Sweetpotato pest challenges and management options

Abstract

Sweetpotatoes, the seventh most important food crop globally, play a crucial role in agriculture due to their starchy, nutrient-rich roots. Their versatility extends beyond human consumption to include animal feed and various industrial applications such as ethanol and biofuel production. In the United States, the Southeast dominates sweetpotato production, with states like Alabama, Louisiana, Mississippi, and North Carolina leading the way. One of the remarkable features of sweet potatoes is their adaptability to tropical and subtropical regions, their resilience to drought, and their ability to thrive in low-fertility soils. These qualities make sweetpotatoes well-suited for organic farming. The increasing popularity of organic agriculture aligns with rising consumer demand for organic products, with vegetables like sweetpotatoes cultivated on a significant portion of American organic farms. However, sweetpotato crops face threats from plant-parasitic nematodes and insect pests, particularly in the Southeast, where the southern root-knot nematode is a major concern. Conventional farming relies on chemical nematicides and insecticides for pest management; however, these are not suitable for organic production. Organic growers utilize biopesticides and cultural practices to manage nematode infestations and insect pest populations. These practices include the use of entomopathogenic fungi and nematodes, as well as cover cropping to improve soil health and control pests. Overall, sustainable sweetpotato cultivation involves a combination of biological control methods and cultural practices to mitigate the impact of pests and maintain soil health, thereby ensuring the viability of sweetpotato production for future generations.

Sweetpotato (Ipomoea batatas (L.) Lam.) is considered the seventh most important food crop in the world (FAO 2023). It is a member of the Convolvulaceae family, which includes morning glory (Mukhopadhyay et al. 2011). This vegetable crop is primarily cultivated for its starchy, nutrient-dense roots which are consumed as human food, animal feed, or for industrial uses (Truong et al. 2018). In addition, sweetpotatoes can be used to produce ethanol, industrial alcohol, and biofuel (Bovell-Benjamin 2010). In 2022, the United States harvested 53,419 hectares (132,200 acres), of sweetpotatoes which was worth $598,424,000 (USDA 2023). American sweetpotato production primarily occurs in the southeastern region of the United States with Alabama, Louisiana, Mississippi, and North Carolina producing 75% of domestic sweetpotatoes. In 2017, Alabama harvested 881 hectares (2,178 acres) of sweetpotatoes (USDA 2023). These plants are cultivated around the world and are best suited to tropical and subtropical regions (Mukhopadhyay et al. 2011). They are vegetatively propagated and often grown from vine cuttings called slips (Jatala and Bridge 1990). Sweetpotatoes prefer sandy, well-drained soils, which allow their storage roots to expand without restriction (Mukhopadhyay et al. 2011). Due to its well-developed root system which can reach deep into the soil profile, sweetpotato is considered a drought-tolerant vegetable that can be grown in low-fertility soils (Jatala and Bridge 1990, Fuentes and Chujoy 2009). These desirable agronomic qualities make this crop well-suited to low-input production systems, common throughout the developing world. Additionally, sweetpotatoes are highly nutritious, with orange and yellow-fleshed cultivars recognized as good sources of carotene and vitamin C (Mukhopadhyay et al. 2011). Interestingly, their vibrant colors make sweetpotatoes useful as food colorants used during food processing (Mukhopadhyay et al. 2011).

Organic Agriculture

The number of organic farms in the United States has increased to fulfill growing consumer demand for certified organic products (Lotter 2003). Vegetables, which include sweetpotatoes, were grown on 57% of American organic farms in 1997, and they continue to be popular crops for organic farmers today (Greene and Kremen 2003). Organic producers have identified insects as a significant pest of vegetables. (George et al. 2024). Since synthetic pesticide use is prohibited in organic systems, farmers employ ecologically based pest and fertility management strategies (Greene and Kremen 2003). However, these strategies are often less effective than conventional practices for pest and fertility management. In fact, organic farmers cite ‘effectiveness of organically allowable inputs and methods’ as a key production constraint on their operations and identify the need for further research into organic management of insect and nematode pests (Walz 1999, George et al. 2024). Despite these limitations, many farmers continue to produce organic products that can command a higher market price. (Lotter 2003). Other benefits of organic farming include improved soil health, lower pesticide use, greater ecological harmony, and reduced energy inputs (Greene and Kremen 2003).

Plant-parasitic Nematodes

Parasitic nematodes are obligate parasites that require living plant hosts to develop and reproduce (Oka et al. 2000). These nematodes attach themselves to plant roots where they withdraw water and nutrients, resulting in a loss of plant vigor and a reduction in plant health (Oka et al. 2000). Despite being among the most widespread and costly pests, comprehensive data on the economic impact of plant parasitic nematodes remain limited, particularly for crops produced in resource-poor areas. In tropical and subtropical climates, nematodes cause an estimated 14.6% crop production loss, compared to 8.8% in developed countries. More critically, only about 0.2% of the crop value lost to nematodes is allocated to nematological research aimed at mitigating these losses (Sasser and Freckman 1987). In fact, an international survey with 371 responses estimated crop losses of up to 10.2% in sweetpotato yield were the direct result of plant-parasitic nematodes (Nicol et al. 2011, Palomares-Rius and Kikuchi 2013). The most economically important plant-parasitic nematodes affecting sweetpotato are Meloidogyne spp., Pratylenchus spp., Rotylenchulus reniformis (Linford and Oliveira 1940) (Fig. 1), and Ditylenchus destructor (Jatala and Bridge 1990). To a lesser extent, M. arenaria (Neal) Chitwood, M. hapla Chitwood, and M. javanica (Treub) Chitwood (Overstreet 2013) also may affect sweetpotato. Recently, a new species, M. enterolobii, (Yang & Eisenback) has emerged as a severe root-knot nematode pest due to its extensive host range, prominent level of virulence, and capability to overcome the Mi-resistant genes in a wide selection of vegetable crops (Brito et al. 2020, George et al. 2024). Root-knot nematodes (Meloidogyne spp.) are the most serious pest in this crop because they can drastically reduce sweetpotato yield and quality (Kim and Yang 2019). Looking forward, plant-parasitic nematodes could become even more problematic pests. In fact, researchers in Asia have reported increased plant-parasitic nematode damage to sweetpotato due to climate change and global warming (Kim and Yang 2019).

