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Potatoes - Solanum Tuberosum

Potatoes are cool-season vegetables that rank with wheat and rice as one of the most important staples in the human diet. They are a native American species that were cultivated from Chile to New Grenada at the time the Spanish explorers reached this continent. The Spaniards introduced the species to Europe soon after 1580, and the popularity of potatoes spread all over Europe and the British Isles by the end of the 17th century. First introduced to New England by Irish immigrants in 1719, the white potato is now referred to as the "Irish potato" because of its association with the potato famine in Ireland in the 19th century.

Potatoes are not roots but specialized underground storage stems called "tubers". Maximum tuber formation occurs at soil temperatures between 60 and 70 degrees F. The tubers fail to form when the soil temperature reaches 80 degrees F. Potatoes will withstand light frost in the spring and can be grown throughout Arkansas.


Varieties

There are more than 100 varieties of potatoes. The horticulture varieties generally have white flesh and light brown or red skin. There are different types of potatoes with yellow to blue flesh and all different skin colors. An interesting one being grown now is yellow firm.

Russet Burbank is one of the most important varieties produced in the United States, but the weather in Arkansas is too warm and the moisture fluctuation is too great for the production of smooth tubers and good yields

The following varieties will produce well in Arkansas:

- Kennebec - 110 days to maturity. Smooth, oblong white tuber, heavy yields, good quality.

- Irish Cobbler - 95 days to maturity. Round white tuber, early, well adapted.

- Pontiac - 100 days to maturity. Round, oblong red tuber, heavy yields.

- Superior - 90 days to maturity. Early white round tubers, moderate heat tolerance.


When to Plant

Potatoes are among the earliest vegetables planted in the garden. Early, mid-season, and late varieties may be planted in early March to early April. Mid-season and late varieties may be planted as late as July to August 1 for fall production. Late potatoes are best for winter storage.


How to Plant

Potatoes are started from "seed pieces" rather than from true seed. Although there is a potato variety, Homestead Hybrid, which is true potato seed. Plant them in a window box or greenhouse to transplant, or plant directly in the garden about April 5-10. These seed pieces may be small whole potatoes or potatoes that are cut into 1 1/2 to 2-ounce pieces. Plant the pieces soon after cutting. Be sure that there is a least one good "eye" in each seed piece. Some garden centers and seed suppliers sell potato eyes that weigh less than an ounce; these are not the desired size. Small, whole, certified seed potatoes are the best choice for home gardeners.

Plant seed pieces 10 to 12 inches apart and cover in a furrow between 2 and 3 inches deep. Space row 24 to 36 inches apart. The 24-inch spacing is often beneficial because the plants will shade the soil and prevent high soil temperatures that inhibit tuber development.


Care

The soil should be fertile and well drained. Clay soils should be improved with organic matter and plowed in the fall. Use raised beds to improve soil drainage.

Mulching is usually helpful in growing potatoes. Organic mulch can be applied after the potato plants have emerged. Its benefits are to conserve moisture, help keep down weeds, and cool the soil. Some gardeners cover rows of early potatoes with clear plastic film at planting to warm the soil and promote early growth when the soil temperature is low. After the plants emerge, remove the film to allow the plants to grow.

After the potatoes break the surface of the ground, gradually build up a low ridge of loose soil by cultivation and hoeing. This ridge, which may become 4 to 6 inches high by summer, reduces the number of sunburned (greened) tubers. The object of potato cultivation is to eliminate competition from weeds, to loosen and aerate the soil, and to build up the row. Misshapen potatoes occur when they develop in hard, compacted soil.

Irrigate to assure uniform moisture while the tubers are developing. A uniform moisture supply also helps to cool the ground and eliminate knobs caused by secondary growth.


Harvesting

Harvest potatoes after most of the vines have died. Handle as gently as possible during harvest. Leave the tubers exposed to the sun just long enough for the soil to dry and fall off. Too much direct sunlight will blister the tubers, cause them to green and cause rot in storage. Since the tubers develop 4 to 6 inches beneath the soil surface, a spade fork is a useful tool for digging potatoes.

