Guide to Commercial Sweetpotato Production in Alabama

Transcription

1 A L A B A M A ANR-982 A & M A N D A U B U R N U N I V E R S I T I E S Guide to Commercial Sweetpotato Production in Alabama A labama sweetpotato growers make significant contributions to the American sweetpotato industry. The sweetpotato (Ipomea batatus (L.) Lam.) is ranked as the world s seventh most important food crop. In 2001, Alabama ranked fifth nationally in total sweetpotato acreage. This accounted for $8.3 million in cash receipts representing 3 percent of the total cash receipts among all horticultural crops in the state. Sweetpotato production is centered in Baldwin and Cullman counties. Economically, the sweetpotato is one of the most important vegetable crops produced in Alabama. Anyone interested in commercial production of sweetpotatoes must thoroughly understand the crop to be a successful producer. The sweetpotato is a root crop that developed in the coastal mountain valleys of Peru and Ecuador. Being of tropical origin, sweetpotatoes can only be grown during the warmest months in Alabama. To understand what sweetpotatoes are, we first must differentiate them from what they are not: They are not related to Irish potatoes. In fact, the sweetpotato is a swollen, underground root, while the Irish potato is a swollen, underground tuber, or stem. Spell sweetpotato as one word, not two: sweetpotato not sweet potato. They are not yams. The sweetpotatoes Americans are accustomed to have sweet, moist, orange flesh. Yams are dry, starchy tubers of a tropical plant unrelated to sweetpotatoes. Yams usually weigh from 3 to 8 pounds when mature; sweetpotatoes only weigh 18 to 36 ounces. Two general types of sweetpotatoes are grown in the United States. These types are differentiated by their flesh and skin types. The most common type has a coppercolored skin with orange flesh. The Figure 1. Roots with black rot lesions (left and right) and uninfected roots (center). Figure 2. Internal and external views of roots infected with black rot. Figure 3. Cross section of root with symptoms of Fusarium root rot (left) and Fusarium surface rot. Figure 4. Internal and external views of roots infected with Rhizopus soft rot. Figure 5. Internal and external views of roots with Java black rot at different stages of development.

2 other, less common type has red or magenta skin with white flesh. Nutritionally, the sweetpotato is a significant source of many vitamins, phytonutrients, and minerals. Sweetpotatoes provide twice the recommended daily allowance of beta-carotene and more than one-third of the requirement for vitamin C. The sweetpotato is also an important source of vitamin B 6, iron, potassium, and fiber. Sweetpotatoes contain virtually no fat, are low in sodium, and stand out from nuts and avocados as a source of vitamin E because they are fat free. This information can aid growers in expanding their marketing opportunities. Selection of a variety to grow depends on the intended market. Most varieties require 90 to 150 days to produce maximum yields. Alabama growers should consider several cultivars: Beauregard, Hernandez, Jewel, O Henry, and White Delite. Table 1 provides descriptions and attributes of these varieties. Further information regarding yield and relative performance of several new and standard sweetpotato varieties can be found in the latest edition of the Vegetable Variety Trials published by the Alabama Agricultural Experiment Station, Auburn University. This report summarizes vegetable variety trials conducted throughout Alabama. Contact your county Extension office for a copy of this report or go to the following link to view it online: auburn.edu/aaes/communications/ publications/fruitsnutsvegs.html. Cultural Requirements Climate. Because sweetpotatoes are of tropical origin, they adapt well to warm climates and grow best during summer. Sweetpotatoes are cold sensitive and should not be planted until all danger of frost is past. The optimum temperature to achieve the best growth of sweetpotatoes is between 70 and 85 degrees F, although they can tolerate temperatures as low as 65 degrees F and as high as 95 degrees F. Soil. Well-drained sandy and sandy loam soils will produce the best-shaped sweetpotatoes. Avoid using heavy soils. Soils with high levels of organic matter can promote scurf, a disease characterized by roughened skin (see Extension publication ANR-917, Field and Storage Diseases of Sweet Potatoes ). Use long rotations to decrease the incidence of scurf and infection from Fusarium wilt. Avoid fields that have produced a crop of sweetpotatoes in the past 2 years and fields that have high nematode populations and are seriously eroded or grassy. Select a soil that is well drained but not prone to drought. Waterlogged, poorly drained soils prevent roots from obtaining sufficient oxygen, which can cause souring of roots. Fertilizer and Lime. Test your soil to determine whether it has the proper level of nutrients necessary to produce high yields of quality roots. Send a representative Table 1. Varieties and Corresponding Characteristics of Selected Sweetpotato Varieties Variety Beauregard Hernandez Jewel Roots Foliage Skin Flesh Green, heartshaped leaves Green, arrowshaped leaves with purple stems Green stems, bushy Characteristics Disease and Insect Resistance Rose Orange White grub; soil pox; Fusarium wilt Orange Copper Deep orange Deep orange Fusarium wilt; root-knot Root-knot; internal cork O Henry -- White Cream White grub; soil pox; Fusarium wilt White Delite Green, heartshaped leaves Purple Cream Root-knot; Fusarium wilt Flood Damage Resistant but roots may be misshapen Flood damage may result in lenticel formation (black pimples) Susceptible Other Weaknesses Susceptible to rootknot nematodes; bacterial soft rot; low sprouting; russet crack Forms lenticels that have a tendency to turn black when washed; slow sprouting; poor taste Mutations, soil pox; skinning Other Strengths Stores well; high percentage of #1s Produces few jumbos but high percentage of #1s Storage life, root shape Very susceptible Wet soil conditions can cause high percentage of rot Stores well; good taste Adapted from the following sources Sweetpotato Varieties, North Carolina Cooperative Extension Service, Horticulture Information, Leaflet No. 23-D; and 2 Alabama Cooperative Extension System

