided by the total number of plant counted, multiplied by 100% (7/5000) × 100% = 0.14%. Same for procedure for calculating percentage of Mosaic: (13/5000) × 100% = 0.26%

The total virus incidence = 20/5000 × 100% = 0.4%. If this total virus incidence was obtained on final inspection, the eligible class to be assigned would be Foundation if Elite 4 or a higher class were planted.

7.1.3. Field Plots Planted in Tuber Units

a) Where tuber units are identifiable, count 100 units. This is considered as one count, e.g., if tubers were cut in 4 pieces and planted consecutively, count 400 plants and diseased plants are counted by unit (one unit = 4 infected plants in sequence). Growers may choose to use tuber uniting as good crop management practice although it is no longer a regulated procedure.

Note: Tuber unit sizes may vary, as a tuber can be cut in several pieces, (2, 4, 6, etc.). As long as the tuber seed pieces are planted consecutively and can be identified as one unit, they would be considered as units. Also, the planting of whole seed can be considered as tuber united.

b) Where the grower has indicated tuber unit planting on the application and the inspector is unable to accurately identify the units, consultation with the grower must take place to verify the method of tuber uniting. If difficulty persists and units are still not identifiable, treat the field as scatter/mass planted where a count is 100 plants and each infected plant is counted as one plant.

7.1.4 Patterns for Walking the Field to Ensure Random Sampling

The inspector shall walk the field and make a random examination of the plants including/covering both sides of the field either following a figure "X" pattern or an inverted "V" pattern, except when inspecting 100% of the plants. These two patterns generally give the inspector the highest degree of randomness and allow the greatest surface area of the field and the highest number of plants to be covered while reducing the chances of omitting a pocket of infestation. If the field configuration does not allow for this type of pattern it is up to the discretion of the inspector to use the most random method possible to ensure the field is properly covered.

For fields using centre pivot irrigation, a "V" or triangular pattern to the center of the pivot should be followed for each inspection. This should be followed by two additional and shorter modified "V" patterns to complete the inspection. Sample patterns for inspecting circles and irregular shaped fields are given below (Figure 7-1). The pattern in the field and entry point (as noted in section 4.3) should be predetermined and varied between inspections. These two patterns, as illustrated below, will give the highest degree of randomness and allow the greatest surface area of the field and highest number of plants to be covered while reducing the chances of omitting a pocket of infestation.

The following images are sample patterns for inspecting circular and irregularly shaped fields.

Figure 7-1 Sample Patterns for Inspecting Circles and Irregular Shaped Fields
image - Figure 7-1. Description follows.

This is an image of two seed potato fields, one L-shaped, the other round. The fields contain arrows demonstrating the initiation of inspection counts 5 to 10 metres inside the edge of the field and then sample walking patterns used during CFIA inspection of seed potato fields of an irregular shape to adequately assess the entire crop.

When entering the field, the inspector should proceed a distance of approximately 5 to 10 m before initiating plant counts. This is intended to negate the edge effect. However do not overlook the initial 10 m as there may be evidence of insect damage and/or varietal mixtures if this is where planting started in that field. From this point the entire field should be viewed, varietal integrity determined, and notes should be made on potential problem areas to investigate as the inspection progresses. Special attention should be paid to areas in the field which are susceptible to biotic or abiotic interactions. The inspector should observe low or high spots and areas along hedgerows which could contribute to disease problems or preservation of volunteers (potatoes that over wintered and grew in the following season). Remain aware of potential presence of foreign varieties in these areas from tubers that may have over wintered in the field.

Although the diagrams in Figure 7-1 indicate a diagonal pattern, as this is basically how you are moving through the field, this is accomplished by an inspector moving through the field in a zigzag type of pattern (see Figure 7-1-a), moving over several rows after each count, to commence a new count. i.e. count or pace a hundred plants, move over several rows, do another count, move over again, and continue as such. This will help ensure that the field is adequately covered.

The following image is an example of a zigzag method for taking counts through the field.

Figure 7-1-a. Taking counts through the field - Zigzag
image - Figure 7-1-a. Description follows.

This is an image of a rectangular seed potato field containing arrows and straight lines representing walking patterns used by a CFIA inspector during field inspection. This zigzag pattern shows how the CFIA inspector moves over several rows after each count, to start a new count, ensuring that the seed potato field is adequately assessed.

Note: Each bold line indicates a count (or 100 plants)

In general, an inspector must be familiar with the variety to be inspected, disease expression, ensure the timing of their inspections are suitable, ascertain variety integrity, cover the field adequately, take an appropriate number of counts, assess the crop properly and assign the appropriate class at the end of final inspection.

