POTATO
Solanum tuberosum L.
 

 


(photo provided by California Agriculture, University of California)

Family Characteristics | Crop History and Development | Plant Characteristics | Propagation Methods
Cultural Practices | Insects | Diseases | Harvesting | Post Harvest | Marketing

FAMILY CHARACTERISTICS
    The Solanaceae family, with about 85 genera and 2,800 species all over the world, includes herbs, shrubs, trees, or vines. The plants in this family are dicotyledons and are of considerable importance for food, drugs, weeds, and poisonous plants.
    Other family members include:
        Capsicum spp.; Chilies, Pepper
        Lycopersicon esculentum Mill.; Tomato
        Physalis spp.; Husk Tomato, Cape Gooseberry
        Solanum melongena L.; Eggplant, Brinjal
        Solanum muricatum Ait.; Pepino
        Solanum tuberosum L.: Potato

CROP HISTORY AND DEVELOPMENT
        The potato originated in the Andean regions of Peru and Bolivia and was utilized by the Incas about 2,000 years before the arrival of Spanish explorers. Carbon 14 dating of starch grains found in archaeological excavations indicated potatoes were used at least 8,000 years ago.
    The name "potato" is believed to be derived from the Inca name "papa". The association with Ireland is thought to be responsible for the name "Irish potato", which is retained even though potatoes are grown almost all over the world. "White potato" is the most popular name used today. Although some cultivars are white fleshed and have white skins, that name does not account for the internal and external color variations that occur. Nevertheless, although neither white nor Irish is accurate, that association persists.
    The potato was introduced into Spain from South America about 1570. From Spain, the potato was taken into neighboring European countries and in less than 100 years was being grown fairly extensively in many regions of Europe. Distribution beyond Europe soon occurred with the introduction into India about 1610, China in 1700, and Japan in 1766. Scotch-Irish immigrants introduced the potato into North America in the early 1700s.  When first introduced into Europe, the potato was regarded as poisonous because of its foliar resemblance to nightshades (Solanum species). Acceptance was also poor due to low productivity. Andean introductions (Solanum tuberosum subsp. andigean) obtained from low latitude regions performed poorly because they were not adaptable to European temperate latitudes, although in Southern European regions, productivity was better. The Chilean (Solanum tuberosum subsp. tuberosum) sources were not present until the 19th century. At the beginning of the industrial age, the crop became a subsistence staple for the peasant population. Its value as a human food soon was recognized, along with the potential to produce more calories at a lower cost than grain crops. Therefore, potatoes were increasingly grown to meet the food needs of the expending European population.
    The increased dependency on this food source resulted in an extension of production areas, a development that contributed to the severity of the potato crop failure and resulting Irish famine during 1845 to 1846. Many years of extensive cultivation, especially in Ireland, with limited crop rotation and increased land area in potato production made the potato crop highly vulnerable to diseases, such as blight fungus (Phytophthora infestans). About one million people died of starvation in Ireland during this period. Because of this famine, massive migrations of the population occurred, as well as considerable economic disruption in Ireland and other European countries. An effect of the crop failure was the introduction during the 19th century of better adapted Chilean potato types replacing the initial Andean sources. This formed the genetic base which is now referred to as S. tuberosum subsp. tuberosum.
 

Potato varieties
(photos provided by California Agriculture, Univeristy of California)

PLANT CHARACTERISTICS
   Overview Solanum tuberosum L. is an herbaceous perennial cultivated as an annual, and is susceptible to frost and freezing. In production, it is a cool season crop, with optimal average temperature between 50° to 65°F. In most regions, yields are best when the plant date falls shortly after last frost. The commercially significant portion of the plant is the tuber, which is a swollen underground stem. The swelling of the tuber is due to the translocation and storage of photosynthates (carbohydrates) which occurs as the aerial portion of the plant reaches maturity. Lodging of the aerial plant signifies the filling of the tubers and the readiness for harvest. Tubers are used in commercial propagation since the true seed are heterozygous and highly variable, used primarily in crop improvement.

