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Commercial Sweetpotato Production in Mississippi

The sweetpotato [Ipomoea batatas (L.) Lam.] is one of the top three vegetable crops in Mississippi. The infestation of the sweetpotato weevil (Cylas formicarius elegantulus) has led to a quarantine for much of the state south of Interstate 20. Commercial production is currently concentrated in Calhoun and Chickasaw Counties in north-central Mississippi although sweetpotatoes are grown commercially in more than half of Mississippi’s counties. They can be grown throughout the state since all areas provide a frost-free period of more than 150 days.

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Varieties

Consider market preference when selecting varieties. Where markets have been developed for certain characteristics, new varieties should incorporate the expected features.

Beauregard was released by the Louisiana Agricultural Experiment Station in 1987, and by 1992 was produced on a major portion of the commercial acreage in Mississippi. It consistently produces a higher yield of marketable roots than other varieties, and storage root growth is faster so that harvest is sooner.

Beauregard has a light-rose skin. It has moderately deep-orange flesh, and is consistent in shape. It is suitable for fresh market and canning. It is resistant to some of the important sweetpotato diseases (Table 4). The moderate level of resistance to soil rot is of special importance. It is susceptible to southern root knot nematodes and is more susceptible than Jewel and Centennial to bacterial soft rot. Beauregard is an excellent sprout producer; however, length of time until the first plants are ready in field beds is longer than other varieties because of slower growth with cool temperatures.

Centennial has a light-copper-colored skin and deep- orange flesh. It produces high yields of smooth, attractive roots suitable for both fresh market and processing. It is tolerant of fusarium wilt and internal cork diseases, but is susceptible to most other diseases and insects. Centennial is a poor sprout producer.

Jewel has a deep-copper-colored skin and orange flesh. It is excellent for baking and is acceptable for canning. Red Jewel is a red-skinned mutant of Jewel and is similar in other characteristics. Jewel is more resistant to fusarium wilt, root knot nematode, and internal cork than Centennial. It is a better sprout producer than Centennial.

O’Henry was released by C.C. Farms, Vardaman, Mississippi, to fill the need for a tan-skinned cultivar with cream-colored flesh. It originated as a single-root selection for off-type skin and flesh color out of Beauregard as a result of somatic mutation. It is similar to Beauregard for most characteristics except root color and produces high yields, comparable to Beauregard.

Several other varieties of sweetpotatoes are grown on small acreages in Mississippi. These serve small, limited demand areas. Varieties are generally old and possess an unusual characteristic that allows this specialization.

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Site Selection

A well-drained soil is the most important element in sweetpotato production. The most productive soils are sandy loam and silt loam soils underlaid by finer textured subsoils. Do not use soils that will stand in water for 24 hours.

Do not plant sweetpotatoes on areas just broken from sod or pasture. Land that has had a clean-tilled crop such as cotton or soybeans the previous year can be used if there are no herbicide carryover problems. Avoid excessive amounts of organic matter.

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Soil Preparation

Take soil samples at least one month before bedding, or 10 weeks before the anticipated planting date. Results will tell the pH of the soil and how to adjust it. Use dolomitic limestone to increase pH if soil magnesium levels are low. Plow the soil 6 to 8 inches to incorporate limestone or to loosen topsoil. Make beds at least 8 inches high and as wide as equipment will allow. Narrow beds dry quickly and may reduce yield. High beds will aid in preventing excess water damage.

Avoid soils that have had sweetpotatoes grown on them in the previous 4 years. Several of the soil-borne insects and diseases have no effective means of chemical control. Avoid fields with a history of morning glory problems for two reasons: there is no effective control of morning glory in sweetpotato, and morning glory can be a host for the sweetpotato weevil.

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Fertilization

The best return on investment in agriculture is the soil test. Follow the recommendations that come with your soil test results. Sweetpotatoes have a high requirement for potassium. You can band fertilizer materials in the bed so the bands are at least 5 inches from the slip. Apply fertilizer material before bedding or at bedding so the fertilizer is distributed throughout the bed. Keep fertilizer material away from the transplanted slip.

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Plant Production

Sweetpotatoes are started in the field from transplants, known as slips or draws. These are produced in plant beds from seed stock. The choice of seed stock influences the success of the sweetpotato crop. Seed stock roots are selected from the previous year’s crop. Select roots for seed stock that have these characteristics: true to varietal type, free from disease or insect damage, firm orange flesh, bright skin color, and free from veins.

Virus-tested foundation seed stock is available from the Pontotoc Branch Experiment Station. Virus-tested plants are produced by taking extremely small tip explants and culturing them on media in test tubes (micropropagated). The plants are then tested for the presence of viruses at least three separate times by grafting to Brazilian morning glory, the indicator plant for sweetpotato viruses. The sweetpotato plant is considered virus tested if no symptoms develop on the indicator plant at any time; however, plants cannot be designated virus free since it is impossible to prove that no virus is present.

Cuttings from these plants are then evaluated in field tests for trueness to variety. Those meeting all standards are nuclear stock plants. Foundation virus-tested plants are cuttings from nuclear stock plants produced in a screened greenhouse under strict isolation in which only virus-tested plants are grown. These plants and greenhouse plants produced from them are generation zero (G0) and are used to establish field plantings to produce generation 1(G1) virus-tested seed stock. G2 virus-tested seed stock is that produced by plants from G1 seed stock.

Improvement in yield and quality from virus-tested seed stock is dependent on maintaining strict conditions of isolation and management during G0 plant production and subsequent seed stock production. Approval of adherence to virus-tested certification standards by the Mississippi Seed Improvement Association is required for virus-tested designation of seed stock. A buyer should require certification before purchase of virus-tested seed stock.

Growers should obtain foundation seed every 4 years to maintain uniformity and high quality.

Harvest the seed crop before any danger of frost. Discard all damaged roots or roots less than 1 1/2 inches in diameter. Select hills for seed that produced at least four No. 1 potatoes.

Seed stock produced from vine cuttings are less likely to transmit diseases. To produce vine cuttings, select healthy shoots, cut them one inch above the soil surface, and hold 24 to 48 hours in an upright position to promote rooting before transplanting.

