NC State Extension Publications

Key Management Practices

  • Position sweetpotato in a rotation of crops to minimize weed competition and pest and disease losses.
  • Choose sweetpotato varieties that perform well in your area and fit target markets.
  • Test soil for fertility requirements and nematode presence.
  • Use good implement sanitation practices to limit the spread of diseases and nematodes among fields.
  • Purchase disease-free seed or slips from a certified sweetpotato seed producer.
  • Avoid planting too early.
  • Manage weeds appropriately at critical periods in sweetpotato growth and storage root initiation.
  • Minimize handling damage when harvesting to limit postharvest disease and unmarketable roots.
  • Use recommended root curing conditions.

Overview

North Carolina is the largest producer of sweetpotatoes in the United States, producing over 50 percent of the crop nationwide. Organic production of sweetpotatoes is an important part of several growers’ businesses; however, the percentage of acreage dedicated to organic versus conventional production is relatively low. There appears to be room to grow the organic sweetpotato sector of the market. Although sweetpotatoes have pest challenges, there seem to be fewer concerns about disease and insect pests than for many other vegetable crops. Control of weeds can be one of the most challenging pest control aspects when growing organic sweetpotatoes, especially Palmer amaranth and nutsedge weeds.

Variety Selection

Variety selection is critical for any crop because the variety not only needs to meet production criteria but also must meet market demands. Sweetpotato is no exception. And pest management considerations are very important criteria, especially when considering organic production. Pest control methods are more limited when growing a crop organically, and thus consideration of pest resistance or susceptibility is of the utmost importance. Agronomic characteristics such as yield, production time in the field, and plant production are also very important. Finally, if one can grow a high-yielding sweetpotato variety that has a white flesh and skin but cannot sell it because the market demands an orange flesh with red skin, it does the producer no good. Thus, one must produce a variety that will sell and produce a high-quality product for which costs don’t exceed reasonable profits. This recipe can work for any grower of any commodity, including organic sweetpotato producers.

Table 8-1 provides organic sweetpotato producers with a choice of varieties and their key characteristics and pest control considerations. Covington is the most grown sweetpotato variety in North Carolina and accounts for about 90 percent of the state’s production. It is grown conventionally and organically. Some key reasons Covington is the number one variety grown in North Carolina is that it is highly adapted to production throughout the state. It has high yields (not the highest), but its roots are somewhat short, and the pack out of this variety is very high. In other words, few culls or misshapen roots are produced, so most of the sweetpotatoes harvested can be sold. It also can be stored for a long period (up to 12 months) if proper curing and storage conditions are maintained. The skin color is a light-rose, and the flesh is orange, which is what most markets are used to and often demand. Covington is a later plant producer and only produces an average number of plants. Its disease package is good; however, it is susceptible to insects, especially the complex of pests that includes certain wireworms, rootworms, and flea beetles, known as wireworm, Diabrotica spp., Systena spp. complex (WDS). These pests are very challenging in producing sweetpotatoes. One general management tool is to avoid fields in which corn was grown the previous season as corn wireworm can be a serious pest if sweetpotatoes following corn.

Some other commonly grown sweetpotato varieties are Beauregard and Orleans, which are high yielding. Orleans is very similar to Beauregard but tends to produce less misshapen roots than Beauregard. Like Covington, Beauregard and Orleans have a light-rose skin and orange flesh. Both Beauregard and Orleans are early and prolific plant producers. Unlike Covington, these varieties are susceptible to root knot nematode and flea beetle. Both Beauregard and Orleans produce very smooth skins and cure and store well.

Averre is a new variety released by NC State University in 2018. It has very high yields, a short growing season, and is an early and high production plant producer. Like Covington and Beauregard, Averre has similar light-rose skin and orange flesh. It is susceptible to root knot nematode, WDS complex, and flea beetle, and has a relatively short storage life. It should be sold by Christmas.