Schematic of a potato plant
USDA Agricultural Handbook 267
   Root System Potatoes produce a fibrous root system arising from initials along the underground portion of the stem. These roots can extend to a depth of 22 inches, but are not highly effective at penetrating the soil layers in search of water.

Root system
    Tuber Morphology  The potato tuber is an enlarged portion of an underground branch of a stem called a stolon or rhizome.
 
USDA Agricultural Handbook 267


     Approximately 30-60 days after the "seed" is planted, tuber formation begins with a swelling at the lowest nodes of axillary stolons in the region immediately distal of the hook end of individual stolons. The absence of light, and favorable temperature and moisture conditions influence tuberization. It is thought that initiation occurs in response to shortened day length and/or cool night temperatures when tuber forming substances (kinins) are produced more readily. Another factor of tuberization is the critical starch concentration sensed by the stolon. Overall, tuber growth and development is dependent on the presence of sufficient foliage to produce the necessary assimilates and adequate supplies of water and mineral nutrients. If initiation occurs before there is sufficient foliage, a 'Little Potato' disorder occurs. 
Once the tubers are initiated, the growth of all the other organs is retarded and the tubers become the dominant meristems and sinks for organic nutrients. A presence of gibberillic acid (GA) will inhibit tuber growth after initiation by inhibiting starch deposition. Growth retardants can hasten tuber formation, and may prevent the synthesis or action of GA. Tuber initiation may be delay by early irrigation or application of nitrogen fertilizers; however, the eventual rate of tuber growth and its duration are increased because of the increased aerial plant mass and persistence. Those tubers attaining the greatest weight are usually produced by the lowest stolons.
    The epidermis of the swelling stolon begins to thicken as a cortical layer; the thickness thereof serves as an indicator of the capacity of the tuber to store starch, thereby providing a measure of quality. This epidermis is covered with lenticels which appear as small dots. In dry conditions a suberized layer forms below the complementary cells of the lenticel, while under conditions of increasing soil moisture and water content of the tuber, a swelling of the cortical cells occurs and eventually the suberized layer ruptures. This results in the opening of the lenticels (these are often the entry sites for pathogens). Interior to the epidermis is the periderm (the skin), next the cortex which encompasses the parenchyma, phloem, xylem, and pith. New tissue arises within the phloem layers, and act as storage cells.
    In response to the wounding of the tuber, a wound periderm (suberin) forms within one day of the wounding. Formation of the corky periderm layer uses starch from the area surrounding the wound, thereby lowering the tuber's starch content. This creates a resistance to some bacterial and fungal diseases in the periderm tissue.
    Each tissue layer is connected to each eye. Eyes appear in higher concentration nearest the "rose end" of the tuber, decreasing in size and concentration towards the "heel end" where a stolon scar is visible. The eye of the tuber is a leaf scar with a subtended lateral bud, in which there are at least three buds arranged in the form of an obtuse triangle (this is significant in the shoot initiation of a "seed" tuber). During the growth of the tuber, the eyes remain dormant.
    Aerial Plant The potato plant has a short life span ranging from 80 to 150 days from planting to maturity, with differences existing between varieties.
    Stem When grown from "seed pieces, " several shoots arise from one seed piece. This occurrence may cause physiological disorders in the developing tubers, such as tuber greening and growth disorders if the stem density is too high. The growth of the stem is erect in early stages, reaching 2-5 feet in height. Density of stems also influences the stem height, with an increasing height as the density increases, and with this there is much decrease in the axillary branching, which decreases the photosynthesis potential. As the plant matures, the stem weakens and lies prostrate, eventually yellowing and dying back at the end of the growing season.
    Leaf Leaving patterns are pinnately compound, alternate, with 7-9 ovate leaflets (one terminal leaf). The margins are serrated or entire. Often many smaller secondary or tertiary leaflets are found growing between the primary leaves. The rate of formation of these leaves is indicative of the potential yielding ability of a cultivar.