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Plant Beds and Bedding

Heated beds were used in the past for early plant production, but most transplants are now produced in field beds. Hoop-supported plastic field bed covers are used on a limited basis for early plants. Regardless of type of bed used, seed stock should be presprouted before bedding to increase plant numbers and earliness. Holding seed stock at 75-85 °F for 2 to 4 weeks with ventilation is optimum. Length of time needed will vary depending on the specific conditions of storage, but growers should presprout until most roots have sprouts one-fourth of an inch long.

Heated Hotbeds

Even though use of heated beds has declined, electrically heated hotbeds are preferred because of uniform heating and ease of construction. Gas or other fuel heats distribute heat unevenly and lead to uneven plant development.

Heated beds are constructed with masonry or cement head and foot walls with treated wood used for sidewalls. The bed can be all wood construction, but masonry and cement last longer and require less maintenance. Do not use creosote-treated wood as the creosote will delay plant growth.

Figure 1 shows a typical plant bed. The head, or north wall, is 18 inches above the level of the heating cable. Side walls slope to the foot, or south wall, at 1 inch per foot. If the bed is the normal 6 feet wide, the foot wall will be 12 inches above the level of the cable.

If drainage is a problem, excavate the area to an 8-inch depth and put in a layer of gravel covered with burlap or some other porous material. On top of this material, lay 2 inches of sand.

The heating cable is the next step. Cable rated to deliver 10 watts per square foot of bed has been adequate in Mississippi.

Place the cable on the soil or sand in loops. Spacing between loops is important. Determine proper spacing by the following formula:

Spacing (in inches) = 12 × watts per foot
of cable/wattage required per foot of bed.

The spacing between the wall and the outside cable is one half of the loop spacing. Be sure that the cable does not cross itself and has no kinks. After the cable is in position, cover it with a 2-inch layer of loose sand. Then place a 1/2-inch-mesh hardware cloth on top of the sand. Have the cable hooked to a proper power supply by a qualified technician. Make all connections to the heating cable watertight. Place a thermostat in the circuit. Place the temperature sensing part of the thermostat at the same depth as the seed stock. The thermostat must be two-stage to turn on and turn off the heating cable.

Place seed on top of sand in one layer, packed as closely as possible without touching. Cover roots with at least 3, and preferably 4, inches of sand into which 8-8-8 or a similar fertilizer has been mixed at the rate of 4 pounds per 100 square feet. Lightly water the covering sand to settle the soil. Cover plant bed with clear plastic. Reinforce the plastic sheets by placing 1- by 2-inch sash supports from head to foot walls and by using light plastic or wooden strips to provide a framework.

Each bushel of seed stock will produce 1,000 to 1,500 slips in 10 to 15 square feet of bed. Each acre requires 10,000 to 12,000 transplants; therefore, about 185 square feet of bed space are required for an acre’s worth of slips.

Maintain temperature in the bed at 80 to 85 °F. The temperature must always remain above 55 °F or root breakdown will occur, causing a loss in slip production. If the temperature rises above 85 °F, open the bed to provide ventilation.

Keep bed moist, but not wet. Overwatering will cool the soil and will increase the incidence of seed bed diseases. Harden slips a few days before cutting by reducing the amount of water, but do not allow plants to wilt.

Plants will be ready for cutting within 6 weeks of planting the seed. Vine cutting is recommended rather than pulling slips to reduce the incidence of potato-borne disease in the production field. Make cuts at least 1 inch above the bed surface and trim slips to 10 to 12 inches.

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Transplanting

Do not transplant sweetpotato plants until all danger of frost is past and the soil temperature at a 4-inch depth
is 65 °F. This is usually late April or early May in most of Mississippi’s commercial areas. Sweetpotatoes can
produce adequate yields when planted as late as July, but highest yields are made when planting is complete by June 15.

Plant slips 12 to 16 inches apart in the beds previously described. Apply 4 to 8 ounces of water to each slip.

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Water Requirements

Although sweetpotatoes are drought tolerant, they need adequate moisture throughout the growing season. During dry years, supplemental irrigation significantly increases sweetpotato yield and quality. Dry periods retard growth and reduce root yield by causing deformed, smaller roots. Uneven and excess moisture cause excessive vine growth and elongated roots. Throughout the growing season, provide 1 inch of water per week from stored soil moisture, rainfall, or supplemental water.

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Weed Control

Few effective herbicides are labeled for use in sweetpotatoes. The most important weed control measure is to choose a field that does not have a history of serious weed problems. Use cultivation to control weeds during early stages of growth. Sweetpotato vines can compete with most weeds after the root system becomes established. See Table 1 for herbicides labeled for weed control.

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Harvesting

Sweetpotatoes can be harvested any time after roots reach marketable size. The earliest varieties such as Beauregard may be ready for harvest in 90 days. Most varieties are ready for harvest in 120 to 130 days.
Delaying harvest leads to a high percentage of jumbo or over-sized roots that have a lower market value. Dig sweetpotatoes before first frost or before soil temperature falls below 55 °F. Chilling injury occurs at temperatures below 55 °F. Sweetpotatoes dug during warm, fair weather will be easier to dig, grade, and store.

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Economics

Since each sweetpotato producer has something unique in his operation, it is difficult to find an average cost of production or profitability statement. Table 2 is an estimate of costs and returns from an acre of sweetpotatoes.

Use the estimates as a guide to determine costs and returns from sweetpotatoes. Only the expense record of an individual grower and the price received can determine the profitability of each crop.

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Marketing

Marketing can be the most difficult part of making money from producing sweetpotatoes. The first step in marketing is field grading of the roots to remove damaged and diseased potatoes. Sell roots for processing immediately after harvest if possible. Size roots immediately and place fresh-market potatoes into storage. Roots between 1 and 1 3/4 inches in diameter and at least 5 inches long are used for canning. Fresh-market potatoes are between 1 3/4 inches and 3 1/2 inches in diameter and between 3 and 9 inches in length. Anything larger is called a “jumbo.”

Fresh-market potatoes have three grades: U.S. Extra No. 1 with a weight ceiling of 18 ounces; U.S. No. 1 with a weight ceiling of 20 ounces, and U.S. Commercial with a weight ceiling of 36 ounces. Most Mississippi sweetpotatoes are marketed as U.S. No. 1.

Roots are removed from storage as needed, washed, graded, and sometimes waxed before marketing. Some scarring and bruising can be eliminated if bins are dumped into a holding tank containing 50 ppm chlorine water rather than a hard surface.