Bayou Belle was developed for french fry processing and has very high yields, a short growing season before roots are ready for harvest, and high plant production in which plants are ready early. The skin of Bayou Belle is red and its flesh is orange. It is not a mainstream fresh market variety but nonetheless has very good eating quality. The roots it produces vary in shape. Bayou Belle is resistant to fusarium and moderately resistant to root knot nematode, while it is susceptible to the WDS complex and flea beetle.

A sweetpotato with red skin is what a substantial number of small-acreage sweetpotato growers produce. The variety they often produce organically is Carolina Ruby, and clientele have found this variety to have excellent culinary characteristics. Carolina Ruby has high yield and is ready for harvest early in the production season. It has very high plant production, and those plants are produced early in the season. It has a dark-red skin and orange flesh. Carolina Ruby is susceptible to root knot nematode, the WDS complex, and flea beetle. The roots of this variety do tend to crack if moisture conditions fluctuate.

There are only two varieties listed in Table 8-1 that have moderate resistance to the WDS complex and flea beetle with good resistance to fusarium and root knot nematode. One variety is Murasaki-29, a variety released by the Louisiana State University Experiment Station that has a purple skin and white flesh. Yields are average, it is ready for harvest late in the season, and it has very high, early plant production. NC04-531 is a clone developed at NC State that should be released within a year and is well-suited for organic production due to its very good resistance to many diseases and insects. As compared with Murasaki-29, it has a dark-rose skin and orange flesh. NC04-531 has an upright growth habit which makes it more conducive for cultivation and potential improved weed control. Although NC04-531 has good pest resistances, several yield studies have been conducted indicating that it has below average yields.


Table 8-1 Sweetpotato variety agronomic and insect pest and disease resistance characteristics.

Variety

Yield1

Days to Harvest2

Plant Production3

Color

Pest Resistance / Susceptibility5

Flesh4

Skin

Fus

SRKN

Strep

WDS

Flea Beetle

Averre

VH

E

E-H

O

Light Rose

R

S

MR

S

S

Bayou Belle

VH

E

E-H

O

Red

R

MR

MR

S

S

Beauregard

H

E

E-H

O

Light Rose

R

S

MR

S

S

Bonita

A

M-L

E-H

W

Cream

MR

R

MR

S

S

Carolina Ruby

H

E

E-VH

O

Dark Red

R

S

MR

S

S

Covington

H

M

L-A

O

Light Rose

R

R

MR

S

MS

Evangeline

H

M

ML-A

O

Dark Rose

R

R

MR

S

S

Georgia Red

BA

L

E-H

O

Red

S

MS

S

S

S

Jewel

A

M

E-H

O

Orange

R

R

S

S

S

Murasaki-29

A

L

E-VH

W

Purple

R

R

MR

MR

MR

NC04-531

BA

L

M-H

O

Dark Rose

R

R

MR

MR

MR

Orleans

H

E

EM-H

O

Light Rose

R

S

MR

S

S

Porto Rico

BA

L

E-VH

O

Light Orange

S

MS

S

S

S

1 Yield: VH = Very High, H = High, A = Average, BA = Below Average.
2 Days to Harvest: E = Early (~90 to 100 days after planting, DAP), M = Midseason (~101 to 115 days after planting, DAP), L = Late Season (~ > 115 days after planting, DAP).
3 Plant Production: E = Early, M = Middle, L = Late; VH = Very High Plant Production, H = High Plant Production, A = Average Plant Production.

4 Color Flesh: O = Orange, W = White.
5 Disease and Insect Ratings: Fus = Fusarium wilt (Fusarium oxysporum), SRKN = Southern root knot nematode (Meloidogyne incognita), Strep = Streptomyces Soil Rot or Po ipomoea), WDS = Wireworm Diabrotica Systena Complex (wireworms, Diabrotica -cucumber beetles, Systena -flea beetles, Sweetpotato flea beetle (Chaetocnema confinis) (R = Resistance, MR = Moderate Resistance, S = Susceptible, MS = Moderate Susceptibility).