    Flower/Inflorescence The potato inflorescence is a broad, flat topped cyme. The primary inflorescence is followed by a second and third order blooming as the previous dies (staggered blooming dates). This can be another means by which to estimate the maturity of the plant. Individual flowers are complete, with calyx, corolla, stamens & pistil. Flower color can range from creamy white to yellow, pink, purple, or striped depending upon the cultivar. Bumblebees are the primary means of cross pollination; self-pollination most often occurs. A significant amount of potato cultivars are either pollen sterile or fail to set fruit because of some other means.
    Fruit/ Seed If established at all, fruits are small (to 1 1/4" diameter) and green, resembling a small tomato. The fruit is a berry with seeds in a mucilagenous pulp. Seeds are flattened and ovate, with up to 300 seeds per fruit. Sexual reproduction is the primary mechanism for crop improvement, outside of this, the fruit and seed are of little value.

PROPAGATION METHODS
     Overview The primary method of commercial propagation is through seed tubers due to the high variability of true seed. True seed is becoming important throughout the tropical world where disease pressures are too high to maintain healthy seed tubers. Vegetative propagation by seed tubers enable a uniform crop. Seed certification regulations have been established to ensure the quality of seed, inspecting the tubers for pest and pathogen, certifying only those which are essentially pest and pathogen free. Specialized growers produce seed stock for commercial growers.
     "Seed Pieces" Vegetative reproduction ensures the integrity of cultivars, essentially planting clones. Growers of seed tubers can either sell whole tubers or precut the tubers. The standard seed piece is 2" x 2" or 2 oz. This size has been found to have the adequate amount of carbohydrate levels for shoot initiation and growth. After the seed pieces are cut, they are allowed to suberize, or cure, for 7 to 10 days at temperatures between 55° and 68°F, and a high relative humidity. If temperatures are too high or too low, seed pieces will not suberize properly, opening the door to decay or an erratic stand in the field. Suberization of the seed pieces allows for a corky protective layer to form around the seed piece. This prevents decay and decreases pathogen and pest penetration. Seed pieces are also treated with a chemical to reduce wireworm and soil pathogen activity.
 Cut potato to form "Seed pieces"
    Temperature of seed storage is crucial in modifying the apical dominance of a seed piece. At a temperature of 59°F apical dominance is complete and at 50°F, several buds will develop simultaneously. A temperature of 34-41°F (over a period of several months) and then placement in a high temperature atmosphere will ensure that all the eye will sprout.
    "Seed Certification" Regulation over seed quality controls the spread of disease and pests, as well as ensuring the potatoes are grown on sited appropriate for their requirements.
    "True Seed" Cross-pollination is manipulated for crop improvement in quality and insect or disease resistance. However, many potato cultivars are pollen sterile, which creates difficulty in breeding programs. The initial crop from such cross results in tubers of less than commercial size.
    Tissue Culture This is currently being used to perpetuate disease free seed stock, which can then be stored "in vitro" until needed. Genetic engineering is also being performed using agrobacterium to produce a more resistant strain.