Southern root-knot Nematode

The southern root-knot nematode (Meloidogyne incognita (Kofoid and White)) is a sedentary endoparasitic nematode with a broad host range of > 3000 plant species, including sweetpotato (Abad et al. 2003, Wednimu 2021). This nematode causes damage that reduces both quality and yield in sweetpotato in addition to many other economically important cropping systems (Jatala and Bridge 1990, Wednimu 2021). The primary visual symptom of root-knot nematode infection (Fig. 1) is the presence of root galls on host plant root systems (Bird 1974). Higher M. incognita population densities are associated with more and larger galls (Overstreet 2013). The degree of galling is variable and associated with sweetpotato cultivar; with some cultivars producing many large galls, while others producing smaller, less inconspicuous galls (Overstreet 2013). These galls injure the root system by interrupting nutrient and water uptake (Abad et al. 2003). Nematode infection results in weak, stunted, and low-yielding plants that display symptoms of nutrient deficiencies (Abad et al. 2003). In sweetpotato, infection with M. incognita results in smaller plants, earlier maturity, lower yields, and fewer marketable potatoes (Agu 2004). Meloidogyne incognita also interacts with pathogens like Fusarium spp. and Pseudomonas solanacearum (Smith) forming disease complexes. These complexes can cause wilting and early death (Jatala and Bridge 1990). All races of M. incognita can infect sweetpotato at varying degrees, but in the United States, race 1 and race 3 are most common in sweetpotato (Taylor and Sasser 1978, Jatala and Bridge 1990). The nematodes invade both feeder and storage roots at similar rates (Jatala and Bridge 1990). Infected feeder roots are typically shorter and have fewer secondary roots and root hairs (Jatala 1991). Meloidogyne incognita infection also causes longitudinal cracking and galling of the sweetpotato root surface, reducing quality, and marketability (Overstreet 2009). Cracks that begin early in the growing season are long and extend deep within the periderm, whereas cracks that begin later in the season are shorter and shallower (Jatala 1991). Especially in the later season, this cracking allows pathogenic organisms to enter the sweetpotato root, causing rot (Lawrence et al. 1986). Meloidogyne incognita females can be observed embedded within the sweetpotato root when roots are sliced at 0.5 cm (Fig. 1). These females are often surrounded by necrotic root tissue and egg masses (Fig. 1) (Lawrence et al. 1986). Meloidogyne incognita continue to colonize the developing sweetpotatoes with 4 to 6 generations per growing season.

Guava Root Knot Nematode

The guava root-knot nematode M. enterolobii, the newest root-knot nematode species, and was first reported in the continental U.S. in Florida in 2002. It has spread to Georgia, Louisiana, North and South Carolinas (Rutter et al. 2019). It is suspected to spread through infected sweet potato slips used for planting. Popular resistant sweetpotato cultivars are ineffective against M. enterolobii, resulting in interstate quarantines on sweetpotato slips and tubers (Hare 2019, George et al. 2024). Preventing M. enterolobii infection is crucial. Growers should use certified clean seed material, even though it is more expensive, as it is free from all root knot nematode species and other pathogens.

Managing all plant parasitic nematodes is challenging once a field is infected. Management is difficult due to its ability to persist in soil for years and quickly rebound when a susceptible host is available. While rotations to non-host crops like sunn hemp can reduce nematode populations, these populations quickly rebound when a sweetpotato is planted (Smith et al. 2017). Some sweetpotato cultivars, such as Covington, can offer moderate resistance to Meloidogyne species but are highly susceptible to M. enterolobii (George et al. 2024). Multiple resistant germplasms have been reported, but no M. enterolobii-resistant cultivars have been commercially released in the United States (Rutter et al. 2021, Schwarz et al. 2021). Prevention is the best management technique. Nematodes are often spread in sweetpotatoes through infected plant material. Certified clean slips can be initially more expensive because they are grown as either first- or second-generation material from sterile tissue culture. Subsequent plants are increased in clean fields, monitored for nematodes and other sweetpotato pathogens. Using certified clean slips can reduce the risk of new nematode infestations.

Insect Pests

Two hundred seventy species of insects are considered pests of sweetpotato around the world, with the majority being foliar feeders, followed by stem, vine, root, and flower feeders (Chalfant et al. 1990). In the southeastern United States, sweetpotato growers identify cucumber beetles, sweetpotato weevils, white grubs, and sweetpotato flea beetles as causing the most economic damage to their crop (Fig. 2). Larval stages of these pests feed directly on the storage roots, causing losses in yield and marketability (Ames et al. 1996). These pest problems often result in reduced income for the growers (Nwosisi et al. 2021).