Treat roots with dichloron (Botran) following label directions to prevent soft rot during the marketing process.

Sweetpotatoes may be sold locally by the grower. To participate in more extensive markets, however, the producer may want to deal with a sales agent or broker. A sales agent works exclusively for the seller, while a broker may work for the buyer or seller. Brokers are responsible to the party who is paying the fee.

Another option available to the producer is to supply sweetpotatoes over an extended period to grocery stores or supermarket chains. In general, these buyers request a specified volume of product over a length of time. These arrangements work well for the producer with storage facilities.

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Table 1. Recommended weed-control herbicides for sweetpotatoes

Herbicide	Broadcast rate per acre	Weeds controlled	Time of application	Limitations, remarks
clomazone Command 3ME	0.75-1.0 lb ai 2.0 to 2.67 pt	See Table 3	Preplant incorporated or within 5 days after transplanting	Do not apply less than 90 days before harvest. Do not apply within 1,500 feet of towns and subdivisions, commercial fruit, vegetables, nurseries, or greenhouses.
DCPA Dacthal W-75	4.5 to 10.5 lb ai 6 to 14 lb	See Table 3	At transplanting or up to 6 weeks after transplanting	Apply to weed-free soil. Thorough agitation required.
fluazifop-p Fusilade 2000 1 lb/gal	0.94 to 0.25 lb ai 0.38 to 1.0 qt	Grasses	After grass emergence	Add either crop oil concentrate at 1% or nonionic surfactant at 0.25%.
glyphosate Roundup 3 lb/gal	0.5 to 2.0 lb ai 1.33 to 5.0 pt	See Table 3	Before crop emergence	Adjust rate to weed species present. At time of application, avoid contact with desirable plants.
sethoxydim Poast 1.5 lb/gal	0.09 to 0.18 lb ai 0.5 to 1.0 pt	Grasses	After grass emergence	Use 2 pt crop oil concentrate per acre. Do not apply within 30 days of harvest.

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Table 2. Estimated costs per acre, sweetpotato, 2-row harvester, custom pack, Mississippi, 2000.

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Table 3. Weed-response ratings for herbicides

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Recommended rates for your soil type or weed species: E = 90% control or better; G = 75-90% control; F = 50 - 75% control; P = 5 - 50% control; N = less than 5% control; L = labeled; and - = unknown.

Note: Always check current recommendations to be sure the herbicide is registered for the crop in question.

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Insects

Insects may feed on sweetpotato foliage or on developing roots. Foliage damage will appear as holes in leaves; however, foliar damage is minor, and treatments are rarely needed except in outbreak situations with large populations of leaf-feeding caterpillars. Damage to roots is another matter because price reductions per bushel can be significant. Damage is often caused by both adult and immature forms. Some adults feed on leaves, while their larval forms attack the roots. Included in the list of pests in Mississippi sweetpotatoes are sweetpotato weevils, wireworms, white grubs, flea beetles, cucumber beetles, whitefringed beetles, cutworms, and other Lepidoptera. Insect injury is often similar in appearance, and it is difficult to make an accurate identification based on injury symptoms alone.

Sweetpotato Weevils

The persistent occurrence of this insect is south of a line that roughly parallels Interstate 20 across Mississippi. Infestations are occasionally found as far as 20 miles north of this line. The insect is a limiting factor in commercial sweetpotato production south of I-20, so this area is under quarantine. The Bureau of Plant Industry maintains sweetpotato pheromone traps throughout the state to monitor the weevils and maintain the weevil-free area.

Adults are ant-like beetles about one-fourth inch long. The head and wing covers are metallic blue-black, and the thorax and legs are reddish orange. They will feed on leaves and stems but prefer roots.

Numerous small holes that occur in groups on the root surface are signs of root feeding. Damage is caused by the grub. It begins when a single egg is placed in a small hole eaten out by the female. Several eggs may be deposited within one sweetpotato. Eggs hatch in about 4 days at summer temperatures. The young larvae (grubs) begin to tunnel into the sweetpotato with tunnels increasing in size as the grubs grow. This feeding makes the sweetpotato unfit for consumption because of the presence of larvae and because of frass in the tunnels. The weevils also impart a strong flavor to the sweetpotato. Weevil grubs feed within the sweetpotato for about 16 days at summer temperatures. They then pupate and emerge as adult weevils in another 4 to 7 days.

The sweetpotato weevil is of tropical origin and does not enter a true hibernation phase during the winter. Weevils can be found crawling about on warm days and must have a food source during their active times. Larvae feed during this time as well and must have some protection from cooler temperatures. They can survive in storage houses and in partially buried sweetpotatoes during warm winters. It is doubtful that the insect can survive in damaging numbers outside of such protected sites.

Control. Insecticides often prove inadequate alone, so a rigorous cultural control program helps to keep this pest in check. The program includes the following recommendations:

• Use certified seed potatoes for slip production, and plant potatoes into an insecticide-treated bed.
• Monitor beds, and spray them regularly for insect pests.
• When harvesting slips, cut them at least 1 inch above the soil line. Never pull slips!
• Destroy the bedding area immediately after slips are harvested.
• Clean storage areas in the spring after seed potatoes are removed. Completely remove and destroy all sweetpotato residue.
• After digging in the fall, be sure that cull sweetpotatoes are exposed to winter temperatures and rain. A mechanical digger usually is adequate for this process.
• Use a protectant while sweetpotatoes are in storage.
• Spray storage houses at least 2 weeks before harvest.

See Extension Publication 2471 Insect Control Guide for Agronomic Crops for recommended insecticides for sweetpotato weevils. Although producers north of I-20 do not have this pest, every effort should be made to keep the area free of weevils. Use extreme care when bringing in seed potatoes, especially if they come from areas (in other states) that may have this pest. Routinely monitor storage houses and seed beds for weevils or weevil damage, and follow recommended treatments.

