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Week 8 (Unit 15)

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Potato

Taxonomy, morphology and reproduction
Crop requirements, adaptive traits, management practices
World production statistics
Genetic resources and potato breeding
Propagation
Harvesting and storage

Diseases and pests
Utilization and quality
Assignments
References

Printer Friendly Version of Unit 15

Much of the information for this lecture was obtained from lectures by Al Mosley, OSU and from the OSU website, Potato Information Exchange:
http://cropandsoil.oregonstate.edu/potatoes/

Taxonomy, Morphology and Reproduction

The potato of commerce (Solanum tuberosum L.) is an annual dicot species. It is an autotetraploid with 4x=48 chromosomes.

  • Solanum tuberosum subsp. tuberosum - from Chile, adapted to long days
  • Solanum tuberosum subsp. andigena - from the Andes, adapted to short days

The potato tuber is a modified, underground stem that is used by the plant for storage. The eyes are buds that can sprout and regenerate new plants.

The potato fruit is actually a berry, much like a small tomato that remains green.

Photo by Scott Bauer, USDA-ARS

Potatoes belong to the Nightshade family (Solanaceae), which includes about 150 species that bear tubers. Most members of this family produce alkaloids in the roots:

  • Potato – solanine, chaconine, etc
  • Tobacco – nicotine
  • Deadly nightshade - scopolamine
  • Tomato – tomatine
  • Petunia
  • Pepper
  • Eggplant
  • Jimsom Weed

Most of these species are graft compatible, so one could develop a nicotine-free tobacco plant by grafting a tobacco scion onto potato rootstock.


Potato Fruits (Berries)
Photo courtesy Al Mosley

Center of Origin

Cultivated potato originated in the highlands of South America, where it has been cultivated for more than 8000 years. There are many wild forms of Solanum spp. in the Andes of Peru, Bolivia, and Ecuador - cultivated potato must have been derived from one or more of these species.

There are also many wild forms of potato in Central America (Mexico, Guatemala, etc.). However, these species never appear to have been cultivated. It is likely that indigenous forms of S. tuberosum found in that region originated in South America.

Wild species of Solanum also occur in North America.

S. jamesii and S. fendleri are found in Colorado, Utah, and New Mexico near Indian middens.

 
Map courtesy A. Mosley


Some Wild Species of Solanum

Photo Courtesy Al Mosley

Historical Perspectives

The Spanish conquistadors first encountered the potato when they arrived in Peru in 1532 in search of gold. The potato was taken from South America to Europe in the late 16th century. By the 19th century, it had spread throughout the European continent. The availability of potato as a staple food crop is thought to have stimulated the rise in population in Europe in the 1700's and 1800's. By the 1800's, the potato supplied about 80 percent of the calories in the diet of Irish peasants, and was also used as fodder for animals. This dependence on one food crop was risky. In 1845, the pathogen Phytophthora infestans (Late Blight) arrived accidentally from North America. Favorable environmental conditions created an epidemic, and about one million Irish peasants died of starvation due to the failure of the potato crop.

European immigrants introduced potatoes to North America several times throughout the 1600's, but they were not widely grown for almost a century. Potatoes were first grown on a large scale in the early 1700's.

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Crop Requirements, Adaptive Traits, Management Practices

Potato is best adapted to the cool temperate zones of the high altitudes in the Andes, at sea level in temperate regions of North America, Europe, southern Chile and Argentina, and at intermediate altitudes within that range.

Potatoes grow best on well-drained loamy soils. Clay soils often result in misshapen tubers, and sandy soils become droughty, which lowers yields. Slightly acid soils (pH<5.2) are preferred to prevent infection of potato scab disease.

  Photo courtesy Al Mosley, OSU

Soil temperatures of 60-70 °F result in maximum tuber formation. Soil temperatures above 80 °F will inhibit tuber formation. A good, uniform supply of moisture is needed to obtain tubers of uniform size and high quality. Fluctuations in water supply can lead to misshapen, cracked tubers, with low specific gravity and poor processing quality. Irrigation may also help to cool the soil during hot, dry periods. In Oregon, virtually all of the crop is grown under irrigation.