Sweetpotato Weevils

Across the globe, sweetpotato weevils (Cylas formicarius Fabricius) (Coleoptera: Curculionidae) (Fig. 3) are considered the most important insect pest of sweetpotatoes in the field or in storage (Chalfant et al. 1990). Cylas formicarius elegantus (Summers) is found throughout the southern United States, from Texas to coastal North Carolina (Chalfant et al. 1990) (Fig. 3). However, in North Carolina, the weevil only attacks the wild Ipomoea spp. found on the Outer Banks (Sorensen 1987). Weevil feeding induces terpenoid and phenol production in the plant that results in bitter, unpalatable sweetpotato roots (Sorensen 2009). Because of this, even low sweetpotato weevil populations can cause extensive economic damage. In fact, over $7 million of crop loss in the southern United States is due to sweetpotato weevil (Sorensen 2009). Sweetpotato weevils complete their life cycle in approximately 33 d at temperatures between 27 °C and 33 °C (Sherman and Tamashiro 1954). Females deposit individual cream-colored eggs into natural cavities created in sweetpotato vines and fleshy roots before sealing each cavity with a fecal plug (Chalfant et al. 1990). Eggs hatch into legless, white first instar larvae after approximately 8 d (Sherman and Tamashiro 1954). Larvae feed by tunneling throughout the sweetpotato root, molting through three instars before pupating (Sherman and Tamashiro 1954). Mature larvae pupate for 7 to 10 d in the sweetpotato root or stem, and the adults emerge by chewing through the plant tissue (Capinera 2018). Adults are 5.5 to 8 mm long, with a black head and abdomen, and reddish-brown thorax and legs. The long rostrum with antennae attached at the midpoint is the sweetpotato weevil’s most striking feature (Capinera 2018). Adult females feed for a day or more before mating, and lay eggs soon afterwards (Capinera 2018). In the United States, strict quarantines have been imposed to limit the spread of the sweetpotato weevil. Restrictions on the shipment of sweetpotato vines and fleshy roots exist at ports of entry and in 14 states (Sorensen 2009). There are additional restrictions in fourteen southern Alabama counties, that are included in the sweetpotato weevil quarantine area (Harden 2015). Sweetpotatoes entering Alabama from other states must be authorized by their state of origin to be appropriately inspected and found to be apparently pest free (Harden 2015). If the sweetpotatoes originated from an area infested with sweetpotato weevil, the products must be properly fumigated to eliminate weevils (Harden 2015).

Wireworm-Diabrotica-Systena Complex

Sweetpotato roots are attacked by a complex of insect pests, including wireworms (Conoderus spp., Melanotus spp., and Heteroderes spp.) (Coleoptera: Elateridae), cucumber beetles (Diabrotica spp.) (Coleoptera: Chrysomelidae), flea beetles (Systena spp.) (Coleoptera: Chrysomelidae), sweetpotato flea beetle (Chaetocnema confinis (Crotch)) (Coleoptera: Chrysomelidae), and white grubs (Plectris aliena (Chapin) and Phyllophaga ephilida (Say)) (Coleoptera: Scarabaeidae) (Schalk et al. 1991, 1993). This group of pests is known as the Wireworm-Diabrotica-Systena or “WDS complex” because the larvae of these beetles cause damage to sweetpotato roots that cannot be differentiated at harvest (Schalk et al. 1993). The WDS complex damage consists of shallow feeding holes ranging from less than 1 mm to 8 mm in diameter (Schalk et al. 1993). However, feeding from these larvae can sometimes reach the vascular cambium, and their holes deepen as the sweetpotato root grows (Fig. 2) (Schalk et al. 1993).

Wireworms

Wireworms are the larvae of click beetles (Coleoptera: Elateridae) (Vernon and van Herk 2022). They are significant pests of many crops, especially potatoes, sweetpotatoes, and corn (Parker and Howard 2001, Hermann et al. 2013). Wireworm species complexes vary with location, but several species cause economic damage to sweetpotatoes in the Southeastern United States (Sorensen 2009). These include the southern potato wireworm Conoderus falli, the tobacco wireworm C. vespertinus, and the wireworm C. amplicollis. Conoderus scissus and C. rudis were found to be the predominant wireworm species in Georgia (Seal et al. 1992). The wireworm Melanotus communis (Gyllenhal) is destructive to sweetpotato along the east coast of the United States (Sorensen 2009). Wireworm feeding reduces sweetpotato quality and results in small holes, narrow tunnels, and scarring to the periderm (Johnson et al. 2008). Wireworms’ life cycles vary from 2 to 3 mo for C. rudis and C. falli to 2 to 3 yr for C. scissus and C. amplicollis (Seal et al. 1992). Most wireworms spend their immature life stages underground, and many species overwinter as larvae in the soil (Willis et al. 2010). Adult click beetles oviposit on the soil near the crop but do not feed on it (Sorensen 2009). Sweetpotatoes are at risk of wireworm damage from the beginning of storage root development in early summer until harvest in the fall (Chalfant et al. 1992, Willis et al. 2010). Thus, few sweetpotato crops are produced entirely free from wireworm damage (Cuthbert 1965).

Cucumber Beetles

The banded cucumber beetle (Diabrotica balteata) and spotted cucumber beetle (D. undecimpunctada howardi) are also members of the WDS complex. Diabrotica balteata is characterized by alternating green and yellow bands on their elytra, while the elytra of D. undecimpunctada howardi have 11 black spots on a yellow-green background (Sorensen 2009). Larvae of this genus chew small holes through the periderm of sweetpotato roots that enlarge as the roots develop (Sorensen 2009). Feeding holes are often found in groups and occur during early root development, resulting in unattractive scarring upon harvest (Sorensen 2009). Adults lay eggs in the soil that hatch in 1 to 2 wk, depending on temperature. The larvae of these species are almost indistinguishable from each other, and this stage lasts for 8 to 30 d but varies with food availability. Pupae are formed in cells just beneath the soil surface and adults emerge after 1 wk. In the warm climate of the Southeastern United States, these insects can overwinter as adults that feed on plants in the Convolvulaceae family (Sorensen 2009).