Wireworms

Currently, wireworms are the most damaging insect pests of Mississippi sweetpotatoes. Damage generally increases in intensity as the season progresses, with earlier harvested sweetpotatoes often exhibiting little damage, progressing to a heavily damaged crop later. Wireworms are immature click beetles. These beetles constitute a large group of insects; however, only about three species in Mississippi may damage sweetpotatoes. These are the southern potato wireworm, the tobacco wireworm, and the gulf wireworm. All three have similar adult, larval, and damage characteristics. Thus, description and biology generally apply to all three. Adults of the southern potato wireworm and the gulf wireworm are uniformly dark brown. The tobacco wireworm is similar but with more pattern on the back. Adults do not feed on the plants but may be found near the soil surface under leaves and trash. They are about one-fourth inch long.

Eggs are laid in the soil in June and July and hatch in about 5 days. The larvae have a variety of color patterns ranging from white to cream or yellowish cream. The head and tail of the larvae are a much darker reddish brown. Under ideal conditions (temperature, soil moisture, food supply, etc.) the larval stage may last only 30 days. During years of prolonged dry weather, the cycle may last 340 days. The southern potato wireworms will usually have two generations per year. The tobacco wireworms and gulf wireworms have only one generation per year.

Table 4. Wireworm life cycles

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sept

Oct

Nov

Dec

spw

L

L

L/P

L/P

E/L/A

E/L/A

E/L/A/P

E/L/A/P

E/L/A/P

L

L

L

tw

L

L

L

P

P

E/L/A

E/L/A

E/L/A

L/A

L

L

L

gw

L

L

L

P

P

E/L/A

E/L/A

E/L/A

L/A

L

L

L

L = larvae; P = pupae; E = egg; A = adult spw = southern potato wireworm; tw = tobacco wireworm; gw = gulf wireworm

Southern potato wireworms usually attack sweetpotatoes late in the season. Injury from wireworms is generally caused by larvae that are hatched in the current year of damage. Tobacco and gulf wireworms usually pupate before roots swell in the spring. Injury consists of irregularly shaped holes. The original holes are usually less than one-fourth inch deep and seldom are as much as one-half inch deep. If the feeding site penetrates the outer surface of the sweetpotato, the depth of the scar will increase as the root enlarges, thereby giving an impression of greater depth and feeding. Newer feeding sites (late season) will have ragged edges, and they will usually contain frass and chewed root fiber. One species of click beetle, Melanotus communis, is reported to feed much deeper into the root than other species. This beetle is said to occur in Mississippi; however, its presence has not been confirmed.

Control. Soil-applied preplant insecticides are commonly used for management of these pests. Always combine the use of insecticides with good cultural practices:

• Harvest the crop as early as possible.
• Destroy crop residue in the fall.
• Deep plow to expose roots to cold temperatures and rain.
• Do not rotate to sweetpotatoes behind grass crops - especially corn, grain sorghum, or pasture.

See Extension Publication 2471 Insect Control Guide for Agronomic Cropsfor recommended insecticides for management of wireworms.

Flea Beetles

These insects have a wide range of hosts, including many weeds. They are usually the first pest seen in Mississippi sweetpotatoes in the spring. Adults move into fields soon after transplanting. Flea beetles that feed on sweetpotatoes (both foliage and roots) are the elongated flea beetle, the pale-striped fleabeetle, Systena frontalis, and the sweetpotato fleabeetle. These beetles have a portion of the hind leg enlarged, and they jump from place to place when disturbed. This jumping action is similar to that found in fleas, thus their name.

Both the elongated fleabeetle and pale-striped fleabeetle are about one-eighth inch long. They are black with two lengthwise white stripes down their backs. The pale-striped fleabeetle is not as dark as the elongated fleabeetle on the front portion of the body. The Systena frontalis does not have the lengthwise white stripes, and the sweetpotato fleabeetle is solid black but is only about one-sixteenth inch long. Foliage damage caused by the beetles may vary from holes eaten in leaves, to narrow channels, or to grooves eaten into the upper leaf surface.

Adults move into sweetpotato fields in spring and summer, feeding at first on young plants. They lay cream to yellow colored eggs in the soil. The eggs hatch in 7 to 10 days onto white larvae, which later become pale yellow to salmon colored, depending on their food, and they migrate to the roots of the plant. The larvae mature in about 20 to 30 days and then form a pupal cell and emerge from pupae in about 7 days. There are at least two generations per year in Mississippi of most species. These pests spend the winter as adults in sheltered areas in ground litter on field edges, hedgerows, and woods lines.

Immature forms of elongated fleabeetle, pale-striped fleabeetle, and Systena frontalis are all soft, white to pinkish larvae. They are about three-eighths inch long at maturity. They eat small holes through the skin of the sweetpotato root and make enlarged cavities with short tunnels under the skin. Late-season feeding by these insects results in a number of small “pinhole” punctures in the roots. Feeding by sweetpotato fleabeetle larvae causes a narrow winding scar on the surface of the root.

Control. Early season spray applications of insecticides are sometimes effective in reducing these pests, but the wide variety of alternate hosts probably negate much of the foliar insecticide activity. Preplant and midseason applications of soil-incorporated insecticides help to protect crops from these pests.

Scout freshly transplanted fields for the presence of these pests, and make timely insecticide applications to developing populations. Setting a threshold for the late season may hold some promise in preventing root damage. See Extension Publication 2471 Insect Control Guide for Agronomic Crops for recommended insecticides for management of fleabeetles.

Cucumber Beetles

Two species of cucumber beetles that may cause problems in sweetpotatoes in Mississippi are the banded
cucumber beetle and the 12-spotted cucumber beetle. The banded cucumber beetle is more prevalent in south Mississippi. The spotted cucumber beetle is universal. The banded cucumber beetle is about one-fifth inch long and is marked with alternating green and yellow bands. The spotted cucumber beetle is greenish-yellow with 12 black spots.

These pests overwinter as adults, and females will generally begin laying eggs as soon as it is warm enough in the spring for plants to become green. Eggs are laid just below the soil surface and hatch in 1 to 3 weeks, depending on temperature. Larvae, again depending on temperature, will mature to pupation in 2 to 4 weeks. Pupae are formed in cells about 1 inch below the soil surface, and adults emerge in 1 to 2 weeks. There may be as many as three to four generations of spotted cucumber beetles and six to seven of the banded. The larvae of the two species are similar and are difficult to separate in the field. They are white to pale yellow with dark heads and a brown spot at the back of the body. Depending on environmental conditions and food availability, the larval phase will last about 20 days. Cucumber beetles eat small round holes into the skin of sweetpotato roots. There may be a slight cavity beneath the skin, but there is no tunnel going into the root. Cucumber beetles may attack sweetpotatoes early in the season, so there is often a “healed-hole” injury.