Another white flesh, white-skinned variety to consider is Bonita. Bonita has a white skin and cream flesh and produces average yields. It has early, high plant production but it not ready to harvest until mid to late season. It has resistance to fusarium and root knot nematode but is susceptible to the WDS complex and flea beetle.

Evangeline is a variety with dark-rose skin and orange flesh that produces high yields. Its plant production is average and can be problematic if the roots are covered too deep by soil during the bedding operation. Evangeline produces plants midseason to late season. It is resistant to many of the key diseases but susceptible to the key insect pests.

Other older varieties that are produced on a more limited basis are Georgia Red, Jewel, and Porto Rico. These varieties still have a following by some clientele. Being older varieties, they are susceptible to key diseases: fusarium, root knot nematode, and Streptomyces species soil rot, as well as the key insect pests that affect sweetpotatoes. These varieties all have an orange flesh; the variety Jewel has an orange skin, the variety Georgia Red has a red skin as its name suggests, and Porto Rico has a light-orange skin. All these varieties have below average yields and produce high amounts of plants early. Jewel is ready to harvest midseason, while roots from Georgia Red and Porto Rico are produced late in the season.

Seed or Plant Considerations

Plants should be obtained from producers who produce and sell sweetpotato seed certified by the NC Crop Improvement Association (NCCAI). Doing so provides assurance that the plants are true-to-type and are the highest quality plants or seed roots. The roots are inspected by the NCCAI to minimize pest incidence, especially diseases such as virus incidence in the G0 plants that are purchased annually by NC Certified Sweetpotato Producers. Plants are maintained by the Micropropagation Center and Repository (MPUR) in tissue culture to reduce the occurrence of mutations and are tested each year for viruses. The NC Certified Sweetpotato Seed Producers obtain new planting stocks every year that have been tested for diseases by the MPUR located on the campus of NC State. When obtaining certified sweetpotato plants each year, the highest quality planting stock is maintained in the industry.

Currently, there is only one seed producer offering several organic sweetpotato varieties for sale: Jones Family Farms in Bailey, NC.

Planting Date

Most commercial sweetpotatoes are planted in late May, and completion of planting is targeted by the end of June. Limited planting is sometimes made in late April; however, plants must be produced in the greenhouse to achieve this early plant date. An early plant date with sweetpotato is not necessarily advantageous as sweetpotato is a tropical-season plant that is grown in the temperate climate of North Carolina. In other words, sweetpotatoes are a warm-season crop and thrive when daytime temperatures are near 90°F and nighttime temperatures are in the upper 60s°F or 70s°F. These temperatures are more common from June into September. Little advantage in growth and shortening the length of season will be gained if cold weather persists into mid-May. Some growers plant in early July. A successful crop can be obtained if the variety selected has a relatively short growing season and growing conditions are favorable. On average, 50 percent of the sweetpotato crop is planted by June 10 with harvest beginning as early as late August through October.

Row Spacing and Plant Population

Sweetpotato between-row spacing depends on the machinery that a grower has, especially when producing another transplanted crop, often tobacco. Thus, spacing between rows will range as close as 36 inches to as much as 48 inches. Most commercial sweetpotato growers use 42-inch or 44-inch spacings between rows. Narrower row spacing potentially results in higher yield due to better land use than wider between-row spacing. Narrower between-row spacing provides the added benefit of covering the row middles with vegetation and results in better weed control.

In-row plant spacing can vary from 8 to 14 inches. Closer in-row spacing allows for more efficient production. Also, placing plants closer together may delay root sizing due to increased competition between plants. Planting plants further apart in-row helps to promote storage roots sizing, thus earliness or less time is needed in the field. Placement of plants in the row may be a way to manage root sizing so that harvest times vary. Close in-row spacing may also aid in weed control by out-competing weeds and reducing light penetration so weed seeds do not germinate. Generally, plant populations per acre vary between 12,500 (when in-row spacing is 12 inches) and 15,000 (when in-row spacing is 10 inches) and between-row spacing is 42 inches. Increasing plant populations results in more plant costs, and allowance should be made so that roots attain adequate size to maximize yields and realize more profits.

