Potato varieties differ from one another in many respects and it is rare to find two that cannot be visually differentiated by some one characteristic whether it be vague or obvious. In general, identification of any one variety is made by knowing its basic features together with vague characteristics peculiar to each variety. The art of variety identification is not learned quickly because some of the characteristic features may be changed or modified by environment. Recent technology has provided means of verifying varieties by their genetic "fingerprints." Since many visual characteristics can be modified by environment, the new identification techniques are extremely important in specifically identifying varieties.

As with all vegetatively propagated crops, however, a potato variety is not static and slight mutations are of frequent occurrence in growing crops of all varieties. Often these mutations are usually to an inferior type of economic plant that produces either a reduced yield, a poorer tuber type, or both. It is therefore highly desirable for a seed grower not only to be able to recognize growing plants of the common varieties, but also to know the differences between desirable and undesirable types within a growing crop. Examples of the most obvious and most easily recognizable variations in any variety are the "giant hill" and the "wilding". The giant hill is taller, more vigorous, and is later maturing than normal plants. It also has smaller leaflets and more flowers. The wilding is a bushy low-growing plant with many weak stems, no flowers, few primary and secondary leaflets, large heart-shaped terminal leaflets and numerous small unmarketable tubers. Tubers from both types of these variations reproduce similar plants.

Not all mutations, however, are to an inferior type. There are a number of mutant selections that have been more popular than the variety they were produced from. The variety that accounts for the most acreage in the U.S. is 'Russet Burbank', a selection from 'Burbank'. Red-skinned sports or deep red skinned mutants such as 'Red Pontiac', 'Red LaSoda' and several selections of 'Dark Red Norland' are examples of mutations that have improved the salability and/or appearance of the original clone. In most cases, the mutants differ from the non-mutated clone in one trait such as skin color, lateness or vigor.

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Variety Identification

Characteristics used to identify individual varieties can, to varying degrees, be modified by environment. Even so, observations of foliage, flowers, tubers, and sprouts are helpful in proper identification. Known disease reaction characteristics can also help identify some varieties. Detailed characteristics are generally published in the American Potato Journal when a new variety is released.

Foliage

Foliage characteristics which assist in variety identification are growth habit, leaves, and stems.

GROWTH HABIT-The general appearance and vigor of the plants can be of great value for varietal identification. Unfortunately, these characteristics are the ones most affected by cultural practices and environmental conditions. However, varietal tendencies for plant height, shape, and vigor can be used in combination with specific information on foliage and flowers to help confirm identification.

LEAVES-A potato leaf is compound and made up of a petiole, a terminal leaflet, and two to four pairs of primary leaflets interspersed with secondary or interjected leaflets and occasionally tertiary or rudimentary leaflets along the midrib (Fig. 4). When pairs of primary leaflets are widely separated and there are a few secondary leaflets the leaf is

described as open (Fig. 5). When primary leaflets are close together and the secondary ones are numerous the leaf is described as closed. The relative denseness (open or close) of potato leaves is a valuable feature in varietal identification. The length of the leaf, the angle between the leaf and stem, and the leaf rigidity are other leaf features used in identification.
 

Figure 5. An illustration of closed and open potato leaves.

Number of primary and secondary leaflets and leaflet characteristics vary by variety. Leaflet size, shape, color, hairiness, smoothness, glossiness, flatness, and rigidity are identifying traits, as is leaflet stalk length.

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STEMS-The angles of the potato stem are extended to form structures called wings. The wings may be prominent or inconspicuous, straight or wavy, and may be so close together they appear as double wings (Fig. 6).

Stems may be uniformly green, reddish, or purple in color or may have varying degrees of mottling. Pigment may be localized at nodes or at certain internodes. Other distinguishing points are thickness, hardness, branching habit, internode length, and node size.

FIGURE 6. Potato stems showing types of wings: (a) straight, double: (b) waved. double; (c) slightly waved, double. Wing characteristics are not generally changed by differences in environment.

Flowers

Flowers and flower color are distinctive features of varieties but by themselves cannot be used to identify varieties. Colors range from white to purple and may be solid or a combination of colors. Anthers are prominent and vary in color. They may be orange, lemon yellow, or greenish yellow. Other floral characters that may help confirm varietal identity are size of flower, profusion of blooms, hairiness and color of buds, and the size, frequency, and persistence of buds. Flower stalks range in color from green to various shades of purple and may be mottled. Leaf-like appendages, called stipples, at the base of the peduncles of the lowest inflorescence vary in size and may be spreading or may entirely clasp the peduncles or petioles.