7.1.5 Assessing Symptoms of Mosaic

Leaf symptoms which are referred to as mosaic are a result of potato plants being infected with certain potato viruses. The symptoms may range in severity from slight mottling to severe crinkling of leaves and stunting of plants. Some of the potato viruses are often referred to as latent viruses because they can be carried in plants or tubers without causing any visible symptoms. Depending on the variety and/or the weather conditions it may also affect symptom expression. Cool, cloudy weather may make the symptoms more pronounced. The presence of more than one virus in the plant may affect the types of symptoms and increase the severity. As well, it is important to note symptoms of the same virus are often expressed very differently depending on the variety.

In general, leaves of virus affected plants are mottled often with some areas of light green to yellow and some darker green than normal. Mottled areas may vary in size and occur both on and between the leaf veins. Leaf margins may be wavy, and the leaves may appear slightly rugose where the veins and interveinal areas are raised. Symptoms caused by different viruses shown below can vary; but they are all scored under the tolerance for mosaic.

The following six images are examples of symptoms of Mosaic on the plants Chieftain, Atlantic, Goldrush, Superior, Red Pontiac and Shepody.

image - Mosaic on Chieftain. Description follows.
Mosaic on Chieftain:
This image is an example of mosaic on Chieftain; it demonstrates its infection through the crinkling of its leaves.
image - Mosaic on Atlantic. Description follows.
Mosaic on Atlantic:
This image is an example of mosaic on Atlantic; it demonstrates its infection through the crinkling of its leaves.
image - Mosaic on Goldrush. Description follows.
Mosaic on Goldrush:
This image is an example of mosaic on Goldrush; it demonstrates its infection through the crinkling of its leaves.
image - Mosaic on Superior. Description follows.
Mosaic on Superior:
This image is an example of mosaic on Superior; it demonstrates its infection through the crinkling of its leaves.
image - Mosaic on Red Pontiac. Description follows.
Mosaic on Red Pontiac:
This image is an example of mosaic on Pontiac; it demonstrates its infection through the crinkling of its leaves.
image - Mosaic on Shepody. Description follows.
Mosaic on Shepody:
This image is an example of mosaic on Shepody; it demonstrates its infection through the crinkling of its leaves.

A) Potato Virus Y (PVY)

PVY has a worldwide distribution and is one of the most important viruses affecting potato production. There is little to no evidence of PVY being effectively mechanically spread between plants by foliar contact, and by tuber-to-tuber transmission under general production practices. However, transmission in a non-persistent manner by aphid vectors is considered to be the most important method of natural spread in the field.

There are several strains of PVY which have been identified in North American potato production. While all are scored as Mosaic,the inspector should be familiar with the following three strains.

  • PVYo: Ordinary Strain

Symptoms can range from very mild mosaic to severe foliar necrosis, sometimes leading to plant death. Symptoms can be on a single branch (leaf) only, which can make field identification difficult. In secondary infections, symptoms can include dwarfing, severe mottle which is variable in size and not necessarily bordered by leaf veins. Leaflets are usually malformed. In some cases, necrotic streaks can develop around veins (vein banding) and stems (stipple-streak). At low and high temperatures, symptoms can be masked. PVYo has been known to cause severe yield loss. Infected leaves frequently develop necrotic spots. Plants are stunted, lower petioles and leaves turn yellow, and dead leaves cling to the stem. Leaves crinkle and curl.

  • PVYn

This is often referred to as the tobacco veinal necrosis strain, which causes very mild or mottling or no visible symptoms in potato plants.

  • PVYntn

Symptoms of this strain include interveinal chlorosis and leaf crinkle. Secondary infection symptoms: distinct mosaic, severe necrosis on stems, leaves and leaf drop; they become more visible under higher temperatures. Secondary infections will exhibit a distinct mosaic, severe necrosis on stems, leaves, and eventually leaf drop. Necrotic ring spots may occur in the tubers of certain varieties, including Yukon Gold.

B) Potato Virus A (PVA)

PVA is transmitted by at least seven different aphid species including the green peach aphid and the potato aphid. It is a non-persistent type virus. Leaves may show mild, crinkle and veinal mosaic, stems tend to bend slightly outward. Rugose symptoms can also be caused if combined with certain strains of PVA and PVY. Similar to other viruses, PVA is transmitted from one generation to the next by planting infected tubers.