Potatoes for use in early summer ("new" potatoes) may be dug before the vines die (usually in July). When the potatoes reach 1 to 2 inches in size, you may wish to dig a few hills to use for soups or to cook with peas, cream, or butter.

Late potatoes are usually dug in July. They will keep in the garage or basement for several weeks in their natural dormancy. Store over the winter in a dark room at a temperature between 38 and 40 degrees F. with high humidity. Check periodically for spoilage.


Common Problems

A number of diseases attack home garden potatoes, and many of these diseases can be prevented. For example, there is a disease of the potato tuber called scab. Potato scab causes the tubers to have a brown, rough, bumpy or corky surface. There is no cure for scab, but frequently it can be prevented by planting scab-resistant varieties such as Agassiz, Bison, Crystal, Norgold Russet or Redsen. Do not use animal manure or straw on potatoes as this will promote scab. Also, do not rotate potatoes with beets on the same land if scab has been a problem.  The potato scab disease (indicated by scabby, rough skin) does not develop when the soil pH is 5.6 or below. Plant resistant varieties when available.

When planting potatoes, here are a few hints that will help prevent disease problems. It is very important to plant good quality seed pieces, as poor quality seed or seed saved from last year's crop can be a source of many diseases. If possible, buy certified potato seed since this will reduce the chances of destructive bacterial diseases such as black leg and ring rot. It will also reduce the chances of getting virus diseases, some which do not even show up but still reduce yield. Other virus diseases, such as rugose mosaic, produce a blotchy or mosaic pattern of light and dark green on the leaves. Whenever this appears, even on just a few plants, you have fairly serious virus problems, because rugose mosaic is a combination of two viruses, both which reduce yields but give no visible signs of their presence, unless they are both infecting the same plant.

Later in the season, when the plants are growing well, watch for leaf spot diseases. There are two that can seriously damage the leaves, causing them to drop prematurely. The most common leaf spot problem is early blight. These spots become large, up to inch or even 1 inch in diameter. They are circular to irregular in shape and dark brown to black in color. Early blight spots have concentric rings arranged in a target pattern. The spots have a definite margin. Leaves with early blight spots turn yellow and drop off. As soon as early blight appears, the potato plants should be sprayed with a fungicide such as Captan (Orthocide) or Daconil 2787 (Ortho Multi-Purpose Fungicide). These fungicides are available at garden care centers. . Early blight is best controlled by weekly applications of suggested fungicides, beginning when the seedlings are 6 to 8 inches high.

Another leaf spot disease develops in years with cool, wet weather. This is late blight. When late blight attacks, the leaves develop large dark-colored wet spots. The whole leaf rots quickly. Late blight spots do not have definite margins. As soon as late blight is found, spray the plants with a fungicide. The same fungicides can be used as for early blight control.

Colorado potato beetles, flea beetles, and fleahoppers can significantly reduce potato yields. These insects can be controlled with suggested insecticides.

Diseases: early blight, scab, late blight, tuber rots, virus complex, fusarium, verticillium, and bacterial wilts.

Insects: Colorado potato beetles, flea beetles, leafhoppers

Cultural: green skin (sun exposure), hollow heart (alternate wet and dry conditions), Black Walnut wilt (too close to a Black Walnut tree).


Scab of Potato Tubers

Scab is a disease of potato tubers that results in lowered tuber quality due to scab-like surface lesions. There are no aboveground symptoms. Two forms of scab occur. Common scab occurs in all production areas and is most severe in soils with a pH above 5.5. Another less common form, called acid scab, is important in acidic soils (below pH 5.5).