Tubers

Characteristics of potato tubers that can be used to confirm identification are skin color, skin texture, shape, eye distribution, and eye depth and specific gravity. These characteristics can be modified by environment and therefore cannot be used alone to identify varieties. Most varieties can be classified as whites, reds or russets. Less common skin colors include pink, purple, and combination of white and red or pink.

Sprouts

When tubers are exposed to diffuse light and warm temperatures for two to three weeks before planting, sprouts with color and shape characteristic of the variety are produced.

SHAPE-There are three distinct parts of a well-developed sprout: the base, the middle and the tip (Fig. 7). Each part may vary in size and shape according to variety. The tip may be long or short and may tend to unfold its leaflets at an early or late stage. Typical shapes are ovoid, globose or cylindrical.

COLOR-Sprout color may be uniform green or green with red or purple pigmentation. Color development can be localized to certain portions of the sprout. In some varieties color is not present in the base of the sprouts until after it is well defined in other parts of the sprout.

HAIRS-The amount of distribution of hairs on the sprouts help to further identify the sprout of a specific variety. Some are markedly hairy and others are essentially without hairs. The hairs may not be distributed over the whole sprout but according to variety may be concentrated on the tip, or the base, or the center.

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Laboratory "Fingerprinting"

Relatively new laboratory procedures in molecular biology have expanded options in identifying potato cultivars. The techniques are similar to the ones used in humans to identify the unique genetic make up of individuals. The standard procedure is referred to as restriction fragment length polymorphisms (RFLP),which are DNAbased markers. A variation to that approach is isozyme analysis. There are advantages and disadvantages to each analysis. However, the results of either are more conclusive than the standard morphological characteristics used to identify potato varieties. As stated previously, morphological characters can be affected by environment where as the marker based procedures are not. However, these analyses should not be used to the exclusion of conventional characters. The real value of laboratory results is in combination with morphological characters. In practice, the standard morphological characters used in certification can be supplemented with these additional discrete characters.

Of the two tests referred to, the RFLP analysis is more precise. However, it is also more time consuming, costly and requires greater expertise.

Isozymes, which are multiple molecular forms of an enzyme derived from a tissue of a plant, have recently been used to characterize varieties in a number of crops. They are usually separated when an electrical charge causes their migration through a starch gel (electrophoresis). They are visualized as discreet bands when the gel is placed in a solution of a proper chemical substrate for the enzyme and the end product of the reaction is stained. Since these enzymes are coded by genes, starch electrophoresis has the advantage of revealing gene activity much more directly than do morphological traits.

Isozymes usually permit unequivocal identification of nearly all genetic classes of a gene. This is accomplished by examination of isozyme banding patterns, whether for alleles at one or many genetic loci. The electrophoretic method is rapid, economical, accurate, and the analysis can be made for nearly all loci in early growth stages of the plant or from the tuber.

Isozymes examined at Michigan State University did not separate sports or line selections of a variety. Duplicated fingerprints were observed for varieties listed in Table 10. In general, duplicate fingerprints were found between line selections or sports of varieties of the same name (i.e. 'Burbank' vs. 'Russet Burbank' or 'Norland' vs. Dark Red Norland').

Disease Resistance

Specific reactions to certain plant pathogens are the basis for another method of varietal confirmation. Tests for what is termed a "hypersensitive reaction" to races of the late blight organism and viruses X, A, and Y (strain C) and perhaps an immune reaction of virus X, can be confirmatory features for some varieties.

Table 10. Varieties with duplicate electrophoretic fingerprints.

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North American Varieties From Which to Choose

A relatively large number of varieties of potatoes is grown in the early, intermediate, and late-crop sections of the United States and the various provinces of Canada. They differ in time of maturity, yield, appearance, cooking and marketing qualities, and resistance to various diseases and insects. All other characteristics being equal, a variety resistant to even one disease or insect is better for planting than a susceptible one.

A variety that is good in one geographical area, however, may be of little value in another. Growers should be warned against buying large quantities of high-priced seed stock of a recently introduced variety or one reported superior in other localities, until it is determined that the variety is adapted to local environmental conditions. The needed varietal information can be obtained from agricultural advisors or small scale grower trials.