Control. Cucumber beetles have many host plants so are not easily confined to one plant, making control more difficult. Early tillage in sweetpotato fields helps to destroy those stages that are in the soil at planting, but alternate hosts provide additional pests for re-infestation. Soil treatments of insecticides at planting also keep early generations of cucumber beetles from developing in sweetpotatoes. Sprays timed to adult peak infestations later in the season will help reduce damage to roots. Regular scouting for beetles using bug-vac or sweepnets easily identifies peak infestations, which can then be treated with insecticides. See Extension Publication 2471 Insect Control Guide for Agronomic Crops for recommended insecticides for management of cucumber beetles.

Grubs

A number of species of May/June beetles are pests of grass and other crops. Included in this list are white grubs, which are also pests of sweetpotatoes. Phyllophaga spp. are most often the species found in sweetpotatoes. These beetles are night flyers, which come readily to lights. The adults are leaf feeders, usually on deciduous trees and weed hosts. They lay their eggs in the soil in the early spring and summer under larval host plants. The fleshy grubs hatch and begin feeding on plant roots. They are usually white to creamy colored with tan head capsules and most often are typically “C” shaped. The last segment is usually darker than the rest of the body. Grubs gouge tracks - broad shallow areas- in the sweetpotato roots, usually down the body of the root. They feed and develop on roots until cool weather comes and then descend deeper into the soil where they overwinter. In early spring, they transform into pupae and emerge in May and June (thus their name May/June beetles).

Damage from these pests is often confused with that of cutworms. Grub damage is usually deeper down the root than cutworm damage and is not as deep into the root. Grubs tend to feed upside down.

Control. Tillage, especially early tillage, destroys habitat, preventing many beetles from surviving in sweetpotato fields. Thus, deep plowing early is a definite positive factor in reducing in-field populations of white grubs. Some grubs may be as deep as 10 inches in the soil and will not be eliminated by tillage. Preplant incorporated insecticides also kill grubs and adult beetles as they come to the surface through the band of insecticide. Mid-season applications of insecticide also help to protect roots from reinfestation of white grubs. See Extension Publication 2471 Insect Control Guide for Agronomic Crops for recommended insecticides for management of grubs.

Whitefringed Beatles

In recent years, whitefringed beetles have appeared as pests in sweetpotato fields. These pests are also classified as grubs and have many of the same characteristics as white grubs, but there is enough difference that they deserve attention. Adults are dark gray beetles, about one-half inch long, with a broad, short snout. The margins of the elytra (hard top wings) are banded with white. The elytra of adults are grown together so they cannot fly. Larvae are yellowish-white, legless grubs with brown heads. They are about one-half inch long.

Large larvae overwinter in the upper foot of soil. They pupate from late May through July. Adults are active from early May until mid-August. No males have been found. Females reproduce parthenogenetically (asexual reproduction). Egg-laying begins 10 to 12 days after females emerge. Each female crawls from plant to plant laying hundreds of eggs in gelatinous masses of up to 60 eggs each on objects on or near the ground. One female may lay up to 3,200 eggs. Eggs hatch in about 2 weeks. Emerging larvae feed below ground until cold weather, when they overwinter. There is one generation per year. Generally, damage from whitefringed beetle grubs is deeper in the soil than other grub damage, and the furrows are not as broad.

Control. Control larval populations with soil applications of insecticides before planting. Adults are easily killed by insecticides. These pests also have many hosts and will often invade from field edges where “goatweed” and other weed hosts allow them to build. Scout field margins and treat field edges when those areas have beetles building. See Extension Publication 2471 Insect Control Guide for Agronomic Crops for recommended insecticides for management of whitefringed beetles.

Cutworms and other Lepidoptera

A number of moths lay eggs in sweetpotatoes, and resulting larvae often cause stand reduction, foliage loss, and direct damage to roots. Cutworms, beet armyworms, southern armyworms, yellow-striped armyworms, hornworms, and others are often pests in sweetpotatoes during the year. Cutworms are probably the most serious of the Lepidopterous pests. The moths lay eggs on weeds in weedy fields before tillage. When fields are tilled in preparation for planting, weed hosts are destroyed, but caterpillars are not. If transplanting is completed too quickly, cutworms begin feeding. Moths may also lay eggs in the crop soon after slips are planted, especially when the weather is cool. The developing larvae feed on slips, cutting stems of the young plants or leaves from larger plants. If the infestation is present before plants begin to run, stand reduction can be severe. Later infestations on running plants are not as damaging to the crop. Late-season damage is also very destructive. The larvae feed on crowns of sweetpotatoes, gouging tracks much like those of the white grubs. In dry years, larvae will get into the cracked soil around sweetpotatoes and actively feed lower on the roots.

Cutworms are categorized as surface, climbing, army, or subterranean. Because of their many similarities, it is often difficult to tell one species of cutworm from another. Some cutworm caterpillars cut off young plants and stunt or kill taller plants by feeding on underground roots and stems. Larvae are nocturnal, hiding in the soil in daylight hours except on dark, wet days. Cutworms overwinter in the late larval or pupal stage. The brownish gray moths may appear shortly after warm weather arrives. The moths are attracted to weedy fields. They may also lay eggs in weedy, overgrown fields before spring tillage. Transplanting should be delayed by 3 weeks after tillage to avoid injury by emerged larvae.

Once fields are planted, young larvae feed on small roots until they are half grown, or about three-fourths inch long. After that, they become more likely to cut off plants at ground level. They may drag part of the plant into the soil where they hide during the day. The larvae can usually be found hiding in the soil in the daytime near a cut plant. The thick-bodied larvae are normally dark colored and curl when disturbed.

Cutworms develop through five to eight larval instars. Pupation occurs in the soil. The number of generations depends on latitude. Generally, there are four generations per year in Mississippi.

Control. In general, soil-applied insecticides have little effect on these pests. Tillage at least 3 weeks before transplanting to remove all weeds from fields will greatly reduce the presence of cutworm caterpillars. Regular scouting of fields after planting consisting of examining plants and surrounding soil will identify the problem easily. Most of the sweetpotato recommended insecticides are effective against these pests. See Extension Publication 2471 Insect Control Guide for Agronomic Crops for recommended insecticides for management of cutworms.