Flea Beetles

Three Systena species, the elongate flea beetle (Systena elongata), the pale-striped flea beetle (Systena blanda), and the redheaded flea beetle (Systena frontalis) feed on developing sweetpotato roots (Sorensen 2009). The larvae of these species produce root damage that includes small holes and winding tunnels under the surface of the periderm (Sorensen 2009). Pinhole sized holes in the root surface are caused by late-season flea beetle feeding (Sorenson 2009). Systena species, specifically Systena frontalis, undergo complete metamorphosis, and their eggs are white, oval-shaped, and approximately 1mm long (Herrick and Cloyd 2020). The larvae are 5 to 10 mm long with creamy white bodies and brown head capsules. Larvae grow through three instar stages in the soil where they feed on plant roots before pupating (Herrick and Cloyd 2020). After pupating, adults emerge and are 5 mm long with shiny black bodies and red heads (Herrick and Cloyd 2020). Adults possess enlarged hind femurs, that allow them to jump like fleas, hence their common name “flea beetle” (Herrick and Cloyd 2020).

Caterpillars

Caterpillars like beet armyworm (Spodoptera exigua Hubner), corn earworm (Helicoverpa zea Boddie), and soybean looper (Chrysodeixis includens Walker) (Fig. 4) can cause defoliation damage late in the sweetpotato season (Jennings et al. 2019). However, infestation by these pests typically occurs after root bulking, so they cause minimal economic damage. Economic damage is possible if caterpillars are present at high levels in the late season and feed directly on exposed sweetpotato roots.

Chemical Management

Pre-plant management of M. incognita is essential to prepare for a successful growing season. Nematode management generally includes three main strategies: nematicides, cultivar selection, and cultural practices (Overstreet 2013). In general, there are two types of nematicides: fumigant and non-fumigant (Liu and Grabau 2022). Fumigant nematicides are highly effective broad-spectrum products that move through the soil as a gas but are highly toxic and can be hazardous for human health (Desaeger et al. 2020). Because of this danger, fumigant nematicides are facing increasing regulatory pressure, including a ban on several widely used fumigants like methyl bromide (Desaeger et al. 2020). Fumigant nematicides authorized for use in sweetpotato production include Telone II (1,3-Dichloropropene) Dow AgroSciences, Indianapolis, IN; Vapam HL (Sodium methyldithiocarbamate) AMVAC, Newport Beach, CA; Dominus (Allyl isothiocyanate) Gowan Company LLC, Yuma, AZ; Pic-Clor 60 (1,3-dichoropropene and chloropicrin) TriCal, Inc, Hollister, CA; and K-Pam HL (Potassium N-methyldithiocarbamate) AMVAC, Newport Beach, CA (Grabau and Noling 2021). Non-fumigant nematicides are applied in liquid or granular formulations and move through the soil as a liquid. In general, these nematicides can be applied several times per year (Desaeger et al. 2020). The non-fumigant chemical nematicides Vydate L (oxamyl) Corteva AgriScience, Wilmington, DE; Velum (fluopyram) Bayer CropScience, Monheim, Germany; Nimitz (fluensulfone) Adama, Ashdod City, Israel; Mocap EC (ethoprop) AMVAC, Newport Beach, CA; and Mocap 15G (ethoprop) AMVAC, Newport Beach, CA; are labeled for control of M. incognita in sweetpotato (Grabau and Noling 2021). The nematicide AgLogic 15G (aldicarb) AgLogic Chemical LLC, Chapel Hill, NC, is restricted for use in sweetpotato and only authorized in Louisiana and Mississippi at this time (Webb 2017).

Soil-directed insecticides are commonly used to manage soil insect populations (Chalfant et al. 1990). The New England Vegetable Management Guide notes that bifenthrin (Brigade 2EC) FMC Corporation, Philadelphia, PA; can be used for the management of wireworms and white grubs (Wallingford 2023). Bifenthrin can be applied as a soil-incorporated broadcast, bed, or in-furrow spray at planting, or as a soil-directed incorporated spray at cultivation or fertilizer lay-by application. This product can also be applied as a foliar spray to manage click beetles (adult wireworms) and May/June beetles (adult white grubs). Rates vary depending on application method and target species (Wallingford 2023). Ethoprop (Mocap 15G) AMVAC, Newport Beach, CA; can be applied and incorporated into the top 2 to 4 inches of soil 2 to 3 wk before planting to manage wireworms and white grubs (Wallingford 2023). Belay (clothianidin) Valent, San Ramon, CA; can be applied at planting and at cultivation to manage wireworms (Coolong et al. 2012). Movento (spirotetramat) Bayer CropScience, Monheim, Germany; is labeled for wireworm management in sweetpotato. This systemic insecticide can be applied to the foliage or by chemigation (Webb 2017). Imidan 70W (phosmet) Gowan Company, Yuma, AZ; is labeled for the control of sweetpotato weevil, banded cucumber beetle, white grub, and wireworm in sweetpotatoes (Webb 2017). The pyrethroid Baythroid XL (beta-cyfluthrin) Bayer CropScience, Monheim, Germany is a restricted-use pesticide labeled for the control of cabbage looper, cucumber beetles, flea beetles, and sweetpotato weevil adults (Webb 2017).

