Many things affect crop growth from the time of planting until harvest. These include fertilization, cultivation, rainfall and/or irrigation, and weed, insect, and disease control.

Cultivation

The purpose of cultivation includes maintaining proper soil aeration, shaping beds to allow space for maximum tuber growth and to prevent sunburning, establishing irrigation furrows, and controling weeds. If a cultivation operation does not accomplish one of these purposes, the operation is a waste of effort.

The kind and amount of cultivation will depend on the planting method, kind and severity of weed infestation, irrigation method used and, to a lesser extent, the potato variety grown.

If potatoes are planted in such a manner as to leave the field quite flat, one or more post-planting bed shapings or billings may be necessary. If potatoes are planted into pre-made beds or if beds are formed at planting, bed shaping may be the only cultivation necessary.

The implements used for shaping vary considerably. Disks, winged cultivator teeth, hilling "spades~ or listers, and rolling cultivators, when used properly, can shape acceptable beds (Fig. 14). No matter which type of cultivating or hilling implement is used, tillage should not take place in wet soils. Working soils that are wet results in compaction and clods that will present problems at time of harvest.
 

FIGURE 14. A rolling cultivator bed-shaping implement. Multi-row equipment Allows for handling large acreage on a timely basis.

Working the ground to aerate the soil should be practiced only if the grower is certain the benefits from aeration will more than offset the compaction in the furrows that results from the operation.

Some varieties tend to set tubers higher in the beds than others. To prevent sunburning, additional soil may be required to cover the tubers. However, late cultivation can also be harmful due to root and stolon pruning.

Cultivation can be a very effective method of weed control. The principal benefits come from post-plant, preemergence cultivation to kill early emerging weeds and cultivation during the first 30-40 days after emergence to control weeds in the furrows and on the sides of the beds. Thereafter, shading and herbicides must be depended upon for weed control.

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Water Requirements and Irrigation

Water management and/or rainfall are probably the most important factors determining yield and quality of potatoes. Knobby tubers, growth cracks, internal necrosis, blackspot, hollow heart, heat sprouting, and other disorders are directly related to amount and distribution of water during the growing season. Diseases such as seed piece decay, rhizoctonia stem canker, and late blight can also be related to amount and distribution of water. Factors to consider are method of applying water (rainfall, sprinkler irrigation or furrow irrigation), timing of irrigation, and quantity of irrigation.

Specific guidelines for irrigation cannot be given here because of the wide diversity of rainfall, temperature, and soil conditions under which potatoes are grown in the United States and Canada. State and local agencies must be relied upon for these specific guidelines. Many general guidelines are pertinent to all potato producing areas, however.

In the eastern one-half to two-thirds of North America most of the potato producing areas receive substantial amounts of rainfall during the growing season. Most of the areas in the western one-third to one-half, except the northernmost coastal areas, must rely upon some type of irrigation.

While the amount of water required for optimum growth of potatoes varies somewhat with variety, humidity, sunlight, and length of growing season, the seasonal requirement for varieties in all areas will be at least 18 area-inches (46 cm) of water. It may be as much as 30-36 area-inches (76-91 cm) of water in some areas. Where irrigation is practiced, the soil profile should be at or near field capacity at the beginning of the season and additional water supplied to the plants in frequent, light amounts during the growing season.

For irrigation management decisions, it should be remembered that: 1) the effective rooting depth of potatoes is 2 ft (60 cm), 2) the soil should not be allowed to dry below 65 percent of field capacity, 3) moisture levels above field capacity will seriously affect yield and quality, and 4) soil types can vary threefold in their respective water holding capacities.

Studies in several different growing areas have shown that daily water needs increase linearly until about two weeks after maximum row coverage is achieved. From this time on the plants' daily water requirement holds nearly constant until the vines begin to mature (cast off, at which time water requirement declines.

The amount and rate of water that should be applied during any given irrigation depends on the infiltration and water holding capacity of the soil, in addition to the amount of water already in the soil and the stage of plant growth, including depth of rooting. In sandy soils, the application of more than two inches of water will result in leaching of nutrients below the plant's root zone, while in heavier textured soils four to five inches of water can be applied without leaching. The amount of runoff can be minimized, if not eliminated, by accurate determination of infiltration rate and adherence to the practice of not applying the water faster than the soil can absorb it.