Other Pests

Crickets are often pests of sweetpotatoes, especially in dry years when the soil is cracked around developing roots. Feeding resembles that of the cutworm in late season. Crickets crawl down into the cracked soil around the roots. They feed on cutworm-damaged roots, expanding the damaged area. Applications of insecticide are not always as effective in controlling these pests because they are protected by the canopy and covered with soil.

Other Lepidoptera, especially armyworms, loopers, and often Heliothines will also feed on leaves and can cause defoliation during the time that roots are growing. Apply insecticides to control foliage feeders when defoliation exceeds 15 percent and when more than 10 caterpillars per 100 feet of row have been collected in a bug vac.

Tortoise beetles and other leaf-feeding beetles also occasionally damage sweetpotatoes, but the damage is light and usually late enough in the season that it is of little consequence.

Other insects that damage sweetpotatoes are the sweetpotato leaf beetle and members of the ant-like flower beetle group. The sweetpotato leaf beetle immatures cause damage similar to that caused by the sweetpotato weevil.

Control. With the exception of wireworms, most damage is caused by larvae that develop from eggs deposited during the current production year. If it is extremely dry during any given year, there could be some carryover of wireworms from one production season to the next.

For early season protection, use preplant incorporated insecticides. However, with sweetpotatoes, the last 30 to 60 days of production can be critical, and most preplant materials will have dissipated by this time.

For late-season protection, it is essential that a layby soil-incorporated insecticide application be made. Over-the-top granules and spray directed to the soil will provide protection for the remainder of the season. These materials must be watered or cultivated into the soil. In recent years, there has been a shortage of materials available for application in this manner and late-season damage has resulted. Newer materials are being tested and will be labeled shortly. These should give added protection to sweetpotato crops during the critical root-development phase of production. Timely applications of foliar materials will also help by killing adults before they can deposit their eggs

Consult Extension Publication 2471 Insect Control Guide for Agronomic Crops for recommended insecticides for management of sweetpotato pests and when to apply them.

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Diseases

Sweetpotato diseases caused by fungi, bacteria, nematodes, and other microorganisms are responsible for reductions in crop yield and quality each year in Mississippi. Some diseases occur in the field; others occur during storage, handling, and marketing.

Although diseases can never be completely eliminated, a preventative disease control program will substantially reduce losses.

General Control Procedures

Field Diseases

• Plant in fields where sweetpotatoes have not been grown in 3 to 4 years. Rotate sweetpotatoes with crops that are not affected by sweetpotato diseases.
• For plant production, use seed stock produced from vine or bed cuttings. Seed stock should be free of disease and selected for type.
• Store seed pieces in new or disinfected containers away from other sweetpotatoes.
• Clean and treat plant production beds and bedding media with registered fungicides or fumigants.
• Treat seed pieces at bedding time with registered fungicides.
• Use resistant varieties when available.
• Control insects that may spread the disease in the field.

Storage Diseases

• Store only sweetpotatoes that have been carefully dug and handled, are as free as possible from disease, and have been placed in clean containers.
• Store in clean, properly constructed storage houses.
• Cure fresh roots at 85 °F and 90 to 95 percent relative humidity for proper healing of wounds. Afterwards, maintain storage at 55 to 60 °F and 85 to 90 percent relative humidity.
• Move seed stock only when necessary during storage to avoid injuries that may cause decay
• Maintain good sanitation at all times around the storage house.

Specific Diseases

Black Rot
(caused by fungus: Ceratocystis fimbriata)

This disease can cause severe losses in storage, in the plant bed, and in the field. Symptoms on the sweetpotato are black, sunken circular spots that often cover more than half of the entire surface (Figure 6). Tissue beneath the margin of the spot is greenish black. Affected sweetpotatoes have a bitter taste when cooked.

The disease is spread in the field by insects, by machinery, drainage water, and diseased roots that are discarded in the field. Black rot continues to develop in storage, and infected roots that appear to be sound may become badly diseased within a few weeks. Diseased roots infect healthy roots stored in the same house. Black rot is spread from one location to another through infected seed, plants, and containers.

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Soil Rot
(caused by bacterium: Streptomyces ipomoea)

Soil rot, also referred to as pox, is the most serious disease of sweetpotatoes in the United States. The disease can seriously lower yield and root quality but does not appear to develop further on sweetpotatoes in storage.

Soil rot is most prevalent on neutral to slightly alkaline soils. Leaves of diseased plants are small and pale green. Black spots occur on feeder roots and the underground part of the stem. Spots may or may not girdle the root. At first, the skin is not broken by the spots, but later the skin breaks and leaves pits one-half inch or more in diameter. Margins of the pits generally have a ragged appearance. If the enlarged root is girdled, it will sometimes have a "dumbbell" shaped appearance (Figure 7).

Losses from soil rot are more severe during dry weather and on poor soils. The disease does not develop well in an acid soil of pH 5.2 or less. The disease is spread in the same way as black rot.

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Bacterial Wilt and Root Rot
(caused by bacterium: Erwinia chrysanthemi)

This disease is also referred to as shell rot or bacterial soft rot. Symptoms include yellow leaves and water-soaked lesions on stems and petioles. In most instances only one or two branches collapse and die, but occasionally the entire plant dies. Many roots decompose completely before harvest; others rot in storage or in shipment to market. In some cases, the inner portion of the root is rotted while the outer portion appears healthy (Figure 8).

Losses to bacterial wilt and root rot can be severe during high humidity, excessive rainfall, and resultant saturated soils. Under such conditions, soft rot symptoms can often be observed in infested fields within 48 hours after a heavy rain followed by warm temperatures.

The bacterium that causes the disease can be transmitted from symptomless seed stock to slips, which in turn introduce the disease organism into a field. It can be moved from one field to another by drainage water and harvesting equipment.

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Scurf
(caused by fungus: Monilochaetes infuscans)

Each year scurf appears in Mississippi sweetpotatoes. The disease is skin deep and develops only on the surface of the sweetpotato. The disease is usually worse during wet weather.

Diseased roots lose moisture and become shriveled under low relative humidity conditions during storage.

Scurf is spread when healthy sweetpotatoes come in contact with diseased roots or contaminated containers and storage houses.