Varietal Management

Varietal selection is a very important decision for sweetpotato growers. There are hundreds of sweetpotato varieties that are divided into groups based on the color of the skin and flesh (Coolong et al. 2012). Covington and Beauregard are the current dominant cultivars in the United States, but Orleans is a newer orange-fleshed cultivar that has impressive yields (La Bonte et al. 2012) and is increasing in acreage (George et al. 2024). Some commercially available sweetpotato cultivars have greater disease and insect tolerance. In commercially available varieties, nematode resistance is most common to M. incognita (Jatala and Bridge 1990). ‘Covington’ is a commonly planted sweetpotato variety developed at North Carolina State University that is resistant to M. incognita race 3 and yields similarly to standard susceptible variety ‘Beauregard’ (Yencho et al. 2008). ‘Bonita’ and ‘Evangeline’ were developed by researchers at the Louisiana Agricultural Experiment Station and are both rated as highly resistant to M. incognita race 3 (La Bonte et al. 2011). ‘Hernandez’ and ‘Jewel’ have been rated as moderately resistant to M. incognita race 3 (La Bonte et al. 1992). At this time the M. incognita resistant sweetpotato cultivars are susceptible to M. enterolobii (George et al. 2024). Although less common, some sweetpotato varieties carry resistance to key insect pests. The varieties Murasaki-29 and NC04-531, a clone developed at North Carolina State University, are rated as moderately resistant to WDS complex and flea beetle damage (Jennings et al. 2019). The sweetpotato cultivars ‘Excel’, ‘Regal’, ‘Resisto’, and ‘Southern Delite’ are considered resistant to WDS complex damage, and ‘Jewel’ and ‘Centennial’ are intermediate (Schalk et al. 1993).

Cultural Management

The main purpose of crop rotation is to minimize the damage caused by pests by growing susceptible host crops away from the targeted pest population. This is achieved by planting non-host plants, reducing pest numbers below economic thresholds (Widmer et al. 2002). Crop rotation with a non-host of M. incognita is effective in reducing nematode populations; however, crop selection can be difficult due to its broad host range (Abad et al. 2003). As a result, developing and designing cultural management strategies like crop rotations can be challenging (CABI 2020). Peanut is a non-host of M. incognita, indicating that it is a good crop rotation partner with sweetpotato to reduce M. incognita populations in the Southeast (Davis and Webster 2005). Bahiagrass was found to be a good rotation crop with sweetpotatoes producing the highest yields with 2 yr of bahiagrass followed by one year of sweetpotato (Guertal et al. 2013). Other crops like cabbage, mustard, and radishes are moderately resistant to M. incognita, making them suitable to rotation for nematode management (Bilgrami and Khan 2022). For sweetpotato home gardens, certain marigold varieties have been reported effective in suppressing M. incognita populations in soils (CABI 2012).

Rotational cropping systems with sweetpotatoes have been found to influence the abundance of wireworms (Seal et al. 1992). Since wireworms have long and varied life cycles, long-term crop rotation away from hosts of wireworms is an effective method to mitigate root damage (Jennings et al. 2019). Growers should avoid rotating with corn and small grains since wireworm species preferentially oviposit in these crops, and instead should consider planting soybean, which is a less desirable host plant (Jennings et al. 2019). Additionally, weedy fallow fields can be a risk factor for wireworm damage, as weeds can be important alternate hosts for wireworm larvae. Winter weed management can reduce the risk of root damage due to overwintering larvae in subsequent sweetpotato planting (Jennings et al. 2019). Growers should avoid planting sweetpotato in fields that have been recently converted from pasture, as grasses are preferred hosts of several economically important wireworm species. The interval between pasture conversion and planting of a sensitive root crop like sweetpotato should be several years to minimize the risk of damage.

Cover Crops

Cover crops are defined by the Sustainable Agriculture Research and Education program as “a plant that is used primarily to slow erosion, improve soil health, enhance water availability, smother weeds, help control pests and diseases, increase biodiversity, and bring a host of other benefits to your farm” (Clark 2015). These crops are grown with the intention of incorporating their residue into the soil (Fageria et al. 2005). Both grasses and legumes are grown as cover crops, but they affect the system differently. Leguminous cover crops can reduce the need for nitrogen fertilization in the upcoming cash crop due to their ability to biologically fix nitrogen (Fageria et al. 2005). Grass cover crops are often used as tools to reduce erosion and NO₃ leaching (Fageria et al. 2005). In the Southeastern United States, winter cover crops are established after harvest of the preceding cash crop, typically in late summer or early fall (Timper et al. 2006). The cover crops in this region are living and growing through the winter months and are terminated in the spring by mowing, rolling/crimping, or herbicide application (Timper et al. 2021). Cover crops can be used to suppress plant-parasitic nematode populations either by their non-host status, producing allelopathic compounds, and/or enhancing nematode antagonists present in the soil (Wang et al. 2006). Non-host winter cover crops minimize the reproduction of plant-parasitic nematodes over warm winters, reducing nematode populations for the subsequent cash crop season (Timper et al. 2006). However, plant-parasitic nematode host status is variable depending on the variety of the cover crop and the nematode present in the specific field. Some hosts and varieties support higher levels of nematode reproduction than others (Timper et al. 2006). For instance, rye (Secale cereale L.) was found to be a relatively poor host of M. incognita when compared with crimson clover (Trifolium incarnatum L.) and hairy vetch (Vicia villosa Roth) in a greenhouse evaluation (Timper et al. 2021). Rye produces benzoxazinoids, secondary metabolites that are toxic to plants, microorganisms, insects, and nematodes (Timper et al. 2021). Rye was also found to decrease M. incognita reproduction and reduce root galling on the following cash crop (Timper et al. 2006). Radish (Raphanus sativus L.) has been classified as a poor host of M. incognita with a reproductive factor of 0.9 and low galling index, which indicates that it could be a suitable winter cover crop for M. incognita management (Anwar and McKenry 2010). Additionally, mustard (Brassica nigra L.) has potential as a winter cover crop for M. incognita management due to its low reproductive factor of 0.7 and low galling index (Anwar and McKenry 2010). Seed meals produced from mustard have been found to suppress plant-parasitic nematodes, including those in the Meloidogyne genus (Meyer et al. 2011). The mechanism is thought to be toxin production resulting from the breakdown of glucosinolates contained in mustard plant tissue (Meyer et al. 2011). This shows that mustard holds promise as a M. incognita suppressive winter cover crop.