C) Potato Virus S (PVS)

PVS is typically symptomless, though some varieties show slight deepening of leaf veins, rugosity of leaves, possible stunting, mottling or bronzing. Combined with PVX, PVS leads to more yield reduction than if either virus is present alone. It is believed that mature plants are resistant to PVS. It can be transmitted by seed, aphids (non-persistent), seed cutting, and injury during field operations, leaf-to-leaf contact and sprout contact before planting.

D) Potato Virus X (PVX)

PVX is the most widespread of the potato viruses. Sometimes it is referred to as potato mottle virus and latent mosaic because it is often symptomless. Some strains do not produce visible symptoms, while others will slightly reduce total yield and/or cause a mild crinkle particularly in mixed infections. PVX is carried in tubers, transmitted mechanically and by plant-to-plant contact in the field and sprout contact before planting. There are no known aphid vectors.

E) Tobacco Rattle Virus (TRV)

Tobacco Rattle Virus (TRV) is also known as corky ring spot, stem mottle and spraing. This virus is soil-borne, transmitted by stubby nematodes of the genus Trichodorus, and occurs mainly in lighter soils. Its host range includes plant species from over 50 families. Only certain potato varieties develop spraing symptoms; while others remain symptomless. Appearance and severity of symptoms vary considerably with varieties, but they are manifested in the form of alternating green/pale green to bright yellow patches. Patches are coarser, more yellow, and more irregular than mosaic.

F) Alfalfa Mosaic Virus (AMV, Calico)

Alfalfa Mosaic Virus (AMV, Calico) is transmitted by at least 16 species of aphids with pea aphid as the most common vector. Infection often occurs on edges of fields located near alfalfa. Little or no secondary spread occurs within the potato field. AMV can also be transmitted to the next generation through the progeny. Symptoms appear as pale to bright yellow mottling or blotching of leaves. Some strains cause severe stunting and necrosis of stems and tubers. AMV produces distinct calico mosaic symptoms on potato and other solanaceous crops.

image - Alfalfa Mosaic Virus 1. Description follows.
Alfalfa Mosaic Virus:
This image is an example of Alfalfa Mosaic Virus. It is an image taken of a bush to demonstrate its leaves which have a light-yellow colour, showing that they have been infected.
image - Alfalfa Mosaic Virus 2. Description follows.
Alfalfa Mosaic Virus:
This image is an example of Alfalfa Mosaic Virus. It is an image taken of leaves demonstrating that their sides and tips have turned to a light-yellow.

7.1.6 Assessing Symptoms of Potato Leafroll Virus (PLRV)

Symptoms scored as Leafroll are caused by the Potato Leafroll Virus (PLRV). Other hosts of this virus include solanaceous crops and weeds, such as, but not limited to, tomato, pepper, eggplant, tobacco, and jimsonweed.

Chronic or seed borne infection occurs when plants are grown from tubers infected with PLRV. Symptoms of chronic infections generally develop on the lower leaves, which will roll upward, become stiff, dry, and leathery, turn light in color, and make a crisp, papery sound when touched or shaken. Plants may be stunted or rigid with light leaf yellowing in the crown and older leaves may become discoloured, turn brown and die.

Current season PLRV infection occurs when plants are infected later in the season when aphids are present, most notably the green peach aphid. If infection occurs early in the growing season, symptoms may appear mainly in the young leaves, often near the top of the plant, which usually stand upright, roll and will become slightly pale. Young leaves may turn pink to reddish in color beginning at the margins on red-skinned varieties; the leaves of white-skinned or russet varieties often turn yellow. Late season infections often do not exhibit symptoms. Current season spread of PLRV can cause tuber necrosis in certain varieties.

The following four images are examples of Potato Leaf Roll Virus in the plants Kennebec, Superior, Red Pontiac, and Goldrush.

image - Potato: Kennebec. Description follows.
PLRV in Kennebec:
This image is an example of Potato Leaf Roll Virus in a Kennebec plant; its leaves are beginning to curve upwards.
image - Potato: Superior. Description follows.
PLRV in Superior:
This image is an example of Potato Leaf Roll Virus in a Superior plant; its leaves are beginning to curve upwards.
image - Potato: Red Pontiac. Description follows.
PLRV in Red Pontiac:
This image is an example of Potato Leaf Roll Virus in a Red Pontiac plant; its leaves are beginning to curve upwards.
image - Potato: Goldrush. Description follows.
PLRV in Goldrush:
This image is an example of Potato Leaf Roll Virus in a Goldrush plant; its leaves are beginning to curve upwards.

7.1.7 Assessing Symptoms of Blackleg (Pectobacterium atrosepticum)

When the plant is infected from diseased seed potatoes, the stems will show an inky black decay, extending up the stem from the seed piece. If the plant becomes infected through wounds on the plant, the rot will extend up and down the stem from the point of infection. Infected plants are often stunted and have a stiff, erect growth, and turn pale green or yellow.