3 soil sample from each field to the Auburn University Soil Testing Lab to determine the soil s needs. The optimum ph range is 5.8 to 6.2. If your soil needs lime, incorporate the appropriate amount several months before planting. This will allow sufficient time for the lime to increase your soil s ph. Use dolomitic lime if the soil s magnesium level is low. Under average soil and water conditions, the current fertilizer recommendation in Alabama for sweetpotato production is 80 to 90 pounds of nitrogen (N), 60 to 80 pounds of potassium (K 2 0), and 150 to 160 pounds of phosphorus (P ) per acre. Broadcast or band half of the required nitrogen (N) before planting and then sidedress the remainder at layby when the vines begin to run. Follow the recommended rate of fertilizer because high fertilizer concentrations may result in salt burn and plant damage. Additionally, applying surplus fertilizer can cause excessive vine growth and be a waste of resources due to added costs that will not result in higher yields. Crop Establishment Propagating. Sweetpotatoes are propagated from sprouts or slips (vine cuttings). Only purchase certified, disease-free slips. For best results, however, produce your own slips. In this discussion, the word seed refers to the roots used for slip production. Select seed that is true to varietal type (no off-types), that is free from insect and disease damage, that has a uniform flesh with bright skin color, and that is free from veins. Your profitability depends on starting with the highest quality seed stock available. Remember, using quality roots for seed is essential for producing quality sweetpotatoes. Quality sweetpotatoes are not produced from poor-quality seed. In most years, it is not possible to purchase sufficient certified seed stock to produce slips for your entire planned production. The best situation is to save seed from each year s crop. When possible, isolate your planting for seed from that of your commercial planting. Save seed from the highest quality roots that you produce. Carefully inspect roots for defects (no off-types), disease, insect damage, etc., as listed above. Each year purchase a portion of your required seed stock for slip production as certified seed, supplementing your total need with seed from the previous year s crop. Establishing and maintaining your own seed program indicates that you are committed to sweetpotato production and gives you confidence in the quality of the slips you use for your crop. Certified slips are available from several growers in Alabama. See Extension publication ANR-1192, Sources for Vegetable Transplants in Alabama, Georgia, and Florida, for a list of these growers and the varieties they produce. Copies are available from your county Extension office or on the Web at Presprouting. Presprouting is a technique that produces two to three times more slips than the typical technique, which uses seed stock that is not presprouted. Some refer to presprouting as waking up the sweetpotatoes after they have been in storage. Presprouting encourages more prolific sprouting in roots. This can decrease production costs by decreasing the total amount of seed stock required. In addition to increasing the number of slips produced, presprouting produces slips faster. Conditions required for presprouting are similar to those required for curing sweetpotatoes. Presprouting involves placing seed stock in a controlled storage area, such as a curing room. You must be able to control temperature and relative humidity and be able to provide ventilation. Be sure that you are able to replace the air one to two times per day because the roots require a significant amount of oxygen to facilitate presprouting. A rule of thumb: if there is not enough oxygen for a match to stay lit, there is likely not enough oxygen for the sweetpotatoes. To presprout, place seed stock in a presprouting room for 21 to 35 days at 70 to 80 degrees F with 90 percent relative humidity. Spraying the walls and floors with water two times per day can help maintain relative humidity. Mechanical humidifiers (automatic humidifiers, misting systems) can help establish and maintain the required relative humidity. Avoid humidity near 100 percent or wetting of the surface of the roots. This can lead to the development of rots. Bedding. Provide 4 to 5 pounds of or type fertilizer per 100 square feet of bed area. After presprouting, place roots into beds, being careful not to damage them. Treat seed with appropriate fungicides to prevent bedding root decay. Be sure to cover roots completely with 2 to 3 inches of soil. Do not be concerned if a few sprouts are above the soil line. Keep beds moist but not wet. After planting roots, cover beds immediately with black or clear plastic to warm the soil. Punch holes in plastic for ventilation as needed. Slips are ready to harvest when they have 6 to 10 leaves and a strong root system. Slips from presprouted roots are generally ready one week earlier. Preparing Slips for Transplanting. To harvest, cut the slips about 1 inch above the bed surface. Cutting is preferred to pulling slips. Always pull the knife up and away from the soil to prevent contamination from the seedbeds from moving into the production field. Clean knives frequently by dipping them into a 1:1 (v/v) solution of bleach and water. This will also prevent the spread of diseases from the seedbed into the field. Set the slips in the field as quickly as possible after harvesting from plant beds. About 500 slips can be produced Guide to Commercial Sweetpotato Production in Alabama 3

