Malformed tubers may take different forms. The tuber may have been exposed to adverse environmental conditions, or have a physiological defect, resulting in a knobby (36), pointed (37), curved (38), creased (39), or dumbbell shaped (40) tuber. In general, irregularities in soil moisture and nutrients are the common environmental factors responsible for malformed tubers. Growth cracks (41, 42) in the tuber 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 extend lengthwise. The cracks vary in size but usually heal over with no rot following the injury. Herbicide damage (43, 44) may result in malformed tubers, depending on the developmental stage of the plants at the time of exposure. Herbicides residues may be remaining in the soil, or drift from adjacent fields.
Tubers are to be scored as malformed when they are significantly different in shape from a typical tuber representative of the variety. Growth cracks are scoreable when they materially affect the bud end of the tuber (affect eyes and are not shallow) or extend more than 2/3 the length of the tuber in the aggregate.
8b: Insect Damage
Some of the more common insects which may feed on, or cause scoreable damage to tubers are listed below.
Wireworm (click beetle) larvae (45, 46, 47), Agriotes obscurus, A. lineatus and A. sputator have overlapping life cycles, so that tuber damage can occur at any stage during tuber development. Early season injury occurs when the tubers are small and the larva causes deep, funnel-shaped cavities which form as the tubers mature. Mid-season injury results in the formation of pits 6 to 12 mm deep and 1.5 to 3 mm in diameter, sometimes lined with discolored scar tissue. Late season injury consists of clean cut round holes and scar tissue is not prominent.
Flea beetle, Epitrix tuberis, may cause surface injury, internal injury or both. Surface injury consists of pimple-like eruptions or rough, winding galleries (trails) approximately 3 mm wide and of varying length. Internal injury consists of single, or groups of narrow, brown slivers or feeding tunnels that extend into the tuber between 5 to 20 mm. Cracks may occur where these tunnels come together.
Grub damage (48, 49) is caused by the larvae of June beetles or cutworms (Polyphylla spp.). The earlier that feeding damage occurs during tuber development, the larger the affected area will be as the tuber develops. Damage consists of small to large feeding cavities between 5 to 20 mm or more in diameter. The irregular-shaped cavities usually are wider than they are deep, with characteristically rough ridges inside them. Half or more of the tuber can sometimes be consumed.
Insect feeding damage is scored under the Malformed and Damage tolerance when either:
feeding damage exceeds an aggregate area of more than 10% of the surface; or
Two or more eyes of the tuber are affected
Live insects and visible feeding damage should be noted in the comments section of the inspection report.
8c: Mechanical and Physiological Damage
50 | Digger cuts on tubers are a result of mechanical injuries, cuts may be deep into the tuber, may be lengthwise or crosswise slab cuts exposing large areas.
51 | Shattering bruises is a term 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
Mechanical damage is scored under the Malformed and Damage tolerance when;
the damage is not well healed and exceeds an aggregate area of more than 10% of the surface; or
the damage affects two or more eyes of the tuber
52 | Air cracks may occur during harvest or packing, and after packing if the packages are too tight or handled roughly. They 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 the Malformed and Damage tolerance when:
they exceed an aggregate area of more than 10% of the surface; or
the surface cracks affect two or more eyes of the tuber
53 - Sunburn is caused when tubers are exposed to the sun during the growing season and 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; this condition may be accompanied by yellow or cream-colored 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.
Sunburn is scored under the Malformed and Damage tolerance when the green colouration causes damage to two or more eyes of the tuber are damaged and tuber vigour may be affected.
54 | 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 causing bulges and cracks in the tubers. Sometimes small new tubers develop inside the mother tuber.
Ingrown sprouts are scored under the Malformed and Damage tolerance when two or more eyes of the tuber are affected
55 | Grass Root Injury is caused by quack grass (Elymus repens) or other similar grass species. The sharp growing points of the rhizomes penetrate into or completely through the tubers. There may be evidence of the rhizome attached to the potato.
Grass root injury is scored under the malformed and damage tolerance when two or more eyes of the tuber are affected regardless of the depth.
56, 57, 58 | 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 it may result in larger necrotic areas forming over the tuber surface. These necrotic areas can be picked out, leaving circular pits of healthy flesh.
Skin spot is scored under the Malformed and Damage tolerance when:
Purplish-black, slightly raised areas which do not penetrate the flesh, affect more than 10% of the surface area in the aggregate; however, if skin spot penetrates the flesh it may be scored as dry rot as appropriate.
8d: Internal Necrosis (Viral)
Internal Necrosis (Viral) - Causal agents:
Tuber necrosis may be attributed to viral infections by Potato Mop Top Virus (PMTV), Tobacco Rattle Virus (TRV), or Potato Virus Y (PVY)N/NTN. Tuber necrosis may also be referred to as spraing depending on the viral agent (TRV, PMTV). Certain potato viruses which cause tuber necrosis are of particular significance to the Canadian seed potato industry.
Internal Necrosis (Viral) - Symptoms:
Generally, affected tubers show rings, semi-circles or spots on the surface, with brown necrotic lines, arcs and rings in the flesh. Tuber symptoms are, on occasion, limited to internal symptoms only. The tuber skin over some of the brown, sunken rings will often crack. The affected tissues are moderately firm and corky. Some mild infections may exhibit symptoms similar to Internal Brown Spot.
Tobacco Rattle Virus (TRV) (59, 60, 61, 62, 63) is the type-member of the genus Tobravirus, and is the causal agent of the disease commonly referred to as corky ring-spot.
Potato Mop Top Virus (PMTV) (64, 65, 66) is a member of the genus Furovirus and vectored by the powdery scab fungus. PMTV may be transmitted to new sites by planting of virus infected seed with powdery scab lesions
8d: Internal Necrosis (necrotic strains of PVY)
Potato virus Y (PVY) is the type-species of the Potyvirus genus, often occurring as a complex of virus strains. Recognized strains include, but are not limited to: PVYO, PVYN, PVYNTN (may include several distinct isolates/groups) and PVYN:O. Visible tuber symptoms are typically caused by PVYNTN (67, 68, 69, 70, 71).
Internal necrosis caused by TRV, PMTV, and necrotic strains of PVY are scored separately under Internal Necrosis, and must be added to other malformed and damage counts for a combined total under the Malformed and Damage tolerance.
8e: Internal Discoloration
Internal discolouration caused by physiological factors including frost, heat or drought:
Internal discolouration caused by physiological factors such as drought or temperature extremes is scored under Damage when the tuber vigour, storage or seed germination may be affected. If the discolouration penetrates greater than 13 mm into the flesh of the tuber or on a tuber cut to expose the largest surface area, if more than 10% of the exposed flesh area is affected by internal discolouration, it is scoreable. If the discolouration penetrates less than 13 mm at the stem end then the discoloration may be scoreable as stem end discoloration. There can be secondary factors present, such as wet or dry rot, and the tubers should not be scored under internal discoloration, but rather it should be scored under the disease, defect or condition affecting the tubers. Mild symptoms of blackheart are scored under the Malformed and Damage tolerance, however if associated with decay, score under soft rot or dry rot as appropriate.