Plants are often "hilled" after emergence, to control weeds, retain soil moisture, reduce temperature, and prevent "greening" of tubers that occurs when they are exposed to sunlight.

A potato crop makes a large demand on the soil for nutrients. Typical amounts taken up per acre by a crop grown under irrigation in the US are:

Nitrogen -- 200 lbs.
Phosphorus (P205) -- 60 lbs.
Potassium (K20) -- 300 lbs.

One-third to one-half of these nutrients are found in the vines and returned to the soil. The remainder is removed with the harvested tubers and must be replaced.

Timing of fertilizer is critical for potatoes intended for frying or chipping. Too much nitrogen fertilizer can reduce yield and specific gravity, increase reducing sugars in the tuber and delay maturity.

The potato is shallow-rooted with 90% or more of the root mass typically in the top foot of soil. Thus, neither fertilizer nor water should be applied in excessive quantities at one time, because they may be lost from the root zone.

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World Production Statistics

World production of potatoes is about 316 million metric tons each year, making it the fourth leading food crop after wheat, rice and corn. Furthermore, production of potatoes is increasing, particularly in developing countries. Yields average about 16 t/ha.

Average annual growth rates in production of selected food crops
in developing countries

Source: CIP Potato Facts and FAOSTAT (1998)

 

Leading producers of potato, based on averages for 2000-2002 (FAOSTAT)

Country Area Harvested (ha) x 1000 Production (Mt) x 1000
China
4,616 65,324
Russian Federation 3,224 33,615
India 1,320 23,619
USA 519 21,390
Poland 1,086 19,684
Ukraine 1,612 17,761
Germany 290 12,368
Belarus 611 7,968
Netherlands 166 7,502
UK 164 6,503
France 162 6,425
Turkey 202 5,190
Canada 164 4,410

China is presently the world leader in potato production. For detailed maps showing area used for potato production, see the CIP World Atlas.

Potato production in the USA

Leading states for potato production in the USA are Idaho and Washington. Most of the crop in the USA is grown under irrigation. High yields are obtained in the Pacific Northwest because days are long and clear and nights are cool during the cropping season. In Oregon, the crop is worth about $132 million annually, and about $358 million in processed forms.

Source: NASS 1997 Agricultural Atlas of the United States

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Genetic Resources and Potato Breeding

CIP maintains the world's largest bank of potato germplasm, including some 1500 samples of about 100 wild species collected in eight Latin American countries and 3800 traditional Andean cultivated potatoes. Many of the wild species were collected by Carlos Ochoa, who personally discovered 80 species of wild potato. The collection is maintained under the auspices of the FAO and is available to plant breeders worldwide free of charge, upon request.
 
Photo courtesy Al Moser, OSU

Potato cultivars

Hundreds of varieties are commercially available for growers, but as few as five account for about 90% of the U.S. acreage. The leading variety, Russet Burbank, still provides about 40% of the U.S. production. Through clonal propagation, this variety has remained essentially unchanged since it was developed by Luther Burbank in 1872. Russet refers to the rough, reticulated skin of this potato, which originated as a mutation of the original Burbank potato.

Varieties differ in a number of characteristics including:

  • yield
  • skin and flesh color
  • tuber shape and eye depth
  • time of maturity
  • disease and pest resistance
  • carbohydrate composition
  • usage potential
  • dormancy and storability

Processors typically have a particular end-use in mind and specify varieties in grower contracts.


Breeding priorities in the OSU potato program are:

  • high-yielding, long, attractive Russet
  • high starch and low sugar content (best for fries and chips)
  • resistance to late blight, viruses, and nematodes
  • good storage and processing properties.

Types of Potatoes

Potato varieties have traditionally been categorized according to shape and color:

  • Round Whites - chipping and direct consumption
  • Russets - frozen processing and fresh market
  • Reds - fresh market
  • "Specialty" and "Gourmet" (varieties not fitting into the three major divisions) - fresh market

'New' potatoes are small potatoes that are dug early before the skins have set.