Symptoms

Scab symptoms are quite variable. Usually, roughly circular, raised, tan to brown, corky lesions of varying size develop randomly across tuber surfaces. Sometimes scab develops as a rather superficial layer of corky tissues covering large areas of the tuber surface. This is called russet scab. Pitted scab can also occur where lesions develop up to 1/2 inch deep. These deep lesions are dark brown to black, and the tissues underneath are often strawcolored and somewhat translucent. More than one of these lesion types may be present on a single tuber. Although scab symptoms are usually noticed late in the growing season or at harvest, tubers are susceptible to infection as soon as they are formed. Small brown, watersoaked, circular lesions are visible on tubers within a few weeks after infection. Mature tubers with a well-developed skin are no longer susceptible, but existing lesions will continue to expand as tubers enlarge. Thus disease severity increases throughout the growing season. Scab is most severe when tubers develop under warm, dry soil conditions. Coarse-textured soils that dry out quickly are therefore more conducive to scab than are fine-textured soils.

A few other conditions can be confused with scab. White, enlarged lenticles, which frequently occur on potato tubers harvested from wet soil, can be mistaken for scab. Usually this condition will disappear when tubers are dried. Patchy russeting, checking, or cracking of tuber surfaces caused by the fungus Rhizocton~a also may be confused with russet scab. A totally different but uncommon disease called powdery scab, caused by the fungus Spongospora subterranea, causes very similar scab-like symptoms. Laboratory examination may be necessary to identify these diseases.

Causal Organisms

Scab is caused by a group of filamentous bacteria called actinomycetes that occur commonly in soil. In soils with a pH above 5.5, Streptomyces scabies is usually responsible for common scab, and is capable of causing all the types of scab lesions described above. It is commonly introduced into fields on seed potatoes, and will survive indefinitely on decaying plant debris once the soil is contaminated. Because the organism can survive passage through the digestive tract of animals and be distributed with manure, it survives well in old teed lots or in fields where manure has been applied. It is easily disseminated in infested soil adhering to farm implements or transported by wind and water. S. scabies also infects other root crops such as beet, turnip, rutabaga, radish, carrot, and parsnip. Acid scab, which develops in soils with a pH from 5.5 to as low as 4.5, is usually caused by the acid-tolerant species S. acidiscabies. This organism has a host range similar to that of S. scabies but is usually carried on infected tubers since it does not survive well in soil. Continuous cropping to potatoes, especially scab-susceptible cultivars, will often increase the populations of scab organisms with resultant increases in disease severity.

Management


Bacterial Ring Rot of Potatoes

Bacterial ring rot is an important disease of potatoes and is one of the main reasons for rejection of seed potatoes from certification programs. This disease is particularly serious because it has the potential to spread quickly throughout a farm and may lead to severe losses if left unchecked. Ring rot was originally found in Germany in the late 1800's. The causal bacteria were introduced into the United States in the early 1930's and by 1940 were found throughout the country.

Symptoms

Severe ring rot can result in wilting of leaves and stems along with yellowing and death of leaves. Lower leaves usually wilt first, are slightly rolled at the margins, and are paler green than healthy leaves. As wilting progresses, leaf tissues between veins become yellow. In the later stages of disease, margins of lower leaves die and become brittle, and eventually entire stems yellow and die. Frequently, only one or two stems in a hill will develop symptoms and, in some cases, there are no above-ground symptoms at all. Ring rot derives its name from a characteristic breakdown of the vascular ring within the tuber. This often appears as a creamy-yellow to light-brown, cheesy rot. The symptom is most frequently observed when a diseased tuber is cut crosswise at the stem end. In severe cases, the vascular ring may be separated, and a creamy or cheesy exudate can be forced out from this tissue when the tuber is squeezed. On the outer surface, severely diseased tubers may show slightly sunken, dry, cracked areas. Infected tubers are often invaded by secondary decay organisms which may lead to complete breakdown. Symptoms of ring rot in the vascular tissue of infected tubers are often less obvious than described above, appearing as only a broken, sporadically appearing dark line, or as a continuous, yellowish discoloration. Because of this, laboratory tests should always be performed to confirm a diagnosis of ring rot.