Where rainfall is the major source of water, efficiency can be improved by not planting on steep slopes, by properly preparing the soil so infiltration is enhanced, and by forming small ridges periodically in furrows to slow the water running down the furrows.

Where irrigation is used, several choices of methods are usually available. The most common include furrow irrigation, solid set sprinklers, wheel line sprinklers, hand-move sprinkler systems, circular overhead sprinklers, and sub-irrigation.

Sub-irrigation is a method used in peat-like soils and/or where the water table is easily raised. For this to be a suitable method, fields must be relatively level and soils uniformly porous. Otherwise, excessively wet and excessively dry areas will occur in the same field.

Furrow irrigation can be efficiently and effectively used where the field does not have much slope (0-2%) and where the length of rows is not long 1600-800 feet (182-244 m)]. Care must be taken to ensure that the water is not applied faster than the soil can absorb it. Uniform application from one end of the row to the other is more difficult to achieve with this method than with sprinkler irrigation methods.

Sprinkler irrigation systems provide the most flexibility and the best opportunity of efficient water application (Fig. 15). Fields need not be flat and application rates can be easily adjusted through nozzle size, pump pressure, and
 

FIGURE 15. Large center pivot or "circle" sprinkler irrigation used widely for potato irrigation. Acreage covered by each unit varies from as small as 20 acres to over 200 acres.

spacing of nozzles. They also increase versatility through applications of some fertilizers and pesticides. They are, of course, considerably more expensive than furrow irrigation systems; but, many studies have proven their economic advantages. Today, most of the irrigated acreage of potatoes in North America is by one of the sprinkler systems; and the highest yielding areas and fields within given areas are almost always under sprinklers.

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Fertilizers

Importance

Most soils require the addition of one or more of sixteen essential elements to produce satisfactory tuber yield and quality.

Numerous research and demonstration plots have shown the importance of supplying fertilizer nutrients to the potato crop. It is not uncommon to increase production of potatoes by 200-300 cwt/acre (22.4-33.6 t/ha) by adding one or more of the essential nutrients. The total amount and balance of the essential elements in the soil are important. Applying a complete fertilizer mix without considering nutrients already present in the soil may result in over-fertilization of a particular element and possible crop injury and is a waste of money and resources.

Potatoes remove large quantities of nutrients from the soil, depending on amount of vine growth and tuber yield. A direct relationship exists between tuber yield and nutrient removal. As the yield increases, nutrient removal increases in a linear manner, with a 600 cwt/acre (67.2 t/ha) yield, removing twice the amount of nutrients are a 300 cwt/acre (33.6 t/ha) yield. Since vine growth varies considerably within areas having similar tuber production, correlation between vine yield, nutrients removed, and tuber yield is poor. Average values for nutrient removal by the 'Russet Burbank' potato are listed in Tables 15 and 16.

TABLE 15. Amount of nutrients removed from the soil by potato tubers.

Source: R. Kunkel, Washington State University

TABLE 16. Amount of nutrients removed from the soil by potato vines.

Source: C.G. Painter, University of Idaho

Although some variation among varieties may exist in nutrient removal, these values can be used to estimate nutrient removal by potatoes in most areas.

The availability of nutrients in individual soils from different areas is influenced by soil development, crop rotation, fertilizer use, irrigation water, and crop residue returned. Therefore, only general information on potato fertilization will be given. Local advice in soil fertility problems is available through university and/or government offices.

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Kind and Amount

Fertilizer recommendations based on plant and soil analyses are the best means of arriving at a sound soil fertility program. Emphasis should be placed on use of soil tests that have been calibrated for the specific areas where the potatoes are being grown. A history of petiole analyses taken during the growing season, coupled with soil analyses, will provide the most accurate prediction of fertilizer needs for a specific field. A history of production level compared to changes in soil nutrient levels as determined from soil samples can also be a useful management tool.

Soil and plant testing services are available through university, government, or commercial laboratories. Laboratories in your growing region will give the most desirable results. Laboratory methods and/or interpretation of results in one area may have little value in diagnosing nutrient needs for potatoes grown in another area.