Breeding methods

Traditional breeding methods involve hybridization of parental clones and the subsequent selection among large numbers of offspring for superior individuals with the desired combination of traits. Single plant selections are then propagated vegetatively and evaluated as clones for relevant agronomic and quality attributes. This breeding approach has resulted in the development of many elite clones which have become highly successful potato cultivars.

Some of the limitations in using traditional methods for potato breeding are:

  • the plant is an autotetraploid, so there are multiple copies of genes of interest
  • cultivars are highly heterozygous, and selfing to produce a true-breeding line results in inbreeding depression
  • some potatoes are sterile or partially sterile, limiting their use as parents.

Several nonconventional approaches have been used in potato breeding.

  1. Diploid parents have been generated from S. tuberosum through various means. Breeding can then be carried out at the diploid level, and the tetraploid reconstituted once the desired traits have been incorporated into the parents.
  2. Biotechnology has been used to incorporate novel genes into potato
    • NewLeaf® potatoes have a modified gene from Bacillus thuringiensis that provides resistance to the Colorado potato beetle
    • Potatoes have been developed that have modified starch for nonfood purposes (only amylopectin rather than the usual mix of amylose and amylopectin starch).

 

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Propagation

The potato is usually propagated vegetatively from tuber seedpieces. The resulting crop will be genetically identical to the plants that produced the tubers. This type of cloning assures genetic uniformity. Plants grown from tuber seedpieces will generally be higher yielding, more vigorous, and mature more quickly than plants grown from seed. One disadvantage of this method of production is that diseases can also be propagated from one generation to the next.

Growers should plant only certified, disease-free tubers of known origin. Tubers marketed for consumption are not a good seed source, because they are often sprayed with growth inhibitors to reduce sprouting. Tubers for planting are cut into small (1-2 ounce) pieces that have at least one eye. These can be planted immediately, or if it is necessary to delay planting for more than a few hours, then the cut surfaces must be permitted to heal (suberize) for 7-10 days before planting.

Potatoes grown in the field can rapidly contract diseases, particularly viruses. Potato Leafroll Virus (PLRV) is spread by aphids. The aphids feed on infected plants, and then the virus multiplies inside the aphid. The aphid moves from plant to plant, spreading the disease throughout its life. A field of disease-free seedlings can become 100% infected in a single cropping season.

In vitro methods are available to break the virus cycle. Meristem culture and heat treatment are combined with sophisticated testing procedures to ensure that plants are disease free.

Meristems are the growing point in buds that divide to generate all of the organs of a new plant. The connections between the meristems and the vascular system of the plant are not well developed, making it difficult for viruses to move into the meristems.

Once the plantlets have proven to be disease-free, they can be multiplied using nodal cuttings to generate planting material for the field. Transplants and/or tubers can be used as planting material.

In vitro techniques have also proven to be very useful for germplasm conservation. Germplasm collections at CIP are maintained as in vitro plantlets or through in vitro production of microtubers.

in-vitro potato propagation
Photo courtesy Al Mosley, OSU

Seed Certification

In the US, all seed potatoes are certified according to a strict limited-generation system.

Pre-nuclear (laboratory/greenhouse)

Nuclear (grower)

Generation 1

Generation 2

Generation 3

Generation 4

Generation 5

Commercial/food crop

Each generation must meet certification tolerances (e.g., no more than 2% virus at Generation 4/5). After five generations, the material can no longer be sold as certified seed and must be used for producing commercial potatoes (those we eat or process).

Harvesting and Storage

Harvesting

Potatoes mature in 90 to 130 days, depending on the cultivar and production region. Vines are often removed before harvest. This allows the tubers to loosen from the stolons, mature more quickly, and to "set skin". It takes up to three weeks for tubers to completely heal once the skin set process begins. A tough skin at harvest provides good protection against disease and bruising and ensures quality during harvesting and storage. Tubers from immature or recently-killed vines typically have relatively low starch and high sugar concentrations compared to mature tubers. Storage decay problems are a common result. Vines can be removed with chemical desiccants or with mechanical vine choppers.