Winter cover crops can also impact soil insect populations. A chief concern of growers when considering adding winter cover crops to their operations is the possibility of a “green bridge effect” (Pellegrino et al. 2021). In fact, despite the benefits of winter cover cropping, adoption of this practice is low in vegetable production systems in the Southeastern U.S. (O’Connell et al. 2015). The green bridge effect occurs when plants are established during a time that the land is typically fallow, allowing a larger population of soil insect pests to survive the winter season (Favetti et al. 2017). Winter cover crops could provide food and microhabitats that promote pest survival and increase the risk of insect damage to the upcoming cash crop (Favetti et al. 2017). This risk is of particular concern in sweetpotato, since it is a root crop, and its primary insect pests are soil-borne. Thus, it is of utmost importance that winter cover crops be assessed for their effects on insect pest damage to the following sweetpotato crop.

Biological Control

Biological control is the reduction of pest populations by natural enemies like predators, parasitoids, and pathogens (Xiang et al. 2017). Beauveria bassiana (Bals.) Vuill. is a naturally occurring soil fungus that causes white muscardine disease upon contact with insect hosts (Groden 2012). This biocontrol agent is environmentally safe and poses zero or minimal threats to human health (Mascarin and Jaronski 2016). It has been formulated into several products including BotaniGard 22WP (Certis Biologicals, Columbia, MD); Mycotrol (Certis Biologicals, Columbia, MD); and Naturalis-L (Fargro, West Sussex, England)(Groden 2012). When applying these products, a high spray volume is recommended so that plants are wetted thoroughly, but the product does not run off the leaves (Groden 2012). The spores of Beauveria bassiana are inactivated by sunlight, so prolonged activity can be gained by using drop nozzles or other equipment that can reach the underside of the leaves (Groden 2012).

Entomopathogenic nematodes can also be effective in the management of sweetpotato insect pests. These species of nematodes often belong to the families Steinernematidae and Heterorhabditidae and are most effective against insects in cryptic and soil habitats (Kaya and Gaugler 1993). The infective juvenile stage enters a host insect through its natural openings (spiracles, mouth, or anus) and penetrates the insect’s hemocoel (Kaya and Gaugler 1993). Entomopathogenic nematodes carry Xenorhabdus bacteria in their intestinal tracts, and upon penetration of the hemocoel, release these bacteria into the insect’s hemolymph (Kaya and Gaugler 1993). Xenorhabdus bacteria populations multiply quickly within the insect and typically kill the host within 48 h (Kaya and Gaugler 1993). The nematodes remain inside the insect cadaver, feeding on its tissue and producing 2 to 3 generations before the next generation of infective juveniles emerge to find new hosts (Kaya and Gaugler 1993). When it comes to sweetpotato insect pests, the entomopathogenic nematode species Steinernema feltiae, S. carpocapsae, and Heterorhabditis bacteriophora were shown to significantly increase sweetpotato weevil mortality in-vitro when compared with a water control (Mannion and Jansson 1992). Additionally, a field trial performed by Schalk et al. (1993) found that the entomopathogenic nematode S. carpocapsae significantly reduced damage on sweetpotato roots from wireworms, Diabrotica spp., Systena spp., and sweetpotato flea beetle when applied three times at monthly intervals (Schalk et al. 1993).

Several OMRI-approved biological insecticide options that have activity on wireworms are available (Jennings et al. 2019). Products that are OMRI-approved are considered suitable for organic production. These insecticides are typically applied and incorporated into the soil before the formation of beds and repeated when applying a lay-by application of fertilizer (Jennings et al. 2019). Thorough soil incorporation is critical to create an insecticidal barrier that will restrict the movement of insects into the root zone. For best results, it is recommended to use the highest labeled rate at a high spray volume to ensure the product is applied uniformly across the soil (Jennings et al. 2019).

Caterpillars can be managed by using OMRI-approved insecticides with spinosad and Bacillus thuringiensis (Bt) active ingredients (Jennings et al. 2019). Using the highest labeled rate, increasing spray volume, and using a spreader-sticker adjuvant can improve canopy coverage. This is essential because the caterpillars must feed on treated leaves for the products to be effective (Jennings et al. 2019). Follow-up applications may be necessary to manage these pests when their populations are high. The organic insecticide azadirachtin can effectively manage defoliating pests. This compound is an insect growth regulator and antifeedant but does not kill adult insects (Webb 2017).

Biological nematicides available to sweetpotato growers encompass a variety of innovative products designed to manage nematode infestations in a sustainable manner. In the United States, these nematicides include Majestene (heat-killed Burkholderia rinojensis strain A396 cells and spent fermentation media) by ProFarm Group, and MeloCon WG (Purpureocillium lilacinum strain 251) by Certis Biologicals (Grabau and Noling 2021). Various biological organisms exhibit nematophagous properties, but products based on these organisms tend to fluctuate in market availability.