4 from one bushel of seed. One bushel of seed requires 20 to 30 square feet of bed area. Transplanting. Avoid planting slips until all danger of frost is past because they are very frost sensitive. Begin transplanting slips March 15 in South Alabama and around April 15 in North Alabama. Beds should be 4 to 8 inches high and as wide as equipment will allow. Narrow beds tend to dry quickly and may reduce overall yields. High beds will aid in promoting drainage, thus preventing water damage to roots. The most economical method to set a large number of plants is with a mechanical transplanter. If you use a starter solution, be sure it is water soluble so it won t clog the transplanter. Use a high-phosphate starter solution ( or equivalent) at the rate of 3 pounds per 50 gallons of water. Space slips 8 to 15 inches apart within rows spaced 3½ to 4 feet apart on row centers. The number of slips needed per acre will depend on your desired spacings (Table 2). Be sure to manage water carefully to avoid transplant shock. To determine the number of transplants required per acre for any spacing, divide 43,560 (number of square feet in an acre) by the product of the desired spacing between slips and the desired spacing between the rows. For example, a 12-inch between-plant spacing on 48-inch bed centers would require 10,890 plants per acre: 12 inches x 48 inches or 1 foot x 4 feet = 4 feet squared; then divide 43,560/4 = 10,890 plants per acre. Irrigating. At transplanting, a light irrigation of 0.50 to 0.75 inches per acre will help establish young slips by providing a ready supply of water to young, developing roots. Although sweetpotatoes are a deeply rooted and drought tolerant crop, they still require sufficient moisture to grow and produce quality roots. Water supply fluctuations can cause the development of small or misshapen roots. These fluctuations can also cause growth cracks to develop in the roots of the sweetpotato. In either case, defects will render sweetpotatoes unmarketable. Excessive moisture can cause extravagant vine growth and undesired elongation of roots. The most critical moisture period for the sweetpotato is during root expansion and the last 40 days before harvest. In dry summers, the ability to provide irrigation can mean the difference between producing high yields of quality roots and poor yields of off-shaped, unmarketable roots. Sweetpotatoes require approximately 1 inch of water per 3 weeks (½1 inch of water per day). Weeding. Be sure to prepare the land in which you are going to plant sweetpotatoes. Maintaining the proper soil ph and applying fertilizer next to the row of sweetpotatoes instead of broadcasting fertilizer through the field will increase the competitiveness of the crop compared to the weeds. Avoid fields with a history of extreme weed pressure. Cultivate to control weeds during the early stages of growth. Chemical controls are effective for the various grasses that can infest a sweetpotato field (with the exception of nutgrass). Chemical treatments for broadleaf weeds, such as sicklepod, common cocklebur, prickly sida, and annual morningglory, are not very effective. Contact your county Extension office for the current year s herbicide recommendations. Table 2. Number of Slips Per Acre Required at Various Spacings Between-row spacing Number of slips required per acre at various in-row spacings 8 inches 12 inches 15 inches 3½ feet 18,670* 12,450* 9,960* 4 feet 16,335 10,890 8,712 *Numbers are rounded. Diseases and Insects Refer to Extension publication ANR-500-A, Alabama Pest Management Handbook Volume 1, for the current year s recommendations for control of diseases and insects. As with any suspected insect or disease problem, consult your county Extension agent if you are unsure of what insect or disease is causing a problem before you attempt to control it. Diseases Black Rot Black rot is caused by the fungus Ceratocystis fimbriata. The disease can cause significant losses in storage, in the transplant bed, and in the field. The pathogen not only reduces yield and quality but also gives sweetpotatoes a bitter taste. Small, circular, slightly sunken, dark brown spots are the initial symptoms of black rot (Figure 1). Spots enlarge and appear greenish black to black when wet and grayish black when dry. Within the spots are small, black fungal structures (perithecia) with long necks that appear as dark bristles. The rot usually remains firm and shallow. If secondary fungi or bacteria invade the tissue, however, the flesh beneath the spot turns black and may extend to the center of the root (Figure 2). Tissue near the discolored area may have a bitter taste. Eventually, the entire root may rot. Roots may appear healthy at harvest but rot in storage, during transit, or in the market. This fungus survives in the soil in crop debris. Infected storage roots escape detection at harvest or bedding. The fungus either colonizes the young shoots or infects the stem. Transplants and, subsequently, the main stem and daughter roots are thereby infected. When slips are pulled for transplanting, the stem carries the pathogen along with the plant. The black rot fungus can produce tremendous numbers of spores during storage. These can 4 Alabama Cooperative Extension System

5 contaminate washing machines, crates, and structures as well as the hands of workers. Contaminated items or dip tanks can be sources of fungal inoculum for new infections. Using contaminated equipment or washing and packing roots infected with black rot before curing may spread the disease. Black rot may develop on sweetpotatoes during transit or in the marketplace. Entire lots may become infected as the fungus spreads quickly to roots surrounding a rotting sweetpotato. As a result, entire crates of roots can be quickly destroyed in storage. Insects, such as the sweetpotato weevil, can also transmit the disease in storage. Development and spread of the disease are rapid at temperatures greater than storage temperatures (55 to 60 degrees F). Control Control black rot with crop rotation because most other crops are unaffected by the disease. Disinfect seedbeds if a clean site is unavailable. Propagate plants from healthy stem cuttings. Cure roots immediately after harvest. Apply a postharvest fungicide. Do not wash and package roots showing symptoms of black rot. Decontaminate equipment that comes into contact with an infected crop. Spray empty washing machines and crates with a fungicide. Fumigate storage structures. Fusarium Surface Rot and Fusarium Root Rot Fusarium surface rot and Fusarium root rot are caused by species of the fungus Fusarium. Fusarium surface rot is common on roots stored for any length of time after harvest. Fusarium root rot is a serious disease of sweetpotatoes in the Southeast. Surface rot occasionally occurs before harvest on roots that have been mechanically injured, split by growth