Starting in 1996, Alexa Internet has been donating their crawl data to the Internet Archive. Flowing in every day, these data are added to the Wayback Machine after an embargo period.
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 OverviewSolanum 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
PrimaryEarly 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.
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
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.