A prime example is Purpureocillium lilacinum, a fungus that parasitizes nematode eggs, thereby reducing nematode populations in the soil (Kiewnick and Sikora 2005, Castillo et al. 2013). Burkholderia rinojensis is known for producing toxic compounds that inhibit plant parasitic nematodes.

Several other biological control products beneficial to sweetpotato growers include Bacillus subtilis (marketed as Serenade) and Bacillus firmus strains I-1582 and GB-126 all by Bayer Crop Science, which help control root-knot and reniform nematodes by colonizing the roots and producing compounds that inhibit nematode development (Castillo et al. 2013, Ghahremani et al. 2020, Wendimu 2021). Pathway Consortia by Pathway Holdings is a mixture of multiple PGPR strains, such as B. subtilis, B. licheniformis, B. megaterium, B. coagulans, P. fluorescens, Streptomyces spp., and Trichoderma spp., which is a biocontrol product for managing Meloidogyne spp. and R. reniformis (Castillo et al. 2013, Xiang et al. 2018). Many more B. firmus products are emerging on the market from startup companies.

Pseudomonas fluorescens produce antibiotics and enzymes that suppress nematode populations and promote plant health. Pochonia chlamydosporia fungi parasitize nematode egg stages (Manzanilla-López et al. 2013). Trichoderma harzianum and Trichoderma spp. (marketed as Rootshield and Rootshield Plus by BioWorks) have been reported to enhance plant growth and provide protection against various soil-borne pathogens, including nematodes. These fungi penetrate the nematode egg mass matrix, reducing its hatching and producing toxic metabolites that directly inhibit nematode penetration and development (Wendimu 2021). Various species of nematode-trapping fungi, such as Arthrobotrys oligospora, capture and digest nematodes in the soil (Castillo et al. 2013). These biologicals have been documented and evaluated on various plant parasitic nematodes and in various crops. By incorporating biological products, sweetpotato growers could potentially manage nematode populations while minimizing the environmental impact associated with traditional chemical nematicides.

Free Living Nematodes

Soil nematodes can be used as bioindicators to evaluate the effects of agricultural management practices on soil health and the soil food web (Wang et al. 2006). The soil food web is key to supporting both soil and plant health, but agricultural management practices can disturb the soil food web (Wang et al. 2011). Healthy soil food webs support nematodes with differing feeding behaviors and life strategies (Bongers and Bongers 1998). Nematologists use the Maturity Index (MI), enrichment index (EI), channel index (CI), and structure index (SI) to monitor soil health and describe the soil food web (Paudel et al. 2021). Organic production can positively impact ecological indices based on free-living nematode feeding groups, which are indeed useful tools for monitoring soil health. Studies have shown that organic farming practices can lead to increased organic matter, enhanced microbial activity, and improved soil structure (Sharma and Chaubey 2024).

The MI is the weighted mean colonizer-persister (cp) value of all the nematodes in a sample, excluding plant parasites and dauerlarvae, which are a specialized larval stage in which development is paused whose existence is triggered by environmental stress (Karp 2018). The MI is given a numerical range from 1 (often encountered after soil fertilization) to 4 (undisturbed environments) based on the colonizer-persister nematode classification (Bongers and Bongers 1998). This ranges from colonizers that have a short lifespan but high reproduction rate, to persisters that have a long lifespan and low reproduction rate (Bongers and Bongers 1998). Colonizers generally have smaller body sizes than persisters, when comparing species at the order level (Bongers and Bongers 1998). The colonizer-persister (cp) scale consists of five classifications: cp1- cp5 (Bongers and Bongers 1998). When nutrients are plentiful, cp-1 nematodes dominate. Under heavy metal toxicity, cp-2 nematodes fare best (Bongers and Bongers 1998). When members of groups cp-3, 4, and 5 are present, it is indicative of low stress and advanced succession (Bongers and Bongers 1998). Nematodes classified as cp-1 are colonizers with a short generation time and are known for their production of many small eggs. They have high metabolic activity and are only active when soil microbial activity is high. They exhibit exponential population growth under food-rich conditions. The cp-1 nematodes are characterized by their ability to form dauerlarvae when microbial activity is low, and food is less abundant. Members of the cp-1 classification include the bacterial feeding nematode families of Rhabditidae, Diplogasteridae, and Panagrolaimidae (Bongers and Bongers 1998). The cp-2 nematodes also have a short generation time and high reproduction rate, but do not form dauerlarvae. They are very tolerant to pollutants and soil disturbance and are found in both food-rich and food-poor conditions. Nematodes of the cp-2 include Anguinidae, Aphelenchidae, and Aphelenchoididae. The cp-3 nematodes have longer life cycles and are somewhat sensitive to disturbances. These cp-3 nematodes include Araeolaimida, Chromadorida, and Diphtherophorida (Bongers and Bongers 1998). The cp-4 nematodes have a long generation time and a permeable cuticle that makes them sensitive to pollutants. Excluding predatory nematodes, members of this group are relatively immobile. Dorylaimids, Alaimidae, and Bathyodontidae are the cp-4 nematodes The cp-5 nematodes are characterized by their long-life cycles and low reproduction rates. These characteristics are indicative of low metabolic activity. They are very sensitive to soil disturbances and pollutants due to their permeable cuticles. This group is made up of the larger Dorylaimids, including predators, omnivores, and plant parasites.