CULTURAL PRACTICES
    Overview Potatoes are best adapted to the more northern temperate regions (or highlands) where the soil is loose so as to provide conditions for tuber swell (loose, friable, clod-free), and the temperature (and to an extent, the photoperiod) are in accord and most favorable to the plant's requirements. Plant density, as well as fertility and moisture availability, greatly determine the productivity of the crop.
    Soil Type Potatoes, as with other root crops, grow best in loose, friable soil for the purpose of preventing abnormal growth. Light, sandy soils are very suitable when crop is to be mechanically harvested; however, such soils require appropriate management in terms of irrigation and fertility to produce satisfactory yields. Well drained mineral or organic soils with medium loam, or light of medium silty textures produce well because of their fertility. Soils which have poor drainage should be tilled to prevent soil saturation and allow for aeration. Potatoes tolerate a wide range of pH, from 5.5 to 7.5 if managed according to the soil type and watering requirements.
    Bed Preparation Cultivation of the seedbed to a depth of 12 inches or greater is important to ensure good drainage and adequate soil aeration for tuber growth. When using mechanized planting and harvesting attempts should be made to avoid clod formation and include clod removal. Mechanical implements with straight lines are preferred over those with curved, to avoid bringing unweathered soil to surface which could increase clod formation.
    Throughout the growing season, the beds should be cultivated, creating a hill (mound) to cover the developing tubers, preventing greening and providing an environment conducive to growth.
    Plant Spacing/Density Seed costs amount to roughly 30-50% of total growing costs, making appropriate spacing of the seeds an essential factor in economic productivity, as it is also that of yield productivity. Plant density is manipulated through the number and size of seed tubers. Increasing the seed density will decrease tuber size. Spacing of seed is also determined by the type of equipment used for harvest and cultivation. Two to three feet between the rows has been recommended to allow for faster rates of harvesting with slightly lower levels of tuber damage. Traditionally, spacing is 8 to 12 inches within the rows and the same between rows.
    In the planting of seed tubers timing and care have marked effects on the crop yield. The chronological age of the seed tuber significantly determines the yielding potential, by affecting the growth and development of the shoot. Best yields are achieved when the seed is 4-5 months. The damaging of sprouts or eyes pre- or during planting delays the emergence and may reduce yields. Late planted seed crops produce tubers which break dormancy later and sprout more slowly, but it has been found that they may give a higher mature yield than seed from crops which are planted earlier in the season.
    Temperature Potatoes are affected by differences in temperatures. Depending upon cultivar, they require a growing season (frost free days) from 90 to over 120 days . Tuberization occurs earlier at lower temperatures, approximately 3 to 5 weeks earlier than those in longer, warmer days. The optimal temperature for tuberization is 55°F, the process decreases above 70°F and with certain cultivars, may stop at 85°F. Higher temperatures may often induce knobbiness and secondary growth in tubers. Maximum yields may be obtained with an average growing temperature between 60° and 65°F . These cooler temperatures cause the rate of respiration to be lower than the rate of photosynthesis, resulting in more accumulation of carbohydrates.
    Photoperiod  A majority of potato cultivars are not day length sensitive. However, as it relates to photosynthesis and respiration, various plant functions are influenced. Long bright days favor photosynthesis and development of top growth. Tuber dry weight increases with an increase in radiation interception by leaves and thereby translocation of photosynthates to the tubers is increased.
    Fertility  Potatoes are comparatively heavy nutrient users. The application of fertilizers ought to coincide with various stages of growth. Through the early stages of growth, potato fields require high amounts of phosphorous because of environmental factors which limit the plant's uptake ability. At seed planting fertilizer may be broadcast and plowed in, or banded 2 inches on either side and below the seed placement. Nitrogen is significant in increasing the number of stolons, in the growth of foliage, and increasing the tuber size; excessive nitrogen may delay plant maturity. During the early growing period (preferably late June) when the plant is roughly 6 inches tall, a foliar analysis should be taken of the most fully developed leaves. At this point, N levels must be at their highest: 4-6%, P at 0.20-0.50%, and K at 4-11.5%. The other key time for foliar analysis is during the final tuber filling stage when the tuber is roughly half its potential size. The sufficient amounts of macronutrients at this time are 3-4% N, 0.25-0.4% P, and 6-8% K. In the application of fertilizer, the form of each nutrient plays a critical role. Potassium sulfate has been found to cause less of a problem with tuber quality than potassium chloride, and liming may induce K deficiencies as Ca+ and K+ compete for root uptake. Micronutrient deficiencies may occur in conjunction with low soil P and K concentrations.
    Fertility programs are best when managed through foliar and soil analysis, which also aid in irrigation requirements, methods, and scheduling.
    Nutrient deficiencies (photos provided by APS)

Nitrogen deficiency -
uniform light green leaves

Nitrogen deficiency -
upward cupping of leaf blades

Severe copper deficiency
upward curling of leaves

Potassium deficiency -
marginal leaf scorch

Sulfur deficiency - light
green younger leaves

Molybdenum deficiency

Phosphorus deficiency - dark green
color and stunted growth

Phosphorus deficiency - 
dark green color
and mild leaf roll

Calcium deficiency -
chlorosis and brown spotting,
blades cup upward

Calcium deficiency

 


Boron deficiency - bushy, droopy
leaves, crinkled, upward cupping
blades bordered by light brown dry tissue

Boron deficiency

 


Iron deficiency - young leaves
become yellow to white, usually
without necrosis

Iron deficiency -
yellowing and green
veining

Zinc deficiency - young 
leaves develop chlorosis
and form narrow, cupped leaf 
blades with tip burn

Zinc deficiency



 


Magnesium deficiency
slight chlorosis with green
veining and brown spotting.