Causal Organism

Ring rot is caused by the bacterium Clavibacter michiganense subsp. sepedonicus. Ring-rot bacteria survive between seasons mainly in infected seed tubers. They are also capable of surviving 2-5 years in dried slime on surfaces of crates, bins, burlap sacks, or harvesting and grading machinery, even if exposed to temperatures well below freezing. Survival is longest under cool, dry conditions. Ring-rot bacteria do not survive in soil in the absence of potato debris, but can survive from season to season in volunteer potato plants. Wounds are necessary for penetration of the bacteria into seed pieces. The pathogen is easily transmitted from diseased tubers to healthy seed pieces during the seedcutting process. A knife that cuts one infected tuber can spread these bacteria to the next 2-100 seed pieces. Likewise, the bacteria may be spread during planting, particularly if a pickertype planter is used. Ring-rot bacteria can be moved in irrigation water and by chewing insects, such as Colorado potato beetles and flea beetles. After the bacteria become established in a plant, they multiply and move throughout the plant via the waterconducting tissues. Fortunately ring-rot bacteria are capable of causing disease only in potato, although they may be able to colonize roots of sugar beets.

Management


Early Blight of Potato and Tomato

Early blight is a very common disease of both potato and tomato. It causes leaf spots and tuber blight on potato, and leaf spots, fruit rot and stem lesions on tomato. The disease can occur over a wide range of climatic conditions and can be very destructive if left uncontrolled, often resulting in complete defoliation of plants. In contrast to the name, it rarely develops early, but usually appears on mature foliage.

Symptoms

On leaves of both crops, the first symptoms usually appear on older leaves and consist of small, irregular, dark brown to black, dead spots ranging in size from a pinpoint to 1/2 inch in diameter. As the spots enlarge, concentric rings may form as a result of irregular growth patterns by the organism in the leaf tissue. This gives the lesion a characteristic "target-spot" or "bull's eye" appearance. There is often a narrow, yellow halo around each spot and lesions are usually bordered by veins. When spots are numerous, they may grow together, causing infected leaves to turn yellow and die. Usually the oldest leaves become infected first and they dry up and drop from the plant as the disease progresses up the main stem.

On tomato, stem infections can occur at any age resulting in small, dark, slightly sunken areas that enlarge to form circular or elongated spots with lighter-colored centers. Concentric markings, similar to those on leaves, often develop on stem lesions. If infested seed are used to start tomato transplants, seedlings may damp off soon after emergence. When large lesions develop at the ground line on stems of transplants or seedlings, the plants may become girdled, a condition known as "collar rot." Such plants may die when set in the field or, if stems are weakened, may break over early in the season. Some plants may survive with reduced root systems if portions of stems above the canker develop roots where they contact the soil. Such plants, however, usually produce few or no fruits. Stem lesions are much less common and destructive on potato.

Blossom drop and spotting of fruit stems, along with loss of young fruit, may occur when early blight attacks tomatoes in the flowering stage. On older fruits, early blight causes dark, leathery sunken spots, usually at the point of stem attachment. These spots may enlarge to involve the entire upper portion of the fruit, often showing concentric markings like those on leaves. Affected areas may be covered with velvety black masses of spores. Fruits can also be infected in the green or ripe stage through growth cracks and other wounds. Infected fruits often drop before they reach maturity.

On potato tubers, early blight results in surface lesions that appear a little darker than adjacent healthy skin. Lesions are usually slightly sunken, circularor irregular, and vary in size up to 3/4 inch in diameter. There is usually a well defined and sometimes slightly raised margin between healthy and diseased tissue. Internally, the tissue shows a brown to black corky, dry rot, usually not more than 1/4 to 3/8 inch deep. Deep cracks may form in older lesions. Tuber infection is uncommon under Ohio conditions.