Storage

To prepare potatoes for storage, they should be cured for a week or two at 65-70º F and 85-90% relative humidity to allow the skin to toughen and wounds to heal.

Tubers should be stored in the dark at 40-45º F and 90% relative humidity. Prolonged exposure to temperatures above 45º F causes sprouting. Potatoes can be stored from 2-9 months, depending on the cultivar and storage conditions.

Because the tuber is a modified stem, potatoes will produce chlorophyll and turn green when exposed to light. Exposure to light also leads to production of solanine, a toxic alkaloid. Production of solanine occurs even at very low levels of light, but the amount of solanine increases with the length of exposure and intensity of the light. Most of the solanine is concentrated in the green skin of the tuber, so peeling the green portion away should remove most of the toxin. Sprouting eyes are also high in solanine and should be removed before eating. Cooking is not a reliable means for destroying solanine.

Much of the U.S. fall potato crop is stored for later marketing and processing. To maintain a constant supply to the fresh and processed potato market, potatoes are treated with a sprout inhibitor. To treat diseases such as late blight in potato storage facilities, disinfectants are either added to the humidifiers or applied directly to the potatoes as a dip.

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Diseases and Pests

For a good reference on potato pests, see Pests of Potato from UC Davis.

OSU Extension maintains an On-line Guide to Plant Disease Control which includes potato diseases.

Diseases

Late blight is caused by the fungus-like Oomycete Phytophthora infestans. Spores transported in the wind or infected tubers carried to new areas can cause infestation. Late blight first appears as a few grayish specks on the plant’s leaves, and then a cottony film appears. Under certain climatic conditions (high humidity and cool to warm temperatures), the disease can easily lead to the destruction of a whole field of potatoes. The disease also affects tubers, and can make the crop unfit for storage.

Late blight is widely distributed throughout the world and was responsible for the Irish Potato Famine of 1845.

  • Bacterial wilt
  • Potato scab
  • Viruses - the three most common of the more than 25 known potato viruses are potato leafroll virus (PLRV), potato virus X (PVX), and potato virus Y (PVY). PLRV and PVY are the most widespread and are responsible for significant crop losses.

Insect Pests


Colorado potato beetle
Photo by Scott Bauer, USDA-ARS

Nematodes

Nematodes are parasitic roundworms that feed on plant roots. Some species feed on potato tubers, which reduces plant vigor and causes blemishes on tubers.

For more information, see the University of California Pest Management Guidelines for potato nematodes. http://www.ipm.ucdavis.edu/PMG/r607200111.html

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Utilization and Quality

Uses of potato

  • Fresh market
  • Fries
  • Potato chips
  • Dried products (instant mashed potatoes, etc.)
  • Animal feed
  Photo by Scott Bauer
USDA-ARS

Potatoes are an excellent source of Vitamin C. The protein content of potatoes is not very high, but it is of good quality. On a per hectare basis, potato produces more protein than the cereal crops.

Content of 100 g of potato

 

Content

% RDA

Water

75 g

 

Energy

97 kcal

 

Carbohydrate

22 gm

 

Protein

2 gm

3%

Fat

0.1 gm

 

Fibre

1.5 gm

 

Calcium

5 mg

 

Iron

0.35 mg

3%

Magnesium

25 mg

 

Phosphorus

50 mg

 

Potassium

391 mg

 

Sodium

5 mg

 

Vitamin C

13 mg

22%

Vitamin B1 (Thiamin)

0.1 mg

 

Vitamin B2 (Riboflavin)

0.02 mg

 

Niacin

1.2 mg

 

Source: USDA National Nutrient Database

Starch and sugar content

Starch has profound effects on product texture and oil consumption during processing. Frying drives much of the water out of chips and replaces it with oil. Therefore, low dry matter/high water tubers tend to absorb more oil, which is typically the most expensive component of fries, and become more soggy and oily.