Nutrition Considerations

Very limited research has been conducted on managing fertilizer practices for organic production of sweetpotatoes. Rather, most work has emphasized the use of synthetic fertilizers for conventional sweetpotato production. For Covington, approximately, 80 lb/acre nitrogen is typically applied in two separate applications. For organic sweetpotato production, nutrition derived from slower-release fertilizers such as poultry litter may require different fertilizer application considerations both in the current growing season or subsequent growing year. Cover crops such as clover or hairy vetch can be used to contribute nitrogen as well. Fortunately, most soils in North Carolina contain plenty of phosphorus. Even though this is the case, about 50 lb/acre is routinely applied to a conventionally grown crop. Potassium is typically available in most North Carolina soils as well, but it is common practice for growers to apply 150 to 200 lb/acre potash for the growing season. As more research is conducted on fertilization and nutrition aspects of growing organic sweetpotatoes, this section will be updated with more detailed information.

Weed Management

Growers rank weed control as the number one barrier in production of organically produced crops. Weed competition during storage root initiation (first three to four weeks after transplanting) and the growth stage of sweetpotato in which storage roots size up directly impact the number of storage roots produced per plant, the size of each storage root at harvest, and the resulting yield and quality (Gajanayake et al. 2013, 2014, 2015; Meyers et al. 2014; Pardales and Yamauchi 2003; Villordon et al. 2012). Control of all weeds is difficult and is often not achievable. A goal of weed control in organically produced sweetpotato is to increase crop competitiveness while also reducing weed competition to an acceptable level. To meet this goal, growers should (1) properly transplant a sweetpotato cultivar with a bunch-type growth habit that is capable of reaching row closure sooner and (2) maintain fertility programs such that sweetpotato growth is enhanced and crop competition with weeds is maximized (Harrison and Jackson 2011). Generally, sweetpotatoes are grown on 9-inch to 12 inch in-row spacing. Sweetpotato has been shown to be more competitive with Palmer amaranth when grown on 6-inch in-row spacing (SC Smith, unpublished data), therefore it might be beneficial to plant sweetpotato at 6-inch in-row spacing in fields with a high population of Palmer amaranth.

Fields with high weed populations should also be planted when temperature is optimum for sweetpotato growth, which is usually during the later portion of the planting season. In addition, growers must start with a weed-free field at transplanting and then control weeds for at least six weeks (around sweetpotato canopy closure) after transplanting. Weeds that emerge after six weeks after transplanting do not normally reduce yield or quality of sweetpotatoes. However, controlling weeds from six weeks to harvest is beneficial as it prevents weed seed being added to the weed seed bank in the soil which will be the source of future weeds.

In North Carolina, cultivation and hand removal are the two most important weed control strategies employed in organic sweetpotato production (KM Jennings, personal communication). Shallow cultivation should begin as soon as the crop is established (usually 10 to 14 days after transplanting). For most organic fields in North Carolina, two to three cultivations will be necessary to control most of the weed populations. Weeds that escape will need to be hand-removed before they reach a large size to not only reduce competition with the crop but also to avoid uprooting the crop. If weeds reach several feet tall, then cutting the weed at the soil surface should be used to avoid uprooting the crop. Weeds that extend above the sweetpotato canopy can be mowed; however, this removal provides limited control. Palmer amaranth mowed above the canopy branches below the cut and results in a dense canopy just above the sweetpotato canopy (Meyers et al. 2017).

Specific weeds

Palmer amaranth

Palmer amaranth is the worst weed in organically produced sweetpotato in North Carolina because it can germinate quickly (within a day) under favorable conditions, grow at a rate of 2 to 5 inches per day, and has the potential across many environmental conditions to grow over 10 feet tall and produce many seeds (a single female Palmer amaranth can produce over 500,000 seeds). Because of these characteristics, it easily competes against the crop for water, nutrients, and light. It can produce viable seeds 30 days after germinating (KM Jennings unpublished data; Legleiter and Johnson 2013).