cracks, or damaged by nematodes, insects, or other soil pests. Lesions on fleshy roots are circular, light to dark brown, firm, and dry. Decay remains shallow, usually not extending beyond the root s vascular ring. Externally, lesions appear solid brown and are often centered on a broken rootlet. When infected roots are stored for an extended period, the tissue around lesions dries and becomes shrunken, and the root eventually becomes hard and mummified. External symptoms of Fusarium root rot may be difficult to distinguish. In some cases, surface rot may be an early stage of the more aggressive root rot. Root rot lesions are circular and commonly exhibit light and dark brown concentric rings. Internal rotting extends beyond the vascular area into the center of the root and may eventually affect the entire root (Figure 3). This pattern distinguishes root rot from surface rot. The tissue near the advancing margins of these lesions varies from orange to light brown and is spongier and moist than either healthy tissue or older lesion tissue, which is dark brown, dry, and spongy. Oval-shaped cavities occur near the root s surface. These cavities often have white fungal growth on their inner surfaces. As the lesions grow, the infected tissue shrinks, dries up, and eventually mummifies. Frequently, rot begins at the ends of the root, a phase known as Fusarium end rot. Another phase, Fusarium stem canker, occurs on sprouts of infected mother roots in plant beds. In this phase, a dark brown to black decay begins at the base of the sprout and progresses up the stem. Species of Fusarium that cause surface rot and root rot can persist in soil for many years. Infection in the field can occur through rootlets or growth cracks. More commonly, however, roots contaminated with the fungus are invaded through wounds that occur during harvest. Surface rot or root rot that develops during storage spreads to other stored roots only when new wounds are made. Surface rot is prevalent when sweetpotatoes are mechanically harvested, when soil is wet and cold at harvest or excessively dry before harvest (causing increased skinning of sweetpotatoes), when sweetpotatoes are exposed to high or low temperatures for extended periods after digging and before curing, or when conditions are favorable for desiccation of wounded tissue. Control Control surface rot to reduce Fusarium root rot. Use sanitary practices and properly handle harvested roots. These are the most effective control procedures for both pathogens. Minimize injury during harvesting and handling, especially if the crop is lifted from wet soil. Cure roots immediately after harvest. Reduce surface rot in the field by controlling rootknot nematodes and insects that can rupture the skin of sweetpotatoes. Reduce spread of Fusarium root rot to sprouts in transplant beds by planting disease-free roots treated with fungicides. Cut transplants above the soil line to avoid transporting the stem canker phase of Fusarium to the field. Rhizopus Soft Rot Rhizopus soft rot is caused by the fungus Rhizopus stolonifer. The disease mainly occurs after sweetpotatoes have been harvested. Guide to Commercial Sweetpotato Production in Alabama 5

6 Infection and decay commonly appear at one or both ends of the root, although infection occasionally begins elsewhere. Rotting may be inhibited under dry conditions, but under humid conditions the affected sweetpotatoes become soft and watery, and the entire root rots within a few days (Figure 4). If humidity is high, the sweetpotatoes become heavily whiskered with a grayish black fungal growth. This feature distinguishes Rhizopus soft rot from other storage rots. The color of the root is not significantly altered, but an odor is produced that attracts fruit flies to the area. Spores of Rhizopus are common in the soil and in the atmosphere. In addition, the fungus can survive in crop debris and, to some extent, on contaminated equipment. The fungus usually infects through wounds made during harvesting and handling when airborne fungal spores or contaminated soil contacts the wounded surface. Once established, the fungus is capable of attacking healthy, uninjured tissue. Infection is especially likely if the relative humidity is between 75 and 85 percent during storage or transport. Also, the longer roots are stored, the more susceptible they become. Chilling and heat damage also predispose sweetpotatoes to infection. Soft rot is very destructive when sweetpotatoes are washed, packed, or shipped to market during cold weather. Control Carefully handle sweetpotatoes during harvest to prevent unnecessary wounding. This is the most important control method for soft rot. Properly cure roots immediately after harvest. Store roots at proper temperature and relative humidity. Avoid handling stored roots because handling can create new wounds. Re-curing is one possible solution to this problem if additional handling is required. Apply a recommended fungicide after harvest. Do not allow sweetpotatoes to be exposed to sunlight for extended periods (to prevent heat damage) or to be chilled in the field. Java Black Rot Java black rot, caused by the fungus Diplodia gossypina, is considered one of the most destructive diseases of sweetpotatoes in the southern United States. The disease is most frequently observed on sweetpotatoes in storage. The disease usually progresses from one or both ends of the root. Affected tissue is first yellowish to reddish brown, turning black as the decay progresses (Figure 5). The decayed area is firm and moist. Infected roots often completely decay within 2 weeks and subsequently dry out, becoming mummified and extremely hard. During early stages of development, Java black rot can be confused with black rot, charcoal rot, or Fusarium rot. In later stages, the skin is pimpled with small, black spore-producing structures (stromata). These black fungal masses break through the surface of the root and take the shape of domes or cushions. Decay in storage is frequently restricted to the tip (½ to 1 inch) of the root. The tissue at the center of the lesion is usually solid black. The fungus survives free in the soil for several years and in crop debris. Infested soil sticks to the broken ends of sweetpotatoes at harvest, and