Nitrogen fertilizer will be needed on most soils to produce a profitable yield of potatoes. Soil having a high amount of nitrate nitrogen from previous fertilizer applications, green manure crops such as alfalfa, peat or muck soils, and where high rates of livestock manure have been applied will require less fertilizer nitrogen than other soils. Excess nitrogen levels decrease tuber quality, grade, and yield.

Phosphorus fertilizer will also be needed on most soils to produce a good crop of potatoes. Soil analysis can generally be used to identify large amounts of residual phosphorus from past fertilizer use and indicate where phosphorus may not be needed for the present crop.

Potassium is required on many soils but this need varies considerably. Cropping practices, past fertilizer use, soil parent material, and source of irrigation water are the main factors affecting the need of potassium fertilizer. In some areas, 10-40 pounds (4.5-18 kg) of potassium per area-foot of irrigation water are being applied along with other nutrients such as nitrogen, sulfur, magnesium, and calcium.

Supplemental calcium and magnesium will be needed in some acid soils where leaching of nutrients has occurred. Soil analyses can usually identify these needs. Sulfur is frequently required where irrigation water does not contain large amounts of this nutrient, where soils are naturally low in sulfur, and where sulfur has not accumulated from previous applications.

Zinc and manganese may be needed in some soils such as the calcareous, alkaline soils in the Northwest. Boron, iron, and copper levels in soils are usually sufficient in most potato growing areas. Certain mucks and peats may be deficient in copper.

The amount of nutrients needed to be applied as commercial fertilizer will depend not only on the level of available nutrients in the soil but on the potential yield as governed by such factors as: variety, seed spacing, moisture, climate, diseases, and insects. On some low fertility soils where 400-600 cwt/acre (44.8 - 67.2 t/ha) of potatoes are produced, 200-400 pounds each of nitrogen, phosphate, and potash/acre (224 - 448 kg/ha) are used. In areas having short growing seasons, low moisture, and yields of 150-200 cwt/acre (16.8 - 22.4 t/ha) only 40-60 lbs/acre (44.8 - 67.2 kg/ha) of N are removed by the tubers. If higher than optimum rates of phosphate or potash are applied, a buildup will occur.

Micronutrients such as zinc and manganese may be soil applied with other fertilizer. Rates of 5-10 lbs/acre (6 -11 kg/ha) of zinc should be sufficient to take care of three years' production of potatoes.

Plant analysis is used in many potato producing areas to diagnose nutrient needs or deficiencies during the growing season. Plant comparison between healthy and unhealthy plants will help determine if the problem is a nutrient deficiency.

Source

The common forms of nitrogen fertilizer are, nitrate, urea, and ammonium. Potato plants can utilize either the nitrate or ammonium form in the soil. The nitrate form is more subject to leaching, thus in irrigated areas where an occasional heavy rainfall occurs, or in heavy spring rainfall areas, pre-plant or planting time applications of ammonium nitrogen will probably be most efficient.. Post-plant nitrogen applied through overhead irrigation systems should be in the nitrate and/or urea forms, since they are less damaging to the irrigation equipment than is the ammonium form.

All of the common carriers of phosphorus fertilizers give satisfactory results in potato production. These include treble super phosphates and ammonium phosphates.

Potassium is usually supplied as either potassium sulfate or potassium chloride. Some evidence supports the use of sulfates, as chloride appears to reduce specific gravity and produce lighter colored skins on tubers of russet varieties than does the sulfate form.

Zinc and copper can be applied as sulfates or as chelates. Manganese should be applied as sulfate. Calcium and magnesium are generally supplied by lime applications. Calcium and magnesium sulfate sources are also available. Sulfur can be applied in elemental form or as a sulfate.

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Time and Method of Application

As a general rule, fertilizer applied more nearly to time of plant need is used most efficiently. However, time of fertilizer application and placement are also a matter of personal preference, convenience, and the availability of materials and equipment.

Part of the nitrogen and all of the other fertilizer nutrients should be applied at or before planting. If leaching is not a potential problem, all of the nitrogen can be applied pre-plant with the other nutrients. In some spring planting areas, phosphate, potassium and zinc fertilizer can be effectively applied in the fall if there is little or no fall and winter precipitation. Banding of ammonium nitrogen will increase the uptake of phosphate, zinc, and manganese. This can help correct deficiencies on some calcareous soils but can accentuate manganese toxicity in acid soils.