The most important varieties and those released in the past ten years are listed in Table 11. The ten most popular varieties in North America are listed in Table 12. Descriptions of the leading 10 varieties grown in North America follows:

'Russet Burbank'

Late. Primarily grown in the Northwestern states, but production is increasing in the Midwest and Maine. Much of this increased production of 'Russet Burbank' is due to its favorable marketing and processing qualities. Tubers are large, long, cylindrical or slightly flattened; skin is russeted, eyes numerous and shallow; flesh is white. Flowers are white and few in number. The variety has some scab resistance but is susceptible to most other potato diseases, especially leaf roll, net necrosis, and verticillium wilt. Storage, cooking and processing qualities are very good.

'Superior'

Early. This variety has wide adaptation. Tubers are medium sized, relatively uniform, round to oblong and slightly flattened; skin is buff and slightly checked; eyes are moderately deep and the same color as skin, flesh is white. Flowers are lavender with white tips. The variety is resistant to scab and net necrosis. It is suitable for fresh market and early chip processing.

Table 12. Ten major potato varieties grown for seed in North America in 1989.

Source: PAA Certification Section and Agriculture Canada

'Kennebec'

Main season. 'Kennebec' has wide adaptation. Tubers are large elliptical to oblong, medium thick; eyes are shallow and skin is smooth, creamy buff: flesh is white. Flowers are white. It is resistant to mild mosaic, and net necrosis and is moderately resistant to late blight but is susceptible to verticillium wilt. 'Kennebec' is considered a general-purpose potato that bakes, boils, and fries satisfactorily. It is grown for French frying or for chipping.

'Norchip'

Medium early. Tubers are medium sized, mostly round, medium thick: skin is smooth, white: lateral eyes are shallow, apical eyes are moderately deep, eyes are the same color as skin: flesh is white. Flowers are white. Possesses some resistance to common scab. Generally produces light-colored chips. Sprouts early in storage.

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'Atlantic'

Main season. Tubers are round in shape and cross section; skin is buff colored and has a scaly net; lateral eyes are shallow, apical eyes are moderately deep; stem end is slightly depressed; flesh is white. Flowers are pale lavender. Tuber specific gravity is high. Atlantic is immune to PVX and net necrosis, resistant to golden nematode but is susceptible to internal necrosis and hollow heart. The variety is well suited for the potato chip market.

'Russet Norkotah'

Early. Tubers are long, uniform in shape, slightly flattened: skin has a medium to heavy russet: eyes are shallow and slightly lighter in color than the skin; nesh is white specific gravity is generally low. Flowers are white. The variety is resistant to common scab and second growth but is susceptible to most other major potato diseases. Grown for tablestock.

'Shepody'

Main season. Tubers bulk early, are oblong to long and slightly Battened; skin is white, relatively thin and smooth; eyes are moderately shallow; flesh is white. Flowers are violet with white tips. The variety is moderately resistant to net necrosis but is susceptible to comman scab and most other potato diseases. Generally grown for early season French fry processing.

'Norland'

Early. Tubers are smooth, medium sized, mostly round; skin is smooth, medium red in color; eyes are relatively shallow and same color as skin; flesh is white. Flowers are purple. Norland is resistant to common scab, but is susceptible to air pollution injury. Tubers sprout early. The variety produces a high percentage of U.S. No. 1 tubers. It is grown primarily for the fresh market. There are several sports of 'Norland' with deeper red skin color.

'Sebago'

Late. Sebago has wide adaptation with pockets of proauction located in Florida and the Southern states. Tubers are large, elliptical to oblong, medium thick; skin is smooth, white; eyes are shallow and same color as skin; flesh is white. Flowers are lilac with slightly lighter tips. The variety is resistant to mild mosaic and net necrosis, and reported to have yellow dwarf resistance in New York and Wisconsin and brown rot resistance in Florida. It possesses a moderate degree of field resistance to late blight and some resistance to common scab but is susceptible to blackleg and has a tendency toward enlarged lenticels. Sebago is generally used as a fresh market variety but some of the southern crop is processed into potato chips.

'Centennial Russet'

Early. Tubers are oblong, slightly Battened; skin is a dark, heavy russet eyes are shallow: flesh is white. Flowers are lavender with white tips. This variety is susceptible to common scab and most other diseases. Grown primarily as a tablestock variety.

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