Causal Organism

Early blight is caused by the fungus, Alternaria solani, which survives in infected leaf or stem tissues on or in the soil. This fungus is universally present in fields where these crops have been grown. It can also be carried on tomato seed and in potato tubers. Spores form on infested plant debris at the soil surface or on active lesions over a fairly wide temperature range, especiallyunder alternating wet and dry conditions. They are easily carried by air currents, windblown soil, splashing rain, and irrigation water. Infection of susceptible leaf or stem tissues occurs in warm, humid weather with heavy dews or rain. Early blight can develop quite rapidly in mid to late season and is more severe when plants are stressed by poor nutrition, drought, or other pests. Infection of potato tubers occurs through natural openings on the skin or through injuries. Tubers may come in contact with spores during harvest and lesions may continue to develop in storage.

Management


Storage

Days to Maturity: 100-120

Harvest: Dig early potatoes when tubers are large enough to eat. Harvest potatoes for storage after the vines die down or just after the first light frost nips the vines, before heavy freezing. Avoid skinning tubers when digging and avoid long exposure to light.

Approximate Yield: (per 10 feet row) 6-15 lbs

Amount to raise per person: 75-100 pounds (plant about 15 pounds of seed potatoes per person)

Storage: medium-cool (40F-50F), moist (90% RH) conditions; 6-8 months. Sprouting is a problem at higher temperatures.

Preservation: usually stored in medium cool, moist conditions.


Questions & Answers

Should I save some of my potatoes for seed potatoes?

No. Saving your own seed potatoes can lead to a buildup of viruses and diseases.

My potato plants flowered and formed green fruits that resemble small tomatoes. What are they?

These small seeds balls are the fruits that contain the true seeds. They are not edible.

What causes green skins on my potatoes?

The green areas on tubers develop where the potato was exposed to the sun. This condition occurs when the potatoes were not planted deeply enough or not covered with straw. The green portions taste bitter because they contain an alkaloid. These green areas should be cut off and discarded. Exposure of potato tubers to fluorescent light or sunlight will cause greening during storage.

Can I make chips from homegrown potatoes?

Yes. Almost any potato variety can be used to make chips when the potatoes are freshly dug. Commercial chips are made from selected varieties that are carefully handled and stored. Chips made from inferior varieties or improperly stored potatoes will be brown and have a dark ring, because they contain excessive amounts of sugars.

I used potatoes purchased at the grocery store as seed for planting and they rotted without sprouting. Why?

Many potatoes sold for fresh market consumption have been treated to prevent sprouting in storage as well as after planting. Potatoes have a rest period which must be broken before the seed will sprout. Cool or extremely warm temperatures can break the rest period and allow potatoes to sprout. Plant certified seed that has been properly stored to induce sprouting.

Can I save the small potatoes from my spring crop for planting in the fall in my garden?

Yes. This is commonly done but sometimes the potatoes saved from the spring garden fail to sprout when planted in the fall because of a natural dormancy in newly harvested potatoes. One recommended procedure for breaking the dormancy includes harvesting the potatoes and placing them in a cool storage area, preferably in the range of 50 degrees F. until about 3 to 4 weeks before the anticipated fall planting date. The small potatoes should be planted whole and not cut to prevent rotting.

What size piece should seed potatoes be cut into?

Each seed piece should contain at least 2 to 3 "eyes." Research has shown that the best size seed piece weighs approximately 2 ounces.

Sometimes my potatoes or the potatoes I see at supermarkets have a green color. Are these potatoes poisonous?

Potatoes that exhibit a green color contain a substance known as Solanine. This substance, if consumed in extremely large quantities can cause severe illness or death. This greening of potatoes is caused by exposure to light during the growing period or excessive exposure to artificial lights at grocery stores or supermarkets. In the garden, this is most common after heavy rains which uncover potatoes near the surface exposing them to sunlight.

How do I know when my potatoes are ready for harvesting?

Potatoes are generally mature when the plant starts to turn yellow. Potatoes require 75 to 140 days from planting to maturity depending upon variety and the season in which they are grown. Immature potatoes will often skin and bruise easily. Dig spring-planted potatoes before the soil becomes hot. Avoid harvesting the potatoes when the soil is wet to avoid storage diseases.