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Foot Rot
(caused by fungus: Plenodomus destruens)

This disease first appears as small brownish-black spots on the stem near the soil line. The disease extends upward 4 or 5 inches and eventually girdles the stem. Affected plants wilt and die in midsummer or later. Generally, diseased plants will not produce sweetpotatoes, or those that are produced will have a brown, firm rot near the attachment end. The disease is spread in the same manner as soil rot.

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Rhizopus Soft Rot
(caused by fungus: Rhizopus stolonifer)

Rhizopus soft rot is a destructive storage disease. The disease may begin early in storage and continue to develop and spread through the storage period. Infection begins in wounds and progresses rapidly. The rot is soft, watery, and stringy, bearing the "whiskery" fungus growth. Diseased roots later become firm, hard, shrunken, and brittle.

The disease is spread by contact and by insects, rodents, or air currents in the storage house. Handling diseased roots will also spread the disease.

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Sclerotial Blight and Circular Spot
(caused by fungus: Sclerotium rolfsii)

Sclerotial blight is primarily a plant bed disease, causing death of sprouts. Circular spot causes a surface rot of enlarged roots at harvest. The above-ground symptoms of sclerotial blight in the plant bed are distinctive in that a cottony web of fungus growth is generally found on lower stems of dead plants.

At harvest, circular spots are apparent on mature roots, and the infected tissue is tan and semi-soft. The spots do not enlarge after harvest. During storage, spots become sunken (Figure 9).

The disease is spread by infected seed and transplants, drainage water, and farm machinery.

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Stem Rot or Fusarium Wilt
(caused by fungus: Fusarium oxysporum f. batatas)

This disease is first seen on young leaves that become somewhat puckered and dull yellow between the veins. The vines wilt and die. Stems are darkened on the inside, and the discoloration may extend several feet from the hill. Sprouts produced from diseased seed pieces will likely become diseased. The disease is spread in the same manner as black rot.

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Nematodes
(chiefly root-knot)

Root-knot nematodes cause galls on feeder roots, roughening and cracking of mature potatoes (Figure 10), and a general decay of the root system. Infected plants appear stunted, yellowed, and generally unthrifty. Cutting a suspected sweetpotato in thin slices may reveal tiny pearly white nematodes inside brownish discolored cavities.

Lesion nematodes cause irregular dark, sunken areas on the surface of the sweetpotato. These spots contain the tiny nematodes.

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Internal Cork
(caused by a virus)

Symptoms of this virus disease appear late in the season as dark, hard, corky spots throughout the flesh of the sweetpotato. The spots are about ¼ inch wide and 2 inches long but may increase in size and number the longer the roots are held in storage and under higher storage temperatures.

Aphids spread the disease from plant to plant during cool weather early in the growing season.

Table 5. Reaction of sweetpotato varieties to important diseases

Variety	DISEASE REACTION*
	Bacterial wilt	Stem rot	Scurf	Soil rot	Black rot	Internal cork	Root-knot
Beauregard	S	R	S	MR	S	R	S
Centennial	S	R	S	S	S	MR	S
Goldrush	S	R	S	S	S	S	S
Hernandez	UD	R	S	MR	S	R	R
Jewel	S	R	S	S	S	MR	R
Nancy Hall	S	S	S	S	S	R	S
Travis	S	R	S	R	S	R	R
Triumph	S	R	S	S	S	R	S
Vardaman	S	R	S	S	S	UD	S

^* = Susceptible
MR = Moderately Resistant
R = Resistant
UD = Undetermined

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Curing and Storing

The primary reasons for curing and storing sweetpotatoes are to maintain good market quality during the winter and spring and to preserve seed potatoes for the next crop. Provide curing and storage facilites with temperature and humidity control. Facilities must be well insulated and constructed so air movement can occur under and around the storage containers. Thoroughly clean, disinfect, and aerate the storage area each year after potatoes have been removed from storage and again before a new crop is stored.

Compartmentalization

Large-acreage growers may need to divide houses into several compartments and fill each area within a week or less. Begin curing when each area is filled. This will allow immediate curing of the potatoes as compartments are filled and will keep storage losses at a minimum.

Small-acreage growers may be able to use smaller buildings and place the entire crop in before beginning the curing process. To keep injuries and decay at a minimum, do not disturb stored potatoes.

Palletization

Palletization offers considerable opportunity for improving curing and storage house operations. Potatoes placed on pallets in the field can be moved directly to the curing house and stacked about three pallets high. After curing, they can easily be moved by forklift as the need arises. This procedure results in less injury to the potatoes.

Curing

Regardless of how gently potatoes are handled during the harvesting operation, some cuts or bruises will occur. Curing immediately after harvest greatly enhances healing.

The best curing conditions are at 85 °F temperatures and a relative humidity of 90 to 95 percent. The length of curing conditions is determined, to some extent, by the weather conditions at harvest. When the outdoor temperature is warm, curing may occur in about 5 days. However, as many as 14 days may be required to provide good wound healing at 50 °F outside temperatures.

Curing protects sweetpotatoes from many storage diseases and excessive shrinkage while starches are being converted to sugars and other flavor components. Do not wash sweetpotatoes before curing or storing.

Temperatures above 90 °F and relative humidity above 95 percent are likely to occur during the curing period. This condition will promote rot. Place a wet bulb-dry bulb thermometer (psychrometer) at about eye level somewhere in the storage area to determine temperature and relative humidity. If the 90 °F and 95 percent relative humidity are exceeded, run the ventilation-cooling fan to reduce both.

Storage

Prolong the storage life of sweetpotatoes by reducing the temperature to 55° - 60 °F and by maintaining the humidity at 80 to 85 percent after the curing period to reduce shrinkage. The lowered temperature greatly reduces the rate of deterioration within the roots. Never allow temperature to drop below 55 °F during storage since sweetpotatoes cannot tolerate cool temperatures. This is most likely to occur at low levels of storage, particularly if intake vents are used near floor level, even if they are closed.

Forced Air Ventilation and Heating

Ventilation for cooling and moisture removal during storage is accomplished by a fan with a time switch and thermostat moving air across the top of the roots. The air is exhausted on the opposite side of the storage from the intake vents. Ventilation requirements should be met if the operation procedures outlined below are followed.