Pre-planting broadcast fertilizer applications have the advantage of more rapid application and the elimination of fertilizer handling during the planting operation-thus full time can be devoted to the potato planting itself. Discing or folding fertilizer into the planting beds should immediately follow broadcast applications. Broadcast applications are not satisfactory where soil fixation of nutrients occurs, where the fertilizer is subsequently plowed below the level of greatest root density, where root restriction prevents nutrient uptake from the area where the fertilizer has been incorporated, or where leaching can occur after application.

Nitrogen can be applied with irrigation water or mechanically side dressed during the growing season. Petiole analyses during the early stages of growth season can be used as a basis for adjusting N applied during the growing season to meet the plants' need. Nitrogen should not be applied late in the growing season. Charts showing optimum levels of nitrate nitrogen in petioles at different stages of growth are available in some areas.

Irrigation management, weed, insect, and disease control significantly influence the response to fertilizer applications and the ultimate crop yield. Over-irrigation and nitrogen leaching are hazards on sandy soils. Under these circumstances high rates of nitrogen or midseason applications may be required to supply the crop's needs. A heavy weed population will compete for fertilizer nutrients with growing potatoes and result in reduced yield. Unhealthy potato plants due to either disease or insects will not be able to take full advantage of optimum fertilizer levels, therefore both disease and insect control programs affect response of potatoes to fertilizer application.

Weed Control

An effective weed control program takes into account the primary weed problem, cultivation, available herbicides, and competitive ability of potato varieties. Although weed problems can be quite specific to given production areas, they can be categorized into three main classes: broad-leaved annuals, annual grasses, and perennials. Broad-leaved annuals, with the exception of nightshade (Solarium nigrum L.), are the easiest to control. An application of a pre-emergence herbicide and/or cultivation as the weeds are germinating and emerging provides effective early season control. The most widely distributed broadleaved weeds of concern in potato fields are: lamb's-quarters (Chenopodium album L.), red root pigweed (Amaranthus retroflexus L.), ragweed (Ambrosia artemistifolia L.) and smartweed (Polygonum pensylvanicum L.). Annual grasses such as barnyard grass (Echinochloa crusgalli IL.I Beauv.), foxtail (Setaria sp.), and fall panicum (Panicum dichotomiJlorum Michx.) germinate later than most broad-leaved annuals. Because of the later germination, a pre-emergence herbicide with residual activity or an effective herbicide that can be applied after potato emergence, is needed for control of these weeds. Potato varieties that develop and maintain a dense canopy when grasses are emerging are also beneficial in controlling these weed pests. The most difficult weeds to control are the perennial weed species. The major perennial problem weeds include nutsedges (Cyperus sp.), quackgrass (Agropyron repens [L.l Beauv.), and Bermuda grass (Cynodon dactylon [L.] Pers.). In addition to causing yield reduction and decreasing harvest efficiency, rhizomes of perennial grasses and nutsedges can penetrate potato tubers causing severe reduction in quality. When perennial weeds are the primary problem, more than the standard number of cultivations may be needed for effective weed control, even though herbicides are used. The additional cultivations can enhance the effect of the herbicides by weakening the perennial weeds. Care should be taken not to spread weed rhizomes from infested to noninfected fields.

If herbicides are used, the choice of which one or ones to use must be tailored to the kinds of weeds present and when these weeds germinate. The method and rate of application should be in accordance with the manufacturer's label and local recommendations (Fig. 16).
 

FIGURE 16. The use of appropriate herbicides at the proper timing and rates can help control weeds.

Methods of application vary from pre-plant soil incorporation, post-plant but pre-emergence, to post-emergence applications. Various herbicides can be applied by ground rig, airplane, or through the sprinkler irrigation system.

Specific herbicides, rates and methods of applications vary throughout the United States and Canada. Most agricultural universities regularly publish specific recommendations for each area. Consult your local pest control advisor and/or government office for specific recommendations.

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Insect Control

The control of insects is an important consideration in production management. A large number of insect pests attack potatoes and cause yield and/or quality reduction. Specific control measures for insects should be obtained from a local source.