Blackleg is carried in or on tubers and can be transmitted to healthy tubers during handling and planting operations. Wind-blown rain and irrigation water may also be sources of inoculum. Additionally when seed harvesting is done in wet conditions in the fall, the Blackleg pathogen may spread during storage. Cool temperatures (10-15°C) and wet soils in the spring followed by warmer temperatures (above 20°C) following plant emergence stimulate development of Blackleg. In cool springs, infected seed pieces may rot before emerging, resulting in uneven stands.

Field symptoms may be expressed at any time during the growing season. Loss of vigour and black, slimy lesions extending from the infected seed piece will occur in warm and wet springs. Plants appear stunted, stiff and erect. Leaves appear chlorotic, and leaflets may roll upwards and cup at the margins. Small water soaked lesions may appear at the base of the stem. Lesions can rapidly become extensive and progress to the upper canopy. Stem piths will usually be decayed well beyond visible lesions. Affected aerial tissue appears soft in humid conditions, and becomes shrivelled in dry environments. Around flowering, some stems may suddenly wilt. Infected tissue is often invaded by secondary bacteria, which produces a fishy odour.

Aerial stem rot can affect all visible plant parts; it does not originate from infected seed pieces. Symptoms appear as a water-soaked green decay that turns light brown or black. On older plants, interveinal yellowing and browning and the upward cupping of leaves are characteristic symptoms.

The following four images are examples of blackleg infection.

image - blackleg infection 1. Description follows.
Blackleg Infection:
This image is an example of blackleg infection; it demonstrates a bush that has been infected – its leaves are beginning to yellow and curve upwards.
image - Blackleg Infection 2. Description follows.
Blackleg Infection
This image is an example of blackleg infection; it demonstrates a bush that has been infected – its leaves are beginning to yellow and curve upwards.
image - Blackleg Infection 3. Description follows.
Blackleg Infection:
This image is an example of blackleg infection; it demonstrates a stem that is green, but in its centre it has become black and split.
image - Blackleg Infection 4. Description follows.
Blackleg Infection:
This image is an example of blackleg infection; it demonstrates a stem that has become almost entirely dark-brown.

7.1.8 Assessing Symptoms of Wilts

Symptoms develop under conditions of warm temperatures, low soil moisture and low fertility. At first the lower leaves wilt, yellow and later turn brown. As time progresses, these symptoms move upwards on the plant. At first only a single stem may show symptoms. The vascular bundles in the lower part of the stem show a brown discoloration that can best be seen if the stems are cut near ground level. Named for the casual pathogen, the two most common wilts are Fusarium wilt and Verticillium wilt, which are both scored under the tolerance for wilts on the form CFIA/ACIA 1298.

image - Fusarium Wilt. Description follows.

A) Fusarium Wilt (Fusarium spp.)

Fusarium overwinters in the soil and on seed potatoes tubers in storage, providing the inoculum that infects developing tubers through root wounds in the spring when temperatures exceed 20 °C. The fungus attacks the vascular tissue in plants, causing wilt. Initial symptoms include stunting and sporadic chlorosis. Tips or margins of lower leaves turn yellow, then tan before drying up and hanging on the plant. Over time, the disease will move up the plant. Infected plants will appear to recover during the night, but show very visible wilt symptoms under high daytime temperatures.

B) Verticillium Wilt (Verticillium dahliae)

This disease is also commonly referred to as Early Dying Syndrome. The causal pathogen is a soil borne fungus called Verticillium dahliae. Inoculum usually originates in the soil as mycelium or resting structures which can be transmitted by water, wind and mechanical movement of soil. The pathogen can also be introduced on contaminated or infected seed tubers. When soil temperatures get warmer in the spring, dormant spores can enter the plant's vascular system through rootlet infection. Development of the fungus is optimal when soil temperatures are high (22°C to 27°C). Initial symptoms tend to first appear in lower leaves usually in mid to late summer. Interveinal tissue in apical leaflets first turns pale green then yellow. Necrosis is then followed by flagging and death of affected stems. Verticillium has also been known to interact with some Erwinia species.

The following two images are examples of Verticillium Wilt.

image - Verticillium Wilt 1. Description follows.
Verticillium Wilt:
This image is an example of Verticillium Wilt; it demonstrates multiple branches that have wilted and have leaves that are light yellow or brown.
image - Verticillium Wilt 2. Description follows.
Verticillium Wilt:
This image is an example of Verticillium Wilt; it demonstrates a branch with leaves on it with some that are a pale beige, green, or yellow colour.