The purpose of a sweetpotato storage house is to provide the most effective combinations of temperature, humidity, and ventilation for long-term storage of the potatoes.Operation Procedure During Sweetpotato Curing and Storage — Forced Air System

• Place sweetpotatoes in storage in crates to a predetermined height, leaving a minimum 2 feet of space between stacks and ceiling.
• Set the thermostat for ceiling-mounted heater to 85 °F for two weeks. During this period, the potatoes will heal the injuries that occurred during harvesting and handling. For this time period, set the ventilating-cooling fan to operate above 90 °F. Respiration heat may cause elevated temperatures, which will result in quality deterioration.
• At the end of the two-week curing period, set cooling thermostat at 55 °F. (Fan should not run below 55 °F.)
• Set the time switch to operate the fan ventilation system between 10 p.m. and 6 a.m. It will take three to seven days to cool the mass of potatoes to about 55 °F.
• Set ceiling-mounted heater thermostat to operate heaters at 55 °F. (Heaters should not operate above 55 °F.)
• Note: Set heating and cooling thermostats to not work against each other — that is, adjust so that a storage temperature of 85 °F to 90 °F is maintained during curing and a temperature of 55 °F to 60 °F is maintained during the holding period. Order plan No. 6228-A from Extension Agricultural Engineering, Box 9632, Mississippi State, MS 39762.

Heating

In Mississippi, the temperatures at harvest, and for a short time thereafter, are usually high enough that supplemental heat is not needed immediately. Later in the storage season, it may be necessary to supply additional heat. When heat is added, it should be distributed so that it provides a uniform warming of all of the roots as heat rises in the room. To accomplish this, one method is to release heat under slatted, false floors located on the cement floor. Trenches allow better heat distribution for this type system than will a few scattered registers, but neither allow as good a distribution as heat released over the top. Floor heating may cause hot spots in the potato stacks. Proper construction of pallets eliminates the need for slatted false floors.

Trenches, usually about 12 inches wide and 12 to 16 inches deep in the floor, permit the use of forklift trucks whose weight would not be supported by slatted false floors. The trenches are usually spaced every 3 to 4 feet apart and covered with short pieces of 2- x 8- or 2- x 10- inch lumber recessed into the floor to keep it level and to permit the use of forklift trucks. The pieces of lumber are spaced to allow the heated air to rise effectively and be distributed evenly throughout the room.

Usually, mechanical circulation is necessary to provide good air circulation. Use of pallets for stacking crates eliminates the need for trenches or false floors.

Ventilation

Ventilation for cooling and removal of condensation during storage can be accomplished by placing air intake ventilators near the floor or below the slatted false floor. Exhaust fans are usually placed in the ceiling or roof. One problem with this system is that cold air drawn into the bottom of the room may cause chilling of the potatoes stacked near the intake vents while temperatures at the top of the room may become too high. High temperatures in the top of the room are usually accompanied by high humidities, which often induce excessive sprouting of potatoes in this area. Cool air from the intake vents may also cause excessive chilling in the bottom of the stacks even with intakes closed.

The most effective ventilation system has fans that exhaust air from the top of the storage rooms at a rate of 1 to 2 cubic feet per minute per bushel of storage capacity — introducing air at the top of the stacks. Exhaust fans are located on the outside walls at ceiling level. A baffle is installed to distribute cool air throughout the stacked sweetpotatoes. The fans are controlled by thermostats that permit them to operate at temperatures above 55 °F. A floor plan and ventilation plan for a storage house are provided. Plans of this house are available on request through your county Extension office.

Floor plan and ventilation plan for a storage house.

Numbers listed correspond to the numbers on the layout plan.

1. Cooling fan — With automatic shutter and capacity to change the air 1 to 2 times per minute.
2. Air intake — Space 5 to 8 feet apart. Allow a minimum of 1.5 times the cross-sectional area of the fan in total area of intakes. Mount above the stacks. See drawing for detail.
3. Stacks — Sweetpotatoes in crates. Make width and length of the stacks approximately 1 1/2 times (minimum) the height.
4. Passageway — For loading 5 feet wide, wider if a forklift is used.
5. Passageway into stacks — 18 to 24 inches.
6. Ceiling-mounted gas heater with blower — The capacity should be enough to maintain 85 °F during curing period, also enough to maintain 50 to 55 °F during the storage. If building is more than 50 feet long, add a circulation fan to distribute heat evenly through building. If a circulation fan is used, install at same height as heater, midway of building length. It should have one to two times the capacity (cubic feet per minute) of the heater circulation fan.
7. Thermostat — Ceiling heater control. Mounted about eye level (5 feet) from floor.
8. Manual switch — For cooling fan control, a thermostat with a sensing bulb inside, in series, with the cooling thermostat can be added to turn the fan on.
9. Cooling thermostat — Mount even with the top of the stacks with the sensing bulb outside. This prevents incoming air below 55 °F from coming in. After the storage temperature of 55 °F is reached, turn off the fan and use only to maintain storage temperature.
10. Time switch — Set to operate the cooling fan between 10 p.m. and 6 a.m. when needed.
11. Psychrometer — Wet bulb-dry bulb thermometers to determine temperature and relative humidity.

Note: Stacked crates—by hand or forklift—should be on pallets. This will enhance vertical air movement through the roots.

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Note: Insect and disease control recommendations listed in this publication were current at time of printing. Future changes in the law, however, could make some of these recommendations invalid. For most recent control guidelines, refer to Extension Publication 554, Disease, Insect, and Nematode Control Recommendations for Commercial Vegetables. When using any pesticide, always follow label directions. References to commercial products or trade names are made with the understanding that no discrimination is intended against other products that also may be suitable.

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By Dr. Paul Thompson, Extension Horticulturist, Dr. Mike Williams, Extension Entomologist, Dr. John Byrd, Extension Weed Scientist, Jim Thomas, Extension Leader, Agricultural Engineering, Dr. Dave Parvin, Professor/Economist, Mississippi Agricultural and Forestry Experiment Station, and Dr. Frank Killebrew, former Extension Plant Pathologist.

Mississippi State University does not discriminate on the basis of race, color, religion, national origin, sex, age, disability, or veteran status.

Publication 1678
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Joe H. McGilberry, Interim Director

(500-06-02)

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Copyright by Mississippi State University. All rights reserved.

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