Specific insect problems vary with production area. However, there are several insect pests that cause varying amounts of damage in most producing areas. Aphids are an example. In addition to direct feeding damage attributable to these insects, the indirect damage, through the spread of potato virus diseases, is more serious. See Table 17 for an understanding of the large number of diseases that are aphid-transmitted. The virus content of seed tubers is especially critical since infected tubers do not produce healthy sprouts and the yield potential of diseased plants can be greatly reduced. Potato leaf roll virus can, in susceptible varieties, also cause tuber net necrosis which makes them unmarketable. Net necrosis is an internal discoloration and is not visible during grading. Aphid control measures include: chemical control, both systemic and contact; the eradication of overwintering host plants such as peach, plum, and other Prunus species; control of weed hosts such as yellow and jimhill mustard; and monitoring of potted nursery plants and flowers to assure that they are aphid-free. Control of aphids does not insure control of potato viruses; but virus control is not feasible without aphid control.

Leafhopper species belong to another group of sucking insects which have wide distribution. These wedgeshaped insects feed on the potato plants and secrete a toxic substance in the process. Feeding of high leafhopper populations will cause a condition known as hopperburn. Characteristic symptoms are an upward curling of the tips and margins of leaves which first appear yellow and then dry to a brown discoloration. Leafhoppers are also vectors of some diseases, such as purple top wilt, witches broom, and curly top.

The most destructive, foliage-eating, potato insect is the Colorado potato beetle (Leptinotarsa decemlineata [Say]). Both adults and larvae feed on potato foliage, but the larval stage is more destructive. If populations are not controlled, leaves will be stripped, leaving only the stalks. The adults are oval-shaped, hardshelled, yellowish beetles with black stripes down the back. The larvae are initially red in color but turn orange as they grow. Two rows of black spots are prominent on the larvae (Fig. 17).
 

FIGURE 17. Colorado Potato Beetle: Larvae are the most destructive, however, high populations of overwintering adults can cause damage to emerging potato plants.

Flea beetles can cause foliar damage to potato plants. Feeding of flea beetle adults appears as small irregular to round holes in potato leaves. The larval stage of the tuber flea beetle, (Epitrix tuberis Gentner), feeds on tubers and can cause a serious reduction in quality. Flea beetle adults are shiny black, small insects that move rapidly when disturbed.

Other insects causing severe tuber defects are potato tuber worm, Phthorimaea operculella (Zeller), and wireworms. The potato tuber worm is primarily a problem in southern producing areas but can be a problem in warm storages in northern states. Even though the larvae cause damage to the growing plant by burrowing in the stems or mining the leaves, the most serious damage is to the tubers. Affected tubers are unsaleable because of the tunnets made by the larvae. Since eggs are laid on tubers, one means of control is to keep tubers from being exposed to the egg laying moths. Proper hilling practices, sprinkler irrigation to prevent soil cracking, and placing tubers in storage or under cover immediately after digging are cultural methods that help control the pest. Chemicals for control of the moths are also available.

Wireworms are larval forms of click beetles. Wireworms are descriptively named since they have wire-like bodies which are jointed and have a brownish color. The larvae feed on developing tubers causing numerous, pencil-point width holes that range in depth from just below the surface to the center of the tubers. Proper crop rotation and pre-plant or planting time applications of approved insecticides are methods of control.

Numerous other insect pests cause damage to potatoes. Many are specific to certain growing areas. Information on insect pests and control measures for a given region should be obtained from the agricultural authorities in that region.

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Disease Control

Numerous potato diseases and disorders are encountered by growers each year. These diseases may be caused by bacteria fungi, viruses, mycoplasms, or may be physiological in nature. Nematodes can also be destructive either by themselves or as vectors of certain viruses.

Table 17 lists the most common diseases and disorders encountered by growers in the United States and Canada; the causal organism; whether it affects the vine, tuber, or root; the most common method of spreading the source of disease from one year to the next; and a brief comment on control.