In contrast to starch, which primarily affects product texture and oil content, reducing sugars (primarily glucose and fructose) play a critical role in the processed color of potato chips and french fries. Even very low levels of sugar cause chips and fries to turn dark brown during frying. For that reason, varieties intended for processing either as chips or fries must have low levels of reducing sugars both in the field and in storage. Sugar levels typically make up less than 3% of total tuber dry matter.

Relationship Between Tuber Dry Matter and Optimum Use

Specific Gravity
Dry Matter
%
Texture
Typical Uses
Below 1.060
(very low)
Below 16.2
Very soggy
Pan frying, salads, canning
1.060-1.069
(low)
16.2-18.1
Soggy
Pan frying, salads, boiling, canning
1.070-1.079 (medium) 18.2-20.2
Waxy
Boiling, mashing; fair to good for chip processing and canning
1.080-1.089 (high) 20.3-22.3
Mealy, dry
Baking, chip processing, frozen french fry processing; some cultivars tend to slough when boiled
Above 1.089 (very high) Above 22.3
Very mealy or dry
Baking, frozen french fry processing, chip processing; tendency to produce brittle chips and to slough when boiled

Adapted from: Mosley, A.R. and R.W. Chase. 1993. Selecting Cultivars and Obtaining Healthy Seed Lots. In: Potato Health Management, APS Press, 1993. Pp. 19-27.

Traditional uses and new products

Potatoes formed the basis of the Aymara Indian and Incan diet in the Titicaca Plateau in the Andes in Bolivia, Ecuador and Peru. Even today, many traditional potato varieties are grown there that are unique in appearance and flavor. Efforts are underway to identify new specialty markets for these varieties.

For more information about traditional potato varieties of the Andes, see http://www.cipotato.org/market/flyerMH_NativPot.PDF and
http://books.nap.edu/books/030904264X/html/93.html

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Assignments

Written assignment

Due May 26th

Choose a developing country where potatoes are grown, and read about potato production practices in that country (http://research.cip.cgiar.org/wpa/index.php).

The Potato Information Exchange provides a great deal of useful information about potato production in Oregon and links to production guides for other states in the US and other countries. Using this resource, choose a state in the US or another developed country and read about potato production practices there.

From the point of view of a farmer, describe some of the major similarities and differences between the potato production systems in the two countries that you have studied. Your discussion should be a minimum of one paragraph and no more than one page. Be sure to include your references.

Submit this assignment via the assignment function on Blackboard.

Quiz

Take the quiz on this Unit on the Blackboard.

References

Berry, R. E., G. L. Reed, and L. B. Coop. 2000. Identification & Management of Major Pest & Beneficial Insects in Potato [online]. Publication No. IPPC E.04-00-1, Oregon State University.
http://ippc2.orst.edu/potato

CIP. 2003. About potato.
http://www.cipotato.org/potato/

CIP. 2002. World Potato Atlas.
http://research.cip.cgiar.org/confluence/display/wpa/Home

Harris, P. 1992. The potato crop. Chapman & Hall, London.

Levetin, E. and K. McMahon. 2005. Starchy Staples. Chapter 14 in Plants and Society, 4th edition. McGraw-Hill, New York, NY. Additional on-line notes and references:
http://highered.mcgraw-hill.com/sites/0072528427/student_view0/chapter14/chapter_outline.html

National Academy Press. 1989. Potatoes. In Lost Crops of the Incas: Little-Known Plants of the Andes with Promise for Worldwide Cultivation.
http://books.nap.edu/books/030904264X/html/93.html

OSU Extension Service. Online Guide to Plant Disease Control.
http://plant-disease.ippc.orst.edu/plant_index.cfm

Purdue University Cooperative Extension Service. Potatoes. HO-62. http://www.hort.purdue.edu/ext/HO-62W.pdf

The Potato, Then and Now. 1999. Digital collection and fun facts about potato from Prince Edward Island.
http://collections.ic.gc.ca/potato/index.asp

Vales, I. 2006. Potato Information Exchange, Oregon State University.
http://cropandsoil.oregonstate.edu/potatoes/

Washington State Potato Commission. 2006.
http://www.potatoes.com/

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