the fungus enters through these wounds or other wounds created elsewhere on the root. Spores of the fungus can also survive on crates from one season to the next and infect newly wounded roots placed into the infested crates. The fungus requires a wound for entry into roots. The fungus does not spread on properly cured roots in storage; however, any subsequent handling can result in new wounds, and secondary infections can spread infection. If contaminated sweetpotatoes are used as seed, the sprouts as well as the daughter roots may become infected. Java black rot is a warm-temperature disease, favoring temperatures between 68 and 86 degrees F. The disease develops over a wide range of relative humidities. Roots become more susceptible to Java black rot with increased storage time or if chilled roots are returned to higher temperatures. Control Wash and disinfect any previously used storage containers before harvest. Do not expose sweetpotatoes to flooding or cold in the field. Minimize wounding during harvesting. Wash harvested roots and dip them in a fungicide. Immediately cure the roots to ensure rapid wound healing. Although temperatures recommended for curing sweetpotatoes are similar to optimal temperatures for Java black rot growth, cure sweetpotatoes immediately after harvest to reduce incidence of the disease. Store sweetpotatoes at proper temperature and relative humidity. Bacterial Soft Rot Bacterial soft rot, also known as bacterial stem and root rot, is caused by the pathogen Erwinia chrysanthemi. Rotting can occur in the field as well as during shipment and storage. The disease is known to occur only in the United States. Roots are affected in the field, or more commonly in storage, by a soft rot that turns diseased tissue 6 Alabama Cooperative Extension System

7 light brown and watery (Figure 6). Lesions on storage roots often have a dark brown margin. Some storage roots appear healthy from the outside but are decayed internally. Infected roots show black streaks in the vascular tissue and eventually undergo a soft, moist decay. Mother roots often decay in plant beds. In the field, brown to black, water-soaked lesions appear on stems and petioles. Eventually, the stem may become watery and collapse, causing the ends of vines to wilt. Usually, one or two vines may collapse, but occasionally the entire plant dies. The bacterium invades the host through wounds. It survives in crop debris and in association with weeds. Sources of inoculum may include soil, infected mother plants, or contaminated wash water and harvesting equipment. The disease is favored by warm, humid weather. Symptoms may not be visible at temperatures below 80 degrees F but may appear rapidly at temperatures of 86 degrees F or higher. Control Carefully handle sweetpotatoes during all stages of production. This is the most important control method for bacterial soft rot. Select mother roots from fields free of the disease. Cull infected roots prior to and during storage. Cut slips above the soil surface. Use a handling system that does not involve immersion of sweetpotatoes in water. Scurf Also known as soil-stain, scurf is caused by the fungus Monilochaetes infuscans. Damage from the disease is primarily superficial but still detracts from market value. Symptoms of scurf begin during the growing season as small, dark brown to black spots that develop on roots and later merge to form irregular lesions (Figure 7). Lesions enlarge until the entire surface of the root is discolored. Copper-skinned sweetpotatoes usually have brown lesions, and red-skinned sweetpotatoes have black lesions. Symptoms are restricted to the skin of storage roots and do not directly affect the underlying tissue. Affected tissue can easily be scraped off. Cracks may develop on severely affected sweetpotatoes resulting in shrinkage due to water loss. Scurf-infected sweetpotatoes are more susceptible to invasion by other fungi. Scurf lesions continue to enlarge when sweetpotatoes are put into storage, and new lesions appear if excessive relative humidity is maintained. The optimum temperature for disease development is 75 degrees F, but scurf can develop to a lesser extent over a wide range of temperatures. Disease development is greatest when soil moisture is optimal for plant growth. Most scurf infections result from the use of infected propagating material. The fungus can also survive in crop debris in the soil for 1 to 2 years. Disease severity is greater and persistence of the pathogen longer in fine-textured soils that have a high level of organic matter. The disease has a narrow host range that consists only of species in the genus Ipomoea. Control Use only scurf-free, fungicidetreated sweetpotatoes as seed. Bed these in soil free of the disease. Cut slips at least 1 inch above the soil line, and dip them in a fungicide. Grow sweetpotatoes on a 3- to 4-year rotation with other crops. Charcoal Rot Charcoal rot, caused by the fungus Macrophomina phaseoli, can cause losses of sweetpotatoes in storage, but serious losses seldom occur. The disease is sometimes confused with black rot and Java black rot. Symptoms in storage begin as reddish brown to brown, firm, moist rot, initially restricted to the area just beneath the sweetpotato skin. As the decay progresses, the pathogen moves toward the center of the sweetpotato, causing further rot (Figure 8). Two distinct zones become apparent within the infected tissue. The leading edge continues as a reddish brown decay, and a zone of black develops behind the zone of active decay. Although the lesions are sometimes restricted, charcoal rot usually consumes the entire root, which eventually dries, becoming hard and mummified. The fungus is soilborne and survives in plant debris or in the soil. There is no evidence showing that the fungus can be carried in roots used for seed. The fungus has a wide host range that includes many crops such as soybean, cotton, sorghum, and corn that are grown with sweetpotatoes. It requires a wound to enter the root. High temperatures (84 to 88 degrees F) appear to favor disease development. Charcoal rot is more common in storage houses that are too warm or in storage crates that are located too close to heaters. Infection is greater if harvested roots are scalded by sunlight before being placed in storage. Control Properly cure sweetpotatoes immediately after harvest to reduce the incidence of charcoal rot. Insects A wide variety of insects feed on sweetpotato foliage. Treatment to control foliar damage is rarely necessary because plants grow vigorously and feeding damage to foliage must be extensive before root growth will be affected, particularly after the root-swell stage Guide to Commercial Sweetpotato Production in Alabama 7