Magnesium deficiency
green veining and
interveinal browning
   

    Irrigation Potatoes have a high water requirement, roughly 1 inch per week. In many of the production areas, this is met through rain. Irrigation before tuber initiation may adversely affect the yield and marketability. A consistent soil moisture of 60-70% of field capacity is crucial especially through tuber development. Fluctuation in soil moisture causes abnormalities in tuber formation. Standing water, often caused by irregular irrigation, can be avoided by leveling the field before planting. Standing water incidents can create environments fostering pathogen activity. Common irrigation methods are furrow irrigation and overhead, center pivot irrigation.
    Maximum benefit from irrigation is only achieved where factors such as nutrient supply and plant population are not limiting yield.
    Weed Control Weeds tend to be a large problem in potato fields, requiring a combination of cultivation and herbicides to best control. Cultivation, or hilling, of the soil covers weeds and seeds. The primary time for weed control is during the 7 to 12 weeks between planting and canopy cover. Once the potato canopy has closed, annual weeds are effectively suppressed. A variety of herbicides are approved for use on potato.
    Crop Rotation  Planned crop rotations with grass or pulse crops helps to keep the soil fertile; maintain a loose, friable tilth; check weeds; build up organic matter; and reduce future crop loss from insect damage and disease. The length of time in rotation can be 3 to 5 years. Shorter rotations of 1 year are in practice, using fall rye which is disked in followed by a green manure crop (legume).

INSECTS
    Overview  Both soil and foliar insects are pests of potatoes. They either cause primary damage of the plant through defoliation or root damage, act as vector of viruses, or make the plant susceptible to the entry of pathogens. The control of insects can be performed by regular application of pesticides or through biological controls such as trap plants or antagonistic (micro-)organisms.
    Primary The most serious insect pest of potatoes in many production areas is the Colorado Potato Beetle (Leptinotarsa decemlineata). Both the bright red, black spotted larvae and the yellow striped beetles cause severe defoliation. The adult beetles lay eggs in clusters on the underside of leaves. The eggs then hatch in ~7 days producing reddish larvae. Chemical control is most effective just after egg hatch and least effective on adult beetles.
    Potato Leafhopper (Empoasca fabae) are small and narrow, about 1/4 inch long, and green to yellow in color. They cause discoloration of the toliage--bronzing of the edges, known as hopperburn-- and sharply defined whitish speckling. Leafhoppers are known to be a vector of virus and mycoplasm diseases.
    Potato Aphids, predominately the Green Peach Aphid (Myzus persicae) and the Potato Aphid (Macrosiphum solanifolii), are soft bodied insects, 1/8-1/4 inch long, green or flesh colored, and with or without wings. In high populations, aphids cause significant loss of plant sap. They also cause rolling of upper leaves ('False Top Roll') or general yellowing of leaves. Viruses can be transmitted by aphids both in the field and in tuber storage. Populations of aphids are monitored with yellow colored traps, and may be controlled with contact materials such as endosulfan and parathion.
    The Tomato Hornworm (Protoparce quinquemaculata) is a Solanaceae family fiend. These moth larvae may reach a size of 4 inches long. They are green with diagonal white stripes on abdominal segments, and are characterized by a single horn on their tail. Tomato hornworms are voracious feeders, but can be effectively controlled because of their sensitivity to a variety of insecticides.
    Pests such as the Potato Flea Beetle (Apitrix spp.) damage the tuber and the foliage. The potato flea beetle feeds in and on tubers, creating networks of tunnels. Damage to the foliage consists of numerous small, circular holes of 1/10 inch in diameter. The leaves may dry and die. Cleanliness and elimination of weeds in and around fields reduces flea beetle food and shelter as well as over wintering sites. Chemical sprays, such as foliar organophosphate sprays can be effective when used while emerging adult cause injury and before they lay eggs. Other chemical controls include carbamates which are applied to the soils as granular insecticides.
    Nematodes have been known to cause significant damage to tubers and root systems. The potato cyst nematode (Globodera pillida and G. rostochiensis) causes a proliferation of fine roots and later formation of white, yellow, or brown cysts upon the roots. Root knot nematodes (Meloidogyne spp.) infect roots and tubers, and are apparent in variable sized "knots" or galls. The infection of the root system causes aerial plant symptoms including stunting of growth, and fewer small, pale green leaves that tend to wilt in warm weather. Soil chemicals can control nematodes, as well as the use of resistant varieties, or the use of various biological controls.
    Other The potato has many insect pests including the Vegetable Weevil, Slugs, Earwigs, Leatherjackets, Bibionid Fly Larvae, Symphlids, Millepedes, Woodlice, Leafminers, Ants, and Nematodes: Needle, Stubby Root, Potato Tuber, Stem, and Root Lesion.
    Chemical Control In the use of chemicals, it is important to use a wide range of pesticide groups in a control program to retard the development of tolerance in the insect population. A wide scale of pesticides are available for the use on potato crops.
    Biological Control Aphids can be controlled with the use of entomophagous fungi. Specific nematodes may be parasitized by fungi: Globodera sp. can be hindered by the parasitization of Catenaria sp. Nematodes (especially Meloidogyne sp.) may also be controlled through crop rotation with cereal crops or fallow periods.