7.1.9 Assessing Foreign Varietal Mixtures

Foreign are any potato plant, of any variety, that is not same as the variety for which crop inspection is being conducted on a particular field. The inspector must record total number and percentage of foreign plants but it is not necessary to identify the varieties of these foreign plants.

7.2 Assessing Other Diseases and Insect Damage

During the inspection of a field, the level of severity for Early Blight, Late Blight, Rhizoctonia, Tip Burn, Aphids, and Colorado Potato Beetles, must be recorded on the Inspector's Field Notes (CFIA/ACIA 1298). Section 7.2 serves as a guide in assessing these diseases and insect damage.

Between counts, the inspector must stop periodically to:

  • look for insect and disease damage;
  • inspect individual plants for aphid infestations by turning over leaf trifoliate;
  • inspect fields for insect damage predominantly close to the edge of the field. This is especially important for determining flea beetle and aphid activity; and
  • examine leaves and stems for symptoms of fungal infestation.

Determining the Level of Disease and Insect Incidence

Levels of disease and insect incidence are rated and recorded as trace, light, moderate, and severe on the field inspection form as specified on the Table 7-2. Figure 7-2 displays leaf samples with 1%, 10%, 25%, and 50% of area damage, respectively. Determine both the incidence and frequency of the pest in order to determine the overall rating. The incidence is determined by either counting the number of pests or determining the percentage of leaf area affected. The frequency is the percentage of plants affected in the field. The final rating is based on the lowest rating level between the incidence and the frequency.

For example, it is possible to encounter severe incidence on a few plants (>25% leaf area affected) however only 2% of the plants are affected (i.e only a trace frequency). In this case the overriding factor would be the percentage of plants affected (i.e frequency), which would result in reporting the above example as trace.

The following table provides different rating levels for disease and insect incidence.

Table 7-2: Rating Level for Disease and Insect Incidence
Rating Level Trace Light Moderate Severe

% of plants affected

<5%

5% to 10%

10%-50%

>50%

# beetles per plant

1

2 to 3

4 to 5

> 5

# aphids per triplet

1

2 to 3

4 to 5

> 5

% leaf area affected

1%

1% to 5%

6% to 25%

> 25%

The following image is an example of a key for assessing different severities of leaf damage.

Figure 7-2: Key to Assessing Leaf Damage Severity
image - Figure 7-2. Description follows.
Description for Figure 7-2

This image is an example of a key for assessing different severities of leaf damage, and provides four examples of leaves. The first example at the top left portion of the image demonstrates a leaf with only 1% of its surface area damaged. The example at the bottom left portion of the image demonstrates a leaf with only 10% of its surface area damaged. The image at the top right portion of the image demonstrates a leaf with 25% of its surface area damaged. Finally, the leaf at the bottom right portion of the image demonstrates a leaf with 50% of its surface area damaged.

7.2.1 Early Blight (Alternaria solani)

Early Blight is found in most potato growing regions of the world. After over-wintering in the soil on decaying plant matter, Early Blight can usually be found each year in varying degrees of severity. Fungal spores are mostly spread to healthy plants by wind and rain. Very wet and humid growing conditions will favour the development of early blight.

Normally, lower leaves are infected first, becoming more severe when stresses such as other diseases, insect damage or nutrient deficiency affect the crop. When the infestation is severe, both stems and tubers may be affected. The disease usually first appears as small, brown, pinhead-like dots on older leaves. These lesions are circular, 3-10 mm across and consist of concentric rings of dead tissue. Lesions become angular in shape when their expansion becomes limited by large leaf veins. Leaf loss in heavily infected crops, particularly during prolonged wet conditions, may cause sufficient loss of leaf area and subsequently reduce crop yields. As the disease spreads, lesions appear on the upper leaves and on the stems, however infected leaves usually remain attached to the plant.

The following four images are examples of Early Blight.

image - Early Blight 1. Description follows.
Early Blight:
This image is an example of Early Blight; it demonstrates an up-close image of a leaf having three main brown spots.
image - Early Blight 2. Description follows.
Early Blight:
This image is an example of Early Blight; it demonstrates a magnified image of en Early Blight spot on a leaf. The spot is dark-brown and has ridges.
image - Early Blight 3. Description follows.
Early Blight:
This image is an example of Early Blight; it demonstrates almost an entire bush with its leaves a yellow and light green colour, with little brown spots.
image - Early Blight 4. Description follows.
Early Blight:
This image is an example of Early Blight; it demonstrates several leaves with the majority of their surface area covered in small or large brown spots.