8 of growth. Damage by root-feeding insects (below ground), however, can cause a serious economic loss and can often be reduced by controlling the above-ground adult stage of the insect. Beetle grubs (whitefringed beetle larvae, white grubs, wireworms, and flea beetle larvae) and sweetpotato weevils are the most damaging root pests on sweetpotatoes. Whitefringed beetle larvae (small, legless grubs) can be a serious problem in localized areas. Feeding damage to roots can appear as round holes similar to wireworm and white grub damage, but whitefringed beetle grubs usually move along the root surface as they feed, creating feeding tunnels or channels (Figures 9a and 9b). If possible, avoid fields where this insect has been a problem because soil insecticides only suppress damage and do not offer complete control. Applications of soil insecticides at root-swell provide better control than applications at planting. Foliar insecticide applications can also be effective in reducing root damage by controlling adults, egg-laying, and subsequent populations of grubs in the soil. The presence of adults on sweetpotato plants can be determined by their characteristic notching feeding damage on leaves (Figure 10). If you observe adults or notching, apply a recommended foliar insecticide spray at 10- to 14-day intervals until 2 to 3 weeks before harvest. Although whitefringed beetles cannot fly, the eggs, larvae, and adults can be transported from one area to another by farm machinery, people, plants, or other objects that come in contact with the soil. White grubs (larvae of June and May beetles) cause large scars by feeding on roots. Avoid fields that were in pasture grass the previous year because grubs are likely to be abundant. Wireworms (long, thin larvae of click beetles) chew small- to moderate-sized round holes into the roots. On harvested roots, early feeding damage appears as shallow to deep, healed-over scars depending on the depth of initial penetration. Late-season feeding by large larvae causes large, round, ragged holes, while smaller larvae produce shallow holes. Larvae of cucumber beetles and flea beetles can cause similar damage. Soil insecticides suppress wireworm damage but may not offer complete control. Placing of bait stations (that is, rolled oats, wheat, or corn seed mixtures) can be used before planting to determine if wireworms are present and to decide whether a soil insecticide is needed. As with whitefringed beetles, properly timed foliar spray applications for wireworm adults can be used to reduce egg-laying and larval damage. Because some wireworm species are especially attracted to weedy fields, good weed control can reduce the possibility of wireworm damage. Flea beetle larvae feed just under the surface of the root skin, creating thin mines, often referred to as writing. Although this damage is only cosmetic, it affects marketability of roots. Sweetpotato weevil is the most serious pest of the sweetpotato. Control measures have made the weevil less of a problem today than it was in the past, but several Alabama counties are still designated as infested areas. The Alabama Department of Agriculture and Industries administers a sweetpotato quarantine, which places restrictions on movement of sweetpotatoes from infested regions to weevil-free regions. Contact ADAI for a list of designated counties with weevil infestations. Although these precautions affect only certain counties, all Alabama sweetpotato growers should be alert for weevil infestations. Pheromone traps are commercially available for monitoring the presence of sweetpotato weevil adults. The adult weevil will feed on any portion of the plant but prefers the root. Sweetpotato weevil eggs are laid on vines or roots. Larvae are white, with light-brown heads (Figure 11a). Larvae create feeding tunnels that begin just under the skin of the root. These feeding tunnels frequently contain larvae, pupae, or newly transformed adults. Adult exit holes in the root are about the size of a matchstick. Adult weevils are antlike in appearance and about ¼ inch long. The head and wing covers are metallic blue (Figure 11b). The thorax and legs are bright orange to red. Infestations of weevils may be found in the field, storage sheds, and propagation beds. Prevention and sanitation are important parts of a sweetpotato weevil control program. Frequent inspections of these areas will identify possible infestations early; notify regulatory officials of ADAI about suspected infestations. The following practices will help control this pest: Use certified seed produced in a weevil-free area. Locate propagation beds away from the area where the past year s crop was produced. Spray with a recommended insecticide on a weekly schedule, if weevils are found in seedbeds. Use slips that are cut 1 inch above the soil line, or vine cuttings, rather than pulled plants. Destroy excess plants and seed in seedbeds once transplanting has been completed. If weevils are found in the field or in pheromone traps, spray plants with a recommended insecticide on a weekly schedule. When cultivating, throw soil around the base of the vines to prevent adult weevils from reaching sweetpotato roots. Destroy all crop debris, including culls. When a sweetpotato crop is not present, destroy volun- 8 Alabama Cooperative Extension System