DISEASES
    Overview Approximately 19% of crop loss is due to disease. Cultural practices are the primary and often most effective way to control disease infestation. These practices include, but are not limited to:
    Using certified seed
    Good rotational practices
    Use of fungicides
    Removal of crop residues
    Good husbandry practices
    Control of insects/vectors
    Use of biological controls--antagonistic (micro)organisms
 
    Primary  Early Blight (Alternaria solani fungal) first develops around blossom time. The primary infection occurs on older foliage early in the season. The inoculum then spreads to immature surfaces, such as young tubers. Early maturing varieties are more susceptible, and may show sever defoliation. Predominate symptoms are brown, angular, necrotic spots; lesions appear first on lower leaves. If managed correctly, plants may grow out of the disease.
Early blight (photo provided by
plantpath.ifas.ufl.edu)
    Late Blight (Phytophthora infestans-fungal) is the single most important disease of potatoes. It occurs under cool, moist conditions. It appears as water soaked lesions on foliage, which turns brown-black within a few days. Lesions occur on leaf, petiole and stem. In damp conditions, white mildew-like sporulation is visible on the lower side of leaves. Tubers may be infected as spores are washed down through the soil. Surface browning occurs throughout the periphery of tuber. Tuber infection may be prevented through hilling, thorough spraying of foliage with fungicide, and permitting vines to die naturally or be killed before potato harvest. Control of late blight is best through the roguing of infected crop, treatment of field with fungicide at key times throughout growing season, use of resistant varieties, and ensuring 