7.2.2 Late Blight (Phytophthora infestans)

Late Blight (Phytophthora infestans)over winters on seed potato tubers in storage and plant debris left in the field. It begins to spread once temperatures are sufficiently warm to allow for production of spores. Initial symptoms usually appear shortly after flowering, following warm and wet or humid weather. Ideal conditions for late blight development include; high humidity, warm nights (10-15°C) and cool daytime temperatures (10-21°C). Once late blight begins to spread, it can be extremely destructive, destroying entire fields if left unchecked. Inspectors should note that late blight spreads through mechanical means as well as through rain and wind. Efforts should be made to avoid spreading the disease while conducting inspections.

Symptoms usually develop 6 to 8 weeks after planting depending on initial inoculum amount and environmental conditions. Dark green, water-soaked leaf tips will become dark brown and brittle within 2 days. Especially on dewy mornings, lesion edges found on the underside of infected leaves will show a fluffy white mycelium. Late Blight infected areas of leaves will cross major veins, whereas Early Blight will not. If infected seed is planted, symptoms can appear on stems and young tissues. When infections occur at the base of the plant, the presence of mycelial growth may help in distinguishing late blight from blackleg.

The following four images are examples of Late Blight.

image - Late Blight 1. Description follows.
Late Blight:
This image is an example of Late Blight; it demonstrates the underside of a leaf that has been infected. The leaf is brown towards the stem and the majority of the underside has a fluffy white mycelium on it.
image - Late Blight 2. Description follows.
Late Blight:
This image is an example of Late Blight; it demonstrates a stem that has been infected and is turning dark-brown and beginning to wilt.
image - Late Blight 3. Description follows.
Late Blight:
This image is an example of Late Blight; on the underside of two leaves there is a spot that is light yellow at its edge and fades to a brown-beige colour.
image - Late Blight 4. Description follows.
Late Blight:
This image is an example of Late Blight; it demonstrates a stem which has become a dark-brown colour and is shrivelled and brittle.

7.2.3 Rhizoctonia (Rhizoctonia solani)

Rhizoctonia solaniis a very common fungus to most potato growing regions of the world. It over winters primarily as sclerotia, which are structures that protect dormant spores, and can be found on the surface of harvested tubers as well as on decaying plant matter in soil. Rhizoctonia development is particularly favourable in cool, wet spring conditions. When infected seed is planted, lesions ranging from reddish to dark brown may develop on sprouts and young stems, and as the disease progresses, sprouts and stems may be pinched off by the infection, resulting in uneven stands following emergence.

Mid and late season infections may result in dark brown cankers on stems which may affect the vascular system leading to the production of aerial tubers in the leaf axils. Other common above ground symptoms may occur, including stunting, swollen stems, chlorosis, rolling of leaflet tips and purple pigmentation of leaves. Late in the season, the fungus produces a white mycelium on the stems just above the soil line.

The following five images are examples of Rhizoctonia.

image - Rhizoctonia 1. Description follows.
Rhizoctonia:
This image is an example of Rhizoctonia; it demonstrates the branch of a plant with its leaves curving upwards with a purple pigmentation on its underside.
image - Rhizoctonia 2. Description follows.
Rhizoctonia:
This image is an example of Rhizoctonia; the plant in the image has multiple red-orange lesions on it, taking up a moderate amount of its surface area.
image - Rhizoctonia 3. Description follows.
Rhizoctonia:
This image is an example of Rhizoctonia; it demonstrates an infected stem with brown canker and a white fungus forming on it.
image - Rhizoctonia 4. Description follows.
Rhizoctonia:
This image is an example of Rhizoctonia; it is a stem which appears white in some areas with dark-brown cankers in others.
image - Rhizoctonia 5. Description follows.
Rhizoctonia:
This image is an example of Rhizoctonia; it demonstrates multiple stems, with some having a white fungus forming near its base.

7.2.4 Tip Burn

Tip burn is a non-pathological condition of the leaves which occurs in many parts of the country and may be confused with early blight. The tips and edges of the leaves turn brown and areas soon become hard and brittle. Tip burning may occur at any time during the crop season and is generally caused by unfavourable conditions surrounding the plant. Stretches of cloudy and damp weather followed by several hot and bright days may result in the burning of the foliage. This is particularly pronounced on soils which hold a low percentage of moisture. When the weather is cloudy and damp the tissues of the potato become engorged with water and weakened. If followed by hot, dry sunny periods, there is a rapid transpiration of the moisture out of the leaf tissues which may occur faster than the water from the roots can reach the leaf. If this continues for any length of time the weaker tissue can collapse, die and dry up. Tip burn may also occur as a result of long periods of dry weather.