9 teer sweetpotatoes and morningglories (alternate host plants). Clean storage houses thoroughly after each season. Postharvest Handling The following information is adapted from The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks (Handbook 66) by the United States Department of Agriculture. It is important to remember that the roots of the sweetpotato are alive. They require sufficient oxygen to survive even in storage. Changes in the environment, including temperature and relative humidity, will affect the shelf life of the sweetpotato. Keeping this fact in mind will help you make good management decisions for your crop. Sweetpotatoes are usually stored in nonrefrigerated commercial or farm warehouses. This method of storing offers the primary advantage of orderly marketing several months after harvesting. Following harvest, sweetpotatoes should be cured. Do not wash sweetpotatoes before curing or storage. Curing promotes the healing of cuts and bruises that occur during harvesting and handling. Curing also protects the roots from many storage diseases and excessive shrinkage while starches are being converted to sugars and other flavor components. Curing the roots increases the postharvest life of the sweetpotato. To cure roots, hold them at 85 degrees F with 90 to 97 percent relative humidity for 4 to 7 days. During curing, ventilation is required to remove carbon dioxide (CO 2 ) and replenish oxygen (O 2 ). Roots consume a significant amount of oxygen (O 2 ) during curing while producing an equivalent amount of carbon dioxide (CO 2 ). After curing, the storage temperature should be reduced to 57 ± 2 degrees F at 90 percent relative humidity. Most properly cured sweetpotato cultivars will keep satisfactorily for 4 to 7 months at this temperature and relative humidity. Due to the sweetpotato s tropical origin, roots will incur chilling injury if held at temperatures below 54 degrees F. Short periods at temperatures as low as 50 degrees F need not cause alarm, but after a few days at 50 degrees F or shorter periods at lower temperatures, sweetpotatoes may develop discoloration of the flesh, internal breakdown, off-flavors and a hard core when cooked, and an increased susceptibility to decay. Temperatures above 60 degrees F stimulate development of sprouts (especially at high humidity), pithiness, and internal cork (a symptom of virus disease) when it is present. Sweetpotatoes are usually stored in 800-pound bulk bins. Palletization of bins and use of pallet boxes facilitates handling. Some of the new storage facilities equipped for palletized handling have separate curing and storage rooms. Sweetpotatoes can be cured in palletized field boxes in a room designed to provide recommended conditions for curing. After curing the sweetpotatoes, carefully move them by forklift to rooms with continuously maintained storage conditions. When ventilation alone will not maintain low enough temperatures, refrigeration is commonly used in some large sweetpotato storages to extend the marketing. Large-acreage growers may need to divide houses into several compartments. Begin curing when each compartment is filled. This will allow immediate curing of the roots 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 them before beginning the curing process. To minimize injuries and decay, do not disturb stored roots. In either case, it is important to keep curing and storage rooms clean. If you are reusing storage containers, be sure to clean them thoroughly. Surfaces can be disinfected with a 10 percent bleach solution. Grading and Packaging. Sweetpotatoes are usually washed and graded, and sometimes waxed, before being shipped to market. Sweetpotatoes are graded into US Extra No. 1, US No. 1, US Commercial, US No. 2, and Unclassified based largely on size, condition, and absence of defects. Table 3 identifies the various parameters surrounding size; see below for further details regarding the standards. US Extra No. 1 consists of sweetpotatoes of similar varietal characteristics that are firm, smooth, fairly clean, fairly well shaped; that are free from freezing injury, internal breakdown, Black rot, other decay, or wet breakdown; and that are free from damage caused by secondary rootlets, sprouts, cuts, bruises, scars, growth cracks, scurf, pox, or other diseases, wireworms, weevils, or other insects, or other means. US No. 1 consists of sweetpotatoes of one type that are firm, fairly smooth, fairly clean, fairly well shaped; that are free from freezing injury, internal breakdown, Black rot, other decay, or wet breakdown; and that are free from damage caused by secondary rootlets, sprouts, cuts, bruises, scars, growth cracks, scurf, pox, or other diseases, wireworms, weevils, or other insects, or other means. US Commercial consists of sweetpotatoes that meet all the requirements of the US No. 1 grade except that an increased tolerance for defects is allowed. (See Tolerances below.) US No. 2 consists of sweetpotatoes of one type that are firm; that are free from freezing injury, internal breakdown, Black rot, other decay, or wet breakdown; and that are free from serious damage caused by dirt or other foreign materials, cuts, bruises, scars, growth cracks, pox, or other diseases, wireworms, weevils, or other insects, or other means. Unclassified consists of sweetpotatoes that have not been Guide to Commercial Sweetpotato Production in Alabama 9

10 Table 3. Sweetpotato Grading Standards Grade Length Maximum Weight Maximum Diameter US Extra No. 1 Greater than 3 Not more than 18 Not more inches but less ounces than 3¼ than 9 inches inches US No. 1 Greater than 3 inches but less than 9 inches* US Commercial Greater than 3 inches but less than 9 inches* classified in accordance with any of the grades listed above. The term Unclassified is not a grade within the USDA standards but is provided as a designation to show that no grade has been applied to the lot. Tolerances. To allow for variations incident to proper grading and handling in each of the USDA grades, the following tolerances, by weight, are provided as specified: Defects US Extra No. 1 and US No. 1 grades. In any lot, 10 percent of the sweetpotatoes may fail to meet the requirements of these grades, but not more than half of this amount, or 5 percent, are allowed for sweetpotatoes that are seriously damaged, including therein not more than 2 percent for sweetpotatoes affected by soft rot or wet breakdown. US Commercial. In any lot, 25 percent of the sweetpotatoes may fail to meet the requirements of this grade, but not more than one-fifth of this amount, or 5 percent, are allowed for sweetpotatoes that are seriously damaged, including therein not more than 2 Not more than 20 ounces Not more than 20 ounces