Control of Palmer amaranth must begin early in the season in sweetpotato, as approximately 10 percent total yield loss can occur if this weed is present during the first three weeks after transplanting. As Palmer amaranth remains in the crop past three weeks after transplanting, total yield loss can increase to 70 percent and number one grade yield loss can reach 90 percent (Smith et al. 2017). Season-long competition of Palmer amaranth at densities of 1 to 13 plants per 6.6 feet of sweetpotato row will likely result in a 35 percent to 80 percent marketable sweetpotato yield reduction (Meyers et al. 2010). Pigweed species such as Palmer amaranth should be removed from fields to also prevent reestablishment and seed production. Palmer amaranth can re-root along the stem if left in the field (LM Sosnoskie, University of Georgia, unpublished data).

Yellow nutsedge

In North Carolina, yellow nutsedge is among the most troublesome weeds in organically produced sweetpotato (Meyers and Shankle 2015b; Webster 2014; SC Smith and LD Moore, unpublished data). Yellow and purple nutsedge are perennial weeds that spread by underground vegetative structures of rhizomes and tubers (Meyers and Shankle 2015b). It is critical to begin controlling yellow nutsedge when it is first detected in a field because a single yellow nutsedge plant from a spouted tuber can form a dense patch (105 shoots per square foot) after six months of growth (Webster 2005). Population density of yellow nutsedge can easily increase as much as 7percent in a four-month period, making it very difficult to achieve control (Meyers and Shankle 2015a). Managing yellow nutsedge to keep its density as low as possible is important because marketable sweetpotato yield losses of 18 percent to 80 percent have been observed. Note that yellow nutsedge density increases from 0.5 shoots per square foot to 8.4 shoots per square foot (Meyers and Shankle 2015a). Yellow nutsedge’s high potential for vegetative growth and reproduction and difficulty in its control necessitates that management strategies for nutsedge be focused on prevention (control prior to transplanting), early detection and treatment (cultivation, hand removal if possible), and integration of control strategies (crop rotation, optimum transplanting date, optimum sweetpotato growth, timely cultivation) to reduce its effects on sweetpotato growth and yield and its spread (Meyers and Shankle 2015a, 2015b; Ransom et al. 2009). Equipment sanitation to prevent spread of tubers from field to field is a critical preventive method (Meyers and Shankle 2015b).

Annual and perennial grasses

In North Carolina, annual (large crabgrass, goosegrass, barnyardgrass, broadleaf signalgrass, fall panicum) and perennial (johnsongrass) grasses are observed often in sweetpotato fields. The grasses generally emerge between field preparation and early in the growing season. All these grasses can grow taller than sweetpotato when they emerge early in the season before extensive sweetpotato growth occurs. Grasses not controlled early in the season must be controlled prior to the last third of the crop growing season when sweetpotato storage roots are sizing up. Otherwise reduced crop vigor, yield, and quality will result. In addition, control should occur before grasses produce seeds (annual and perennial grasses) or reproductive rhizomes (perennial grasses such as johnsongrass).

Annual morningglory (many species)

Entireleaf morningglory, ivyleaf morningglory, pitted morningglory, tall morningglory, and smallflower morningglory are commonly observed in sweetpotato fields in North Carolina. These weeds establish from seeds and have a growth habit that is vining, like sweetpotato. They must be controlled early in the growing season through cultivation and hand removal because as they intertwine with sweetpotato and other weeds, they become impossible to control. As these weeds intertwine with sweetpotato and weeds, they grow to the top of the crop canopy resulting in rapid growth and seed production (Price and Wilcut 2007). Being positioned at the top of the canopy allows morningglory to compete with the crop for light, nutrients, and water, which can reduce sweetpotato yield and quality. Morningglory seeds remain viable in the soil for many years, likely due to their hard seed coat (DeFelice 2001; Elmore et al. 1990). Thus, morningglory should be controlled prior to the weed vining and producing seeds.