While prevention and control of the many diseases affecting potatoes vary widely depending upon the specific disease, some practices apply generally to all diseases. These include the following:

1. Use certified seed.
2. Handle seed properly.
3. Follow a regular and rigorous sanitation program.
4. When applying fungicides, follow the instructions on the label as well as the local authorities' recommendations.
5. Irrigate uniformly and adequately, but not excessively.
6. Control aphids, leafhoppers, and nematodes.
7. Harvest and handle tubers gently.
8. Do not harvest when tuber temperatures are below 45F (7C) or above 85F (30C).
9. If storing, provide environment for wound healing, followed by proper temperature, humidity, and aeration.
10. Warm cold tubers in storage before handling.

Production Management

The foregoing description of cultural practices shows that potato production is a complicated process. This complexity is due, in part, to implementing new production technology and to national and international competitive pressures. Environmental concerns are also complicating potato production practices as growers try to reduce detrimental impacts to the soil and water resources. Finally, consumer demands for safe and high quality fresh and processed potatoes further adds to the complex nature of potato production.

To remain competitive and profitable, growers will have to deal with the increasingly complex nature of potato production. This will require the development of integrated management strategies which will increase production efficiency while protecting and sustaining soil and water resources. A part of this strategy will consider reducing purchased inputs such as fertilizer, pesticides, irrigation, and energy. Reducing inputs can result in reduced costs and reduced potential for environmental impacts. But it also increases the risks of producing potatoes.

An integrated management strategy is based on grower knowledge of potato growth, climatic factors affecting growth, cultural practices used in potato production and how such cultural practices interact to affect potato productivity (yield/quality) and the soil/water resources. Potato growth, climatic requirements and cultural practices have already been described. Recent research and grower experiences have provided considerable information on the interactions between the many cultural practices used in producing potatoes. The interaction of irrigation with other cultural practices provides some excellent examples of how such interactions can impact potato productivity and the soil/water resources.

Irrigation is used in many potato production areas to insure adequate soil moisture for potato growth and tuber development. Its use is essential for potato production on sandy soils which have low available water storage. Over-irrigation, which exceeds crop use and soil water storage, increases the potential for leaching nitrates and pesticides to ground water causing contamination problems. The practice of hilling interacts with irrigation to further increase this leaching potential on sandy soils. In this interaction, hilling changes the water infiltration pattern by causing more water to enter the soil in the furrow area. The significance of this is related to the root system of potatoes. Potatoes are shallow rooted and the density (number) of roots under the furrow is less than that within the hill. Thus water uptake from the furrow is less. Without plant uptake, water infiltrating the furrow becomes a source of leaching water and the potential for ground water contamination increases. Modifying the hill to increase water infiltration into the hill would reduce the leaching impact of this irrigation/hilling interaction.

The leaching problem, associated with over-irrigation and hilling also interacts with fertility practices, especially nitrogen management. Potatoes use relatively large amounts of nitrogen which in the nitrate form readily leaches. Leaching of nitrogen from the root zone can result in nitrogen deficiency and a potential for reduced yields. It also leads to nitrate contamination of ground water. Nitrogen deficiency also results in earlier senescence of the vines. This in turn increases plant susceptibility to early blight which if not controlled can further reduce yields. Irrigation (or other forms of water management) is also associated with other disease problems. Heavy irrigation and/or high soil moisture conditions increase problems with late blight, white mold and aerial blackleg. Lack of irrigation and low soil moisture increases problems with scab and the "early dying complex. Thus, there are now interactions between four cultural practices; irrigation, hilling, nitrogen fertility and disease management. There are more examples, but the above make the point that cultural interactions do increase the complexity of potato production.

Knowledge of these interactions allows the grower to "tailor" a management strategy which maximizes production efficiency and provides environmentally sound cultural practices. Many growers are already developing such strategies. For example, growers will use disease resistant varieties if they adequately meet the market requirements. Pest resistant varieties reduce pesticide use and cost. Growers are placing more emphasis on rotation programs which enhance pest control as well as provide additional fertility. Irrigation scheduling is being used to eliminate over-irrigation thus reducing leaching and energy use.

Adopting IPM techniques is another integration management strategy. Computer software programs are also being developed to assist growers in dealing with cultural interactions and to make effective management decisions. The future will likely see increased use of such computer technology. Whatever the methods used, growers will need to incorporate the impacts of cultural interactions into potato production management strategies.

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