US No. 2 None Not more than 36 ounces* * Unless otherwise specified percent for sweetpotatoes affected by soft rot or wet breakdown. US No. 2. In any lot, 10 percent of the sweetpotatoes may fail to meet the requirements of this grade, including therein not more than 2 percent for sweetpotatoes affected by soft rot or wet breakdown. Off-size In any lot, 10 percent of the sweetpotatoes may fail to meet any specified size, but not more than half of this amount, or 5 percent, are allowed for sweetpotatoes that are below the minimum diameter and minimum length specified. Following grading, sweetpotatoes are generally marketed in 40-pound boxes. Treat roots with a fungicide to reduce decay during marketing. Consumer packaging of sweetpotatoes in film bags or over-wrapped trays is done mainly to aid marketing and should not be done before storage. The shelf life of washed and fungicide-treated roots in consumer packs is only 2 to 3 weeks. Weight loss of roots during marketing is much less in perforated film bags than in mesh bags. Not more than 3½ inches Not more than 3½ inches None Minimum Diameter Not less than 1¾ inches* Not less than 1¾ inches* Not less than 1¾ inches* Not less than 1½ inches* Methods of Cooling and Storage Forced-air Ventilation. To provide ventilation for cooling and moisture removal during storage, use a fan with a time switch and thermostat to move air across the top of the roots. The air is exhausted on the opposite side of the storage from the intake vents. A sweetpotato storage house should provide the most effective combination of temperature, humidity, and ventilation for longterm storage of the roots. Take the following steps to effectively use a forced-air system: Place sweetpotatoes in storage bins to a predetermined height, leaving a minimum of 2 feet of space between stacks and ceiling. Set the thermostat for ceilingmounted heaters to 85 degrees F for 4 to 7 days. During this period, the sweetpotatoes will heal injuries that occurred during harvesting and handling. For this time period, set the ventilating-cooling fan to operate above 90 degrees F as heat from respiration of the roots may cause elevated temperatures in the storage, 10 Alabama Cooperative Extension System

11 which will result in quality deterioration. At the end of the 2-week curing period, set the cooling thermostat at 57 degrees F. (Fan should not run below 57 degrees F.) Set the time switch to operate the fan ventilation system between 10 p.m. and 6 a.m. It will take 3 to 7 days to cool the mass of sweetpotatoes to about 57 degrees F. Ventilation. Ventilation for cooling and removal of condensation during storage can be accomplished by placing air intake ventilators near the floor or below a slatted false floor. Exhaust fans are usually placed in the ceiling or roof. Cold air drawn into the bottom of the room may cause chilling of the sweetpotatoes that are stacked near the intake vents. Higher temperatures at the top of the room are usually accompanied by high humidity, which may induce excessive sprouting of sweetpotatoes. 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 while 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 temperatures above 57 degrees F. Budget The following budget provides information concerning general levels of costs and returns that will need to be adjusted for specific situations (Tables 4 and 5). Quantities of materials listed in each budget are based on survey data from producers or generally accepted recommendations. Direct costs include estimated costs of repairs and maintenance for all machinery and include fuel costs for powered machinery. Direct costs are estimated on an hourly basis and are then converted to a per-acre basis using the performance rate for the particular operation. Direct costs include an estimate of fuel cost based on average fuel consumption per hour of use for the power unit. Other components of direct costs include quantities of materials used in production multiplied by the price per unit of these inputs, custom rates, hourly wage rates, and interest charges on short-term capital. The labor wage rate per hour includes social security, accident and unemployment insurance, and some perquisites. Labor costs are estimated for two labor categories: operator labor ($9.02/hr) and hand labor ($6.44/hr). Operator labor and hand labor represent estimates of labor required to perform the in-field tasks. Operator labor is that labor required for operating all power-driven equipment. Interest on operating capital is determined by using a short-term interest rate obtained from agricultural lenders and making a charge against capital outflows as the production process takes place. Interest is accumulated until the crop is harvested. In Alabama, the 5-year average yield (1998 to 2002) is approximately 160 cwt per acre. This equates to pound boxes per acre. The average price over this same period is $16.14 per cwt or $6.46 per 40-pound box. Based on these figures and cost information in the enterprise budgets, producers would earn a total of $307 per acre above all costs associated with sweetpotato production in the state (Table 6). Guide to Commercial Sweetpotato Production in Alabama 11

12 Table 4. Estimated Costs Per Acre, Sweetpotatoes, Alabama - MALTA 1, 2005 ITEM UNIT PRICE QUANTITY AMOUNT YOUR FARM dollars dollar DIRECT EXPENSES CUSTOM Trapping acre Custom Apply Fert acre FERTILIZER Fert S+B cwt HERBICIDE Roundup Ultra oz Command 3ME pt Poast 1.53 pt INSECTICIDE Lorsban 4E pt Endosulfan 3E pt Imidan 70 WSB lb Penncap-M pt Spintor oz SEED/PLANTS Sweetpotato Plants thous OTHER Crate Sweetpotato each Bin Sweetpotato each Storage Sweetpotato cwt Clean, grade, pack box Box Sweetpotato each Broker Sweetpotato box OPERATOR LABOR Tractors hour Self-Propelled hour Harvest Labor Special Labor hour PLANTING LABOR Special Labor hour HAND LABOR Implements hour DIESEL FUEL Tractors gal GASOLINE Self-Propelled gal REPAIR & MAINTENANCE Implements acre Tractors acre Self-Propelled acre INTEREST ON OP. CAP. acre TOTAL DIRECT EXPENSES FIXED EXPENSES Implements acre Tractors acre Self-Propelled acre TOTAL FIXED EXPENSES TOTAL SPECIFIED EXPENSES Note: Cost of production estimates are based on last year s input price. 12 Alabama Cooperative Extension System