Purslane (common and pink)

Common purslane and pink purslane are low-growing annual weeds and are common in North Carolina sweetpotato fields. Purslanes have thick succulent stems and leaves that contribute to their ability to thrive in many environmental conditions, including dry conditions. They reproduce from seeds or from fragmented stems sections having a node (Holm et al. 1977; Proctor 2013; Proctor et al. 2011). Purslane seeds have reportedly remained viable for as long as 40 years (Darlington 1941). To prevent seed production, cultivation or hand-hoeing of purslane should occur before three weeks after emergence or 125 growing degree days (Haar and Fennimore 2003; University of California 1990). In addition, scouting the field after cultivation is important to verify purslane was controlled and stem sections are not reestablishing. Relatively speaking, while not as competitive as tall weeds like pigweeds, purslanes add to the competitive load of a weed population in an organic sweetpotato field.

Insect Pest Management

Pest control in organic sweetpotato requires an integrated approach that minimizes risk for insect damage. Unlike conventional production that can leverage acutely toxic insecticides to disrupt pests, organic systems depend on longer term reductions in pest populations to achieve economically acceptable levels of damage. Because the number of OMRI-listed insecticides for sweetpotato pests is currently limited, a comprehensive integrated pest management (IPM) program that includes multiple tactics to limit pest infestation is important. A holistic approach that incorporates crop rotation, insect-resistant varieties, and other tactics must be used to mitigate risk for insect damage and economic loss.

Important sweetpotato pests feed on foliage and developing roots below the soil surface; however, many of these pests only cause economic injury when infestation levels are high. Typically, foliage feeding pests cause sporadic problems in organic sweetpotato but can cause significant losses when left uncontrolled. Soilborne insect pests that feed directly on developing roots are the most important and consistent group of pests that cause economic damage to the crop annually.

To understand the pests that can damage the sweetpotato crop, we have organized this section of the chapter by pest complex and the location where problems typically occur (greenhouse, field, or storage unit). A general description of each pest’s interaction with the crop is included to highlight key vulnerabilities that producers can exploit to manage populations.

Greenhouse pests

Aphids and whitefly

Aphids and whitefly are small soft-bodied hemipteran insect pests that feed on plant sap using piercing-sucking mouthparts. When infestations are large, these pests can reduce plant health through direct feeding. In contrast, low numbers of insects can vector plant viruses that harm the plant and cause yield impacts (such as sweetpotato feathery mottle virus, sweetpotato leaf curl virus). Because these insects can spread viruses among plants, purchasing virus-free and vector-free seed or slips from a certified seed producer is a key step to limit the potential for disease spread during the season.

Aphids and whitefly can be an issue during seed propagation in the greenhouse and occasionally in the field. Scouting greenhouses for insect presence is an important strategy to document the establishment and spread of pest populations. Adult whitefly can be observed flying around the plant canopy when disturbed. Looking at the undersides of sweetpotato leaves for whitefly larvae (crawlers), aphids, honeydew, and sooty mold will reveal obvious signs of a growing infestation. For greenhouse seed production, planning a plant-free period before propagating sweetpotato seed can limit carryover of pests from previous greenhouse crops. Several OMRI-approved products are listed for aphid and whitefly control; however, some active ingredients (pyrethrum) may have limited efficacy where resistance occurs.

Mites

Mites are a common pest during greenhouse seed production. Common mite species are twospotted spider mite (Tetranychus urticae) and broad mite (Polyphagotarsonemus latus). Mites use piercing-sucking mouthparts to damage plant cells, a process that causes leaves to curl, brittle, and shrivel. Heavy mite infestations can cause characteristic bronzing and severe deformation of sweetpotato foliage.