Late blight (photo provided by
plantpath.ifas.ufl.edu)
production field is a distance from other host crops - tomatoes or other potatoes. Most potato production areas have late blight warning systems which broadcast the coming of a period of high susceptibility. These are times when fungicides must be applied (generally at a dry time with low wind before a coming low front or occlusion zone.
   Fusarium Wilt and Tuber Rot (Fusarium spp. - fungal) does severe damage to young sprouts causing decay of the sprouts themselves and/or the seed tuber. By delaying planting until the soil becomes warm and by planting shallowly with sprouted seed tubers, there is a decreased risk of seed tuber or sprout decay. This may also prevent damage by Rhizoctonia solani on the stems.
    Mosaic viruses infecting other members of the Solanaceae family are threats to Potato production. Potato Viruses X, S, M, Y, and A, decrease yields by creating a mosaic chlorosis or mottling of the leaves, thereby limiting photosynthesis. Control of such viruses include roguing, seed selection, and insect vector
    Chemical Control A series of chemicals are available as either fungicides or bactericides. As with all chemicals, the extensiveness of the organisms controlled by each chemical should be understood, as side effects might occur.
    Biological Control Various diseases can be controlled biologically.  The agent for the Common Scab (Streptomyces scabies) can be controlled with a suppressive strain of another Streptomyces species when it is applied to the soil.  Other specific fungi may be parasitized or inhibited by bacteria or nematodes: species of Bacillus, Enterobacter, and Pseudomonas may  control Phytophora sp; Rhizoctonia and Fusarium are parasitized by the mycophagous nematode Aphelenchus avenae.
   Trap crops (planted in rows) can reduce amount of disease inoculum by providing alternative food source for aphids and other insect vectors.  Trap crops also act as a decoy during the virulent phase; when the aphids do move into the crops, the virus no longer persists in the insects.
Common scab

HARVESTING
    Potato tubers are harvested from 90 to 160 days after planting and this may vary with cultivars, production area, and marketing conditions. High yields are usually obtained with late maturing cultivars and from long growing periods. Occasionally, harvesting become necessary before foliage senescence or frost kill occurs and tubers are not fully developed. Existing foliage can interfere with harvest, especially when machinery is used. To reduce plant interference with harvest equipment, the tops are destroyed a week or two before harvest by mechanical shredding or with a desiccant. Foliage destruction tends to firm the periderm tissue of immature tubers, thus improving resistance to possible injury during harvest.
    Harvest practices vary from simple hand digging and placement into small container, to the use of highly automated equipment that separates potatoes from the soil and rapidly transfers large volumes into bulk containers or wagons. Mechanization greatly reduces labor and is responsible for the large scale of production in many countries. Proper soil moisture during harvest and soil temperature above 20°C help reduce the incidence of bruising compared to harvesting during low temperatures and dry soil conditions.

POST HARVEST
    Following harvest, potatoes, especially those intended for storage, should be cured by holding at 59-68°F and at high RH for 10 or more days to enhance periderm formation and heal harvest wounds. Wound healing, the formation of a cork-like layer of cells beneath damage tissues, occurs rapidly at 68°F and helps to restrict disease infection and moisture loss. After curing, the temperature is lowered, the amount lowered depends on the expected length of storage and intended use.
    The potato is at its best culinary and processing quality at the time of harvest. Storage extends the availability and thereby assists with orderly marketing, distribution, and 