The following image is an up-close example of a leaf with tip burn; the point of the leaf has become brown and brittle with the rest remaining green.

image - tip burn. Description follows.
Tip Burn: This image is an example of tip burn; it is a close-up image of a green leaf with its tip orange-brown and brittle.

7.2.5 Aphids

The main concern associated with the presence of aphids is their capacity to transmit viral diseases from one plant to the other, notably PVY, PLRV, PVA and PVS. There is tremendous diversity in aphid species as well as aphid host species. Potato crops usually host potato, buckthorn, foxglove and green peach aphids. These soft-bodied insects tend to use plant leaves as feeding sites to access plant sap.

Aphids occur in both wingless and winged forms, and over winter as eggs. In the spring, eggs hatch and wingless aphids start immediately feeding and reproducing without mating; they spawn live wingless individuals. Dense colonies consist of thousands of individuals who collectively weaken plants. In some cases a sticky film of honeydew will cover leaves, and circular patterns of dead plants may be observed in the field. When populations become too dense, winged aphids are produced which migrate to nearby host plants where new colonies are established. As the host crop matures and days become shorter, sexual reproduction produces eggs for the next growing season.

The bottom third of potato plants is their preferred location, the exception being the potato aphids which are often found on the top half of the plant where they prefer feeding on the underside of leaves.

The following three images are examples of Wingless Aphids and a cluster of aphids.

image - Wingless light green Potato Aphid. Description follows.
Wingless Potato Aphid:
This image is an example of a wingless potato aphid; it is a light green colour with nine legs.
image - Wingless Green-Brown Potato Aphid. Description follows.
Wingless Potato Aphid:
This image is an example of a winged potato aphid; it is a green-brown colour with eight long brown legs.
image - Wingless light green and dark green to brown and dark red Potato Aphid. Description follows.
Cluster of Aphids:
This image is an example of a cluster of aphids; they vary from light green and dark green to brown and dark red.

7.2.6 Colorado Potato Beetle (Leptinotarsa decemlineata)

The Colorado Potato Beetle (CPB) is a common insect pest found in many potato producing areas including Canada, with the exception of Newfoundland and Labrador. When spring temperatures rise, adults emerge over a period of 4 to 6 weeks from fields previously planted with potatoes. Immediately after emergence, first generation beetles walk towards the nearest emerging potato crop and start eating. Mating and egg laying occur shortly thereafter.

Females may lay up to 400 eggs over a 4-5 week period. Eggs are yellow/orange, shaped like a tiny grain of rice and are deposited in masses of 25-40 eggs mostly on the underside of leaves. They will turn to a darker colour as they mature and hatch within 4-9 days. Larval development occurs over a period of 10 to 20 days, depending on temperatures and rainfall. Once the 4th instar (final larval development stage) reaches maturity, larvae will drop to the ground, burrow 5-10 cm deep, pupate and emerge as adults 5-9 days later. Summers in many areas of Canada are warm enough to allow for two generations of CPB. Cooler regions will have one to two generations depending on temperature during the growing season.

Typical feeding damage observed will primarily be defoliated plants, which can be severe if populations of CPB are high, notably on field edges. CPB is not known to be a vector of potato plant pathogens, although it appears likely that mechanical transmission of propagative material such as spores could occur.

The following four images are examples of the Colorado Potato Beetle as an adult, as larvae, as egg masses and hatched 1st instar larvae, and in an example of defoliation.

image - Colorado potato beetle adult. Description follows.
Colorado Potato Beetle (adult):
This image is an example of an adult Colorado Potato Beetle; it has an orange and black spotted head, and a black and yellow striped shell.
image - Colorado Potato Beetle (adult). Description follows.
Colorado Potato Beetle (adult):
This image is an example of an adult Colorado Potato Beetle; it has an orange and black spotted head, and a black and yellow striped shell.
image - Egg Masses and hatched 1st instar larvae. Description follows.
Egg Masses and hatched 1st instar larvae:
This image is an example of three egg masses and hatched 1st instar larvae; the mass of eggs closest to the bottom left portion of the image is orange. There is another mass nearby which is both eggs and 1st instar larvae which are black and orange. At the right side of the image there is another mass of larvae that is more dispersed.
image - Defoliation. Description follows.
Defoliation:
This image is an example of defoliation which has occurred due to the Colorado Potato Beetle; it has lost all of its leaves and the beetles can be seen almost everywhere on the plant.