In the greenhouse, mites can infest adjacent plants via touching foliage or by workers handling infested foliage and then touching uninfected plants. Because mites have a high reproduction rate, populations can reach damaging levels quickly if conditions are favorable. Preventive control measures for organic greenhouse systems start with clean seed stock and good sanitation practices when moving between greenhouses. Mineral oil and insecticidal soaps are useful tools to reduce mite populations. Maximizing coverage throughout the plant canopy is essential for adequate efficacy with miticidal soaps and oils. Always use a high rate and spray gallonage to improve coverage on leaves and stems. Frequent reapplication may be necessary to adequately control problematic populations.

Predatory mites can be an effective biological control agent when densities of pest mite populations are low to moderate. Phytoseiulus persimilis is a species of predatory mite that is commercially available for greenhouse mite control and can be effective for twospotted spider mite control. Carefully read all infestation instructions and monitor greenhouse temperature conditions to improve predatory mite establishment.

Thrips

Western flower thrips (Frankliniella occidentalis) are a common pest in many greenhouse crops. In sweetpotato, thrips feed on leaf tissue, often causing silvering when pressure is high. Adults are very mobile, but larvae tend to aggregate near leaf venation and can be observed using a hand lens. To scout for thrips, strike sweetpotato foliage on a white paper plate at several locations throughout the greenhouse. Light-brown, cigar-shaped adults and larvae will be apparent when densities are high.

Thrips do not typically cause significant injury to sweetpotato seed plants in the greenhouse. However, if infestations have been problematic in the past, proactive management is important to minimize damage and maintain plant health. Spinosad is an effective thrips material when applied in a timely manner before large populations become established. Beauvaria bassiana and botanically based OMRI-approved insecticides (azadirachtin and pyrethrins) also have activity on thrips but may require more frequent applications under high infestation levels.

Scout greenhouses weekly for thrips, aphids, and whitefly infestations. Always sample from multiple greenhouse locations and positions in the plant canopy. Because management of greenhouse pests is a challenge, detecting developing pest populations is crucial to a timely and effective response.

Field pests

Wireworms and other root-feeding pests

Wireworms are the primary insect pest of organic sweetpotato production systems in the Southeast. In eastern North Carolina, there are eight different wireworm species that have been documented in sweetpotato fields; however, tobacco wireworm (Conoderus vespertinus Fabricius) is the most common pest of the crop. Currently, very little is known about the wireworm species that infest sweetpotato or white potato in the NC piedmont or mountains.

Wireworms have varied lifecycles that make these pests a significant challenge for organic sweetpotato growers. Wireworms can take between one to four or more years to complete development from egg to adult. The long period spent as larvae (more than 95 percent of the lifecycle) results in multiple generations of wireworms in the soil that can cause different amounts of damage depending on size. Wireworm larvae feed on developing sweetpotato roots throughout the growing season, causing pinhole damage sites on the outside of roots. Sweetpotato plants often compensate for early-season wireworm damage by forming a callous skin over the feeding site, often causing irregular root development.

Crop rotation for wireworm. Because wireworm larvae can persist in the soil for several years, long-term crop rotation away from suitable host plants is one of the most effective methods to reduce the risk of root damage. For most organic growers, planning crop rotations that do not involve corn is one strategy to reduce wireworm damage on sweetpotato. Several key wireworm species in North Carolina will preferentially lay eggs in crops like corn and small grains. Fallow fields with high numbers of weeds in the autumn can be important alternate hosts for wireworm larvae. Separating the sweetpotato crop from these alternate hosts in time will reduce the abundance of larvae and, in turn, the risk for root damage. Rotation to a less desirable host plant (soybean) will allow existing larvae to complete development and leave the system.

Over the past several years, organic growers have transitioned pastureland to certified organic sweetpotato production. While there may be agronomic benefits to planting sweetpotato in recently converted land, the risk for wireworm damage is high because grasses are preferred alternate hosts of several economically important wireworm species. Avoid planting sweetpotato on land that had been pasture during the past several years. Extending the period between pasture and a sensitive root crop such as sweetpotato or white potato will be important to minimize damage risk.