Washed potatoes ready for marketing
utilization. Whereas storage can extend the usefulness of harvested potato crops, quality does diminish proportionally to the length of storage. However, in well constructed and well managed storage, tubers of some cultivars can be stored in marketable condition for more than 10 months. Storage is designed to prevent moisture loss, decay, and early sprouting while removing respiratory heat. Accurate temperature and ventilation management are the most important features. Other factors being equal, tuber quality is extended at temperature of 36-39°F and high RH (90-95%). High temperature decreases storage life because of increased respiration. However, RH is also important, as about 90% of the weight loss is due to moisture loss and 10% is because of respiration. Light is excluded to avoid chlorophyll development that results in tuber greening and the associated formation of toxic and bitter tasting glycoalkaloids.
    Many types of storage are used; those providing precise temperature and humidity control are ideal. Some are highly automated and may also provide controlled atmosphere (CA) management. Others are very simple, such as in situ field holding, field clamps, placement in damp sand and various kinds of pits, cellars, and above or below ground covered structures that rely on ambient temperature management. Even simple storage facilities, if well designed and insulated, can provide satisfactory storage conditions. Storage facilities should be clean and, if necessary, disinfected to minimize diseases.
    Diseased and damaged potatoes should be excluded, and direct contact with moisture avoided, so as to limit the spread of decay. If tubers are to be washed, it is usually deferred until removal from storage.  In many modern bulk storage facilities, potatoes are placed in large piles or compartments. If too large, such piles can interfere with ventilation and cause crushing of tubers at the bottom. Wooden slatted floors or air ducts are commonly used to improved ventilation and to drain moisture that may accumulate. Some storage use large pallet boxes which improve ventilation, lessen damage, and greatly facilitate handling into and out of storage using forklifts.
    Storage conditions vary depending on intended usage. Those stored for table use are typically maintained at a high relative humidity and at about 39°F. For processing, they are also held at a high relative humidity but at somewhat higher temperatures (50-60°F), because starch is converted to reducing sugars at low temperatures. The presence of reducing sugars increasing the tendency for tissue darkening when potatoes are processed by frying or dehydration. At warm temperatures, reducing sugars are converted to starch. With a high ratio of starch to reducing sugars, tissue discoloration is minimized or avoided. In order to extend storage time and thereby provide a continuous supply of potatoes, it is common for most producers to provide low temperature storage. To remedy the starch to reducing sugar conversion, potatoes can be reconditioned before processing. Reconditioning involves removal from low temperature storage and placement for several days or more at 64-70°F and 85-90% RH to accelerate the conversion. There is a marked degree of difference between cultivars in their ability to accumulate sugars. Therefore, cultivar selection is important in producing acceptable quality processed potato products.
    Depending on the length of the storage period, temperature management is not always totally adequate in controlling tuber sprouting. Therefore, to further minimize sprouting, chemical treatments can be applied. Maleic hydrazide is sprayed onto the foliage 2-3 weeks after full bloom or when most tubers have reached a size of 3-4 cm. An application of 1000-6000 ppm is effective in inhibiting sprouting. Chemicals to suppress sprouting can be applied as a dip or aerosol treatment to tubers after harvest and after injuries are healed. Inhibitors should not be applied to tubers intended for seed use.
    Excluding early sprouting, most storage disorders are due to rough physical handling beginning at harvest and detrimental conditions existing within the storage. Disease incidence is usually traceable to preexisting tuber infection prior to storage, although inadequate disinfecting of the storage can also be responsible.

MARKETING
    Quality Characteristics and Toxic compounds Important tuber quality factors include external appearance, size, shape, skin texture and pigmentation, flesh color, eye depth and number, defects and importantly, dry matter. The texture of the cooked potato is greatly influenced by its dry matter content and also by tuber cell size and the ratio of amylose to amylopectin starches. Culinary and processing uses are influenced by these features. In general, tubers with a high dry matter, high amylose to amylopectin ratio, small cell size, and low sugar content are preferred for most processing uses and for preparation by baking or frying. Such potatoes, when boiled, tend to slough and have a mealy texture. Potatoes with a low dry matter are best used boiled because they tend to remain intact. The starch composition tends to have a low amylose to amylopectin ratio. Such potatoes, when baked, tend to have a moist texture.
    Potato plants and tubers contain the toxic glycoalkaloids, alpha-solanine, and alpha-chaconine, which act as cholinesterase inhibitors. When tubers are exposed to light, chlorophyll along with the glycoalkaloids are synthesized. The amount of glycoalkaloids formed depends on exposure length, intensity and light quality (mostly ultraviolet), and temperature; little is synthesized at temperatures below 41°F. These compounds taste bitter, and ingestion can cause illness and death in extreme cases; toxicity depends on the amount ingested. Mechanical injury also induces the formation of these substances.
    Normally, the highest amounts of glycoalkaloids are found in tissues with high metabolic activity such as sprouts and flowers. The content in foliage and stems is higher than in tubers. The tuber skin has the highest glycoalkaloid concentration; peeling removes most but not all of it. Mature tubers contain 2-6 mg glycoalkaloid/100 g fresh weight. The content is high early in tuber development; small immature tubers have the highest glycoalkaloid (14-28 mg/100 g) levels. Heat does not destroy these substances, although some can be leached during boiling. Glycoalkaloid content varies with cultivars. The introduction of some newly developed cultivars, promising in many beneficial characteristics, has been prevented because of high glycoalkaloid content. The acceptable concentration is less than 20 mg/100 g; at that level the bitter flavor is very apparent.

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