I have some seed potatoes left from my spring garden. Would it be all right to eat them?

No. Potato tubers purchased for seed purposes definitely should not be eaten because they frequently have been chemically treated. Like all treated seeds, seed potatoes should not be fed to humans or animals.

After harvesting, how should I handle my potatoes to result in the longest storage time possible?

Dig potatoes when the soil is dry. Be careful not to skin or bruise the tubers. Do not wash the potatoes. Place them in crates or some suitable container and store them in a dark areas for about 10 days at a temperature of 60 degrees F. with a relatively high humidity. After this curing period, keep the potatoes at 40 to 45 degrees F. with humidity near 85 percent and provide good air circulation.

Can potatoes be left in the ground for storage?

Generally no. Cool, humid conditions (38 to 45 degrees F. and 85 percent relative humidity) are best for Irish potato storage. The potatoes would not stay dry enough in the soil to prevent second growth or sprouting. Several weeks at high temperatures can break the rest period in home-grown potatoes after which sprouts will develop on the tuber. It is better to dig the potatoes and put them in a cool, damp area.

Why do home-stored potatoes have a different flavor in the winter than in the summer?

Irish potatoes stored at temperatures below 45 degrees F. will taste sweeter and be stringier that those stored at warmer temperatures. At temperatures less than 45 degrees F., enzymes within the tuber convert starch into sugars causing the sweet taste and stringy consistency. Potatoes to be eaten should not be stored in the refrigerator. Sugars within the potatoes can be converted back into starch by storing the potatoes at temperatures above 65 degrees F. for a week or two prior to use.

My potato plants produced small tomatoes this year. I planted them next to my tomatoes. Could they have crossed or have my potatoes mutated?

The fruit on the potato plant is actually the fruiting structure of the potato plant. The potato and tomato belong to the same botanical family and have similar growth characteristics. The potato flower looks very much like the tomato flower and is pollinated and fertilized identical to the tomato flower.

The stems of my Irish potato plants are decayed. The plants weaken but do not die.

This is Rhizoctonia. It is a soilborne fungus that causes decay in stems and seed pieces. Approved seed-piece fungicides are the best control. Follow the label instructions closely.

After a rainfall, the plants in one area of my garden began to die rapidly. The stems are rotted. A dark discoloration is moving up the stem to the top of the plant, and the stem has a foul odor.

This is black leg of potatoes, one of the major bacterial potato problems. To prevent problem, plant only in well-drained areas. Seed piece treatment will also help prevent the entry of bacteria and other organisms.

When I dug my potatoes, they were covered by small, raised bumps.

These are root knot nematodes and pose a serious problem on potatoes. Control them with crop rotation.

After I dug my potatoes, I found that they were rough with deep scars.

This is potato scab, caused by a soilborne organism. Control by maintaining an acid soil, below pH 5.6, around your potato plants and a uniform moisture level from the time the potato is formed until it is harvested.

The lower foliage on my potato plants is beginning to turn yellow and is covered with brown spots.

This is early blight of potatoes and is similar to blight on tomatoes. Spray with a fungicide as soon as spots are observed and repeat at 7- to 14-day intervals for two to three sprays.

The foliage of my potato plants is distorted, rolled and is not as healthy as it should be.

Several viruses attack potatoes. The best prevention of potato viruses is to plant only certified seed pieces.

When I dug my potatoes, I noticed small holes chewed in the tubers. How do I prevent this?

Several soil-inhabiting insects such as wireworms and white grubs cause this type of damage. Use control measures for these pests before planting. Use insecticides only as directed on the label.

The leaves of my potatoes are disappearing fast. All I see on the plant is some pinkish worms.

These pinkish larvae are immature Colorado potato beetles. They can defoliate plants and can be controlled with insecticides. Use as directed on the label. In small plots, control by hand picking the larvae and destroying them.


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This page was last updated on November 16, 2002