Varietal selection. Choosing the correct sweetpotato variety for the target market is an important decision for an organic farmer. Understanding what specific varietal characteristics fit your market is crucial; however, some varieties do have greater disease and insect tolerance. New sweetpotato varieties developed by the Potato and Sweetpotato Breeding and Genetics Program at NC State have pest and disease resistance characteristics. See Table 8-1 for more information about insect-resistant and disease-resistant sweetpotato varieties.

Chemical management. Controlling wireworms with chemicals is an ongoing challenge for both organic and conventional sweetpotato farmers. For organic systems, there are a limited number of OMRI-approved plant and microbial-based biological insecticide options that have efficacy on wireworms. These insecticides are typically incorporated into the soil before bed forming and again at fertilizer layby. For preplant applications, thorough incorporation is important to establish an insecticidal barrier that will limit movement of existing wireworms into the root zone. Applications at layby also need to be incorporated to create an insecticidal barrier for insects laying eggs in the crop during the growing season (tobacco wireworm). This tandem approach is important to maximize protection. For all applications, consider using the highest labeled application rate and an increased spray volume to ensure application uniformity in the soil profile.

Caterpillar pests

Caterpillars can be a minor defoliating pest late in the sweetpotato growing season. Although several different lepidopteran larvae (caterpillars) species can infest the crop, beet armyworm and corn earworm are common in NC. Infestations are often very sporadic within and among fields, causing minimal economic damage because infestations typically occur after root bulking. Larvae will also feed directly on exposed sweetpotato roots after vine destruction. These larvae are occasionally found on infested roots in storage facilities, causing some concern for postharvest contamination.

Several OMRI-approved insecticides are available for caterpillar control in sweetpotato. Spinosad and Bacillus thuringiensis (Bt) insecticidal formulations have efficacy on most caterpillar species found in the crop. Larvae must be actively feeding on treated leaves, so uniform spray coverage across the canopy is essential. Using an approved spreader-sticker and increased spray volume may improve canopy coverage and rain-fastness of sprays. Always use higher label rates and shorten the time between applications to ensure adequate efficacy under high levels of infestation.

Storage pests

Fruit flies

Damaged or rotting produce are an ideal food source for several different species of fruit flies found in storage facilities. These insects lay eggs in open wounds in the skin of the sweetpotato, where the larvae subsequently develop and pupate. Improving the quality of roots entering storage through proper curing is an important factor to limit large fruit fly infestations and postharvest disease.

Fruit flies can be managed with attract-and-kill stationary fly traps equipped with vinegar and yeast baits. Pyrethrin-based insecticides can also be applied in storage houses using ultra low volume (ULV) foggers, which produce small insecticide droplets that can cover and penetrate spaces around roots. Spaces around roots increase the probability of contacting adult fruit flies. While the number of flies may be reduced using this approach, there is minimal evidence that increasing fly mortality relates to higher postharvest root quality.

Postharvest Curing and Storage

Proper curing and storage of sweetpotato is an important step in the production process that can ensure stability of the product from the field to the consumer. The curing process improves the visual appeal, shelf life, and flavor characteristics of harvested roots. Successful curing requires proper infrastructure to maintain roots at a set temperature (85°F) and humidity (85 to 90 percent relative humidity) for three to five days. Alternatively, sweetpotato roots can be packed and consumed “green”; however, green roots may not possess the same characteristics and storage ability of cured roots.

For more information on sweetpotato storage, see Postharvest Handling of Sweet Potatoes, (NC State Extension publication AG-413-10).

References

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Authors

Associate Professor
Horticultural Science
Associate Professor, Plant Pathology (Cucurbits and Sweetpotato)
Entomology & Plant Pathology
Specialist, Sweetpotato/Curcurbits/Sweet Corn
Horticultural Science
Crop & Soil Sciences
Professor
Horticultural Science
Sweetpotato Breeding and Genetics
Horticultural Science
Assistant Professor and Extension Specialist, Field Crops and Sweet Potatoes
Entomology & Plant Pathology
Graduate Student
Philip Morris Professor
Biological & Agricultural Engineering

Publication date: June 24, 2019
AG-660

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