Since nematodes are abundant and respond rapidly to changes in resource availability, their populations can be used to observe changes from land management practices (Du Preez et al. 2022). These ecological indices, like maturity, enrichment, channel, and structure indices, that are based on nematode feeding groups are useful tools to monitor soil health through soil nematode populations (Bongers and Bongers 1998). These indices allow scientists to describe changes in the soil food web due to nutrient status, soil fertility, and the effects of soil contaminants (Bongers and Ferris 1999). To perform these analyses, identifying nematodes to the genus or family level is efficient and suitable, depending on the taxon (Bongers and Bongers 1998). Analyzing nematode abundance based on feeding group allows scientists to describe changes in decomposition pathways and determine the effects of agricultural management practices like fertilization or winter cover cropping on the soil food web (Du Preez et al. 2022).

In summary, sweetpotato is an economically important crop in the Southeastern U.S. with the potential to be produced organically. However, sweetpotato faces substantial pest pressure in this region from insects like those that make up the WDS pest complex (wireworm spp., Diabrotica spp., and Systena spp.), sweetpotato weevils (Cylas formicarius and C. formicarius elegantus), cucumber beetles (Diabrotica spp.), and flea beetles (Systena spp.). The sweetpotato weevil is particularly damaging, especially in the southern United States, causing significant economic losses. Various insect pests, including wireworms and cucumber beetles, damage sweetpotato roots, leading to reduced quality and marketability. Nematodes like Meloidogyne spp., Pratylenchus spp., and R. reniformis are particularly harmful to sweetpotatoes, with root-knot nematodes (Meloidogyne spp.) being the most severe. This southern root-knot nematode is a significant pest of sweetpotatoes, causing root galls that impair nutrient and water uptake, leading to reduced plant growth, yield, and quality. M. incognita interacts with pathogens to form disease complexes and completes multiple generations per growing season.

Selecting the right sweetpotato variety is crucial for growers, with numerous varieties available, categorized by skin and flesh color. Some varieties offer greater disease and insect tolerance. Notably, many commercial sweetpotato cultivars have resistance to M. incognita, particularly race 3. Sweetpotato varieties ‘Bonita’ and ‘Evangeline’ are considered highly resistant to M. incognita race 3. ‘Covington’ is resistant to M. incognita race 3, with yields similar to the susceptible ‘Beauregard’. ‘Excel’, ‘Regal’, ‘Resisto’, and ‘Southern Delite’ are resistant to WDS complex.

Effective cultural practices, such as crop rotation, play a key role in managing pests like M. incognita and wireworms. Crop rotations to peanut, a non-host of M. incognita, making it a suitable rotation partner in the Southeast. Avoiding planting sweet potatoes in recently converted pastures to reduce the risk of wireworm damage, which is often severe in the first year of sweetpotatoes cultivation. Soybean is a good rotation option as it is a less desirable host for wireworms. Cover crops, primarily used to improve soil health, can also play a role in pest management. Rye is a poor host for M. incognita, capable of reducing nematode populations. Winter cover crops such as cabbage, mustard, and radishes which are moderately resistant to M. incognita, are useful for nematode management. Mustard and radishes show promise due to their glucosinolates that produce nematode-suppressive toxins. The ‘green bridge effect’, where cover crops allow insect pests to survive winter, is a concern. This is particularly relevant for sweetpotato growers due to the crop’s vulnerability to soil-borne pests. Therefore, the impact of cover crops on subsequent sweetpotato crops must be carefully considered.

Biological control involves reducing pest populations using natural enemies, including fungi and entomopathogenic nematodes and there are many products available. Beauveria bassiana, a soil fungus effective against various insect pests, is formulated into products like BotaniGard 22WP, Mycotrol, and Naturalis-L. It is most effective when applied with high spray volume and protected from sunlight. Entomopathogenic nematodes, such as Steinernema feltiae, S. carpocapsae, and Heterorhabditis bacteriophora, are effective against soil pests like sweetpotato weevil and wireworms. Several OMRI-approved biological insecticides are used for managing wireworms and caterpillars. Some are incorporated into the soil to create an insecticidal barrier. Products with spinosad and B. thuringiensis (Bt) are foliar-applied and particularly effective against caterpillars.

Sweetpotato is an economically important crop in the Southeastern U.S. with the potential to be produced organically. However, sweetpotato faces substantial pest pressure in this region from insects like those that make up the WDS pest complex (wireworm spp., Diabrotica spp., and Systena spp.) and plant-parasitic nematodes like M. incognita. Therefore, it is critical to develop effective organic integrated pest management practices for growers. More research is needed to evaluate biopesticides for the management of M. incognita and insect pests, and to determine the efficacy of winter cover crops in the suppression of M. incognita populations and insect pest damage. More information on integrated approaches will enhance crop resilience and supports long-term productivity and sustainability in sweetpotato farming.

Acknowledgments

The authors sincerely thank Dr. John Beckmann and Ms. Janiyah Cotton for their expertise in editing the images of the insects, nematodes, and sweet potato damage symptoms.

Author contributions

Claire M. Schloemer (Data curation [equal], Formal analysis [equal], Investigation [equal], Methodology [equal], Validation [equal], Writing—original draft [equal]), Scott H. Graham (Conceptualization [equal], Data curation [equal], Formal analysis [equal], Funding acquisition [equal], Investigation [equal], Methodology [equal], Resources [equal], Supervision [equal], Validation [equal], Writing—review & editing [equal]), and KATHY Lawrence (Conceptualization [equal], Data curation [equal], Formal analysis [equal], Funding acquisition [lead], Investigation [equal], Methodology [equal], Project administration [lead], Resources [equal], Supervision [equal], Validation [equal], Writing—original draft [equal], Writing—review & editing [equal])

Funding

This work was supported by USDA NIFA OREI (HAW09705-G) and Hatch projects ALA015-214003 and ALA015-19117.

Conflicts of interest. None declared.

References