7.3 Assessing Stand, Vigour and Cultivation

During the course of the inspection, the stand, the vigour and the cultivation of the crop will need to be rated. Stand, vigour, and cultivation are rated as: A= Excellent, B= Good, C= Fair and D= Poor. These are subjective ratings and a field will not fail as a result of these ratings. These categories are valuable for inspectors, producers, and buyers to determine if crop characteristics are due to seed quality problems, a result of natural conditions or crop management. For example, if one individual lot is planted in several fields in different areas and has reduced vigour in all of these fields it may indicate another underlying problem, leading to further investigation.

Stand refers to overall uniformity of the plants in the field (e.g. in colour, height, etc.) combined with the percentage of plant emergence. An excellent stand would show a very even and complete emergence with very few misses in a row.

Vigour is measured by observing the intensity of plant growth in the field. An excellent field should show lush new growth with even colour. In cases where the crop is performing poorly, possibly attributed to drought, variable fertility, herbicide injury or poor quality seed (physiologically aged, chilled, bruised, small seed pieces), the crop will be given a lower vigour rating.

Cultivation is the condition of the field in relation to tillage and weed control. An excellent seed field should be free of weeds and the plants are well hilled with loose friable soil.

Any disease or anomaly, other than those mentioned above that is encountered during the inspection process should also be noted in each of remarks sections in the Inspector's Field Notes (CFIA/ACIA 1298) and the Report of Field Inspection (CFIA/ACIA 1284).

7.4 Field Inspection Tips

The following are a few inspection techniques or tips which may assist an inspector in increasing her/his accuracy in the assessment of a seed potato crop.

  • An inspector should have filled out grower name, field number, variety, hectares, seed source, minimum counts for each field on Inspector's Field Notes (CFIA/ACIA 1298) prior to walking any field.
  • An inspector should try to avoid inspecting rows which are directly at their feet, but rather should view the rows which are one to two rows over from the row next to which they are standing, maintaining the line of sight which allows the inspector to view approximately one to two metres ahead from their pace.
  • An inspector should try to ensure that the plants being inspected are completely visible from top to bottom. This is especially important upon first inspection; however this will become difficult as plants mature. Inspectors should pay close attention for symptoms of Potato Leafroll Virus (PLRV) as the expression may be first seen on the bottom of the leaves when infected seed is planted.
  • An inspector should attempt to maintain the sun at their back to reduce glare and strain on the eyes. This also helps significantly in being able to see symptoms of disease.
  • The inspector should shade suspect plants with either, their body, field book, or hat to more readily view characteristic disease symptoms such as mosaic, interveinal necrosis and mild mottling.
  • The inspector may need to physically handle plants to determine the presence and type of disease/stress related problem. For example, PLRV infected plants show lower leaf curling, that is a lighter colour, but the leaves are generally firm and rattle when shaken. Whereas blackleg infected plants may show leaf curling that is a lighter colour but leaves remain soft. This may require the pulling of a few plants to observe other symptoms and determine the cause.
  • The inspector should examine suspect plants for the presence of insects, and physiological problems which may be attributed to other related factors. For example, nutrient deficient plants could display symptoms similar to mosaic in contrast to healthy plants. Examine the underside of the lower leaves of plants near edges and throughout the field to determine presence of aphids.
  • The inspector should become familiar with the characteristics of the variety being inspected, as some pathogens are capable of expressing themselves very differently on various varieties. For example, mosaic in Hilite Russet, Shepody or Russet Norkotah can be difficult to see, while mosaic found in a variety such as Red Pontiac, Atlantic or Century Russet can express severe symptoms and can lead to plant death.
  • Inspectors should remember that varieties can display symptoms of primary and secondary infection during different periods of the growing season. For example, the Goldrush variety may appear disease free early in the season, but symptoms of mosaic may become evident around 80 - 100 days after planting
  • Inspectors should take note of areas of potential environmental factors which could lead to disease development (e.g. along hedgerows, and other shaded areas, where moisture lingers on the plants, Late Blight symptoms may be more prevalent).
  • Inspectors should determine where the planting was initiated in the field. Varietal mixture and disease spread may be most apparent in this area because planting equipment may not have been properly cleaned. Special attention should be given to these areas.
  • Inspectors should stop periodically, look around and validate the variety of the crop, observe the general health and status of the crop and investigate any areas which give indications of pest or disease problems. Unless part of count, the disease levels noted in these areas are not to be incorporated into the count, but should be noted in the remarks section of the Inspector's Field Notes(CFIA/ACIA 1298)and the Report of Field Inspection (CFIA/ACIA 1284).
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