Pests
Symptoms Small holes and dark, spongy tunnels inside the sweet potatoes, which you usually find only when you dig or slice them. Even a little feeding makes the roots taste bitter and smell off, so they are no longer good to eat. The beetle itself is a slim, ant-like weevil with a long snout, reddish legs, and a shiny blue-black body. It is the worst sweet potato pest, especially in the Deep South.Small holes and dark, spongy tunnels running through the storage roots, found when you dig or cut them. Even light feeding triggers a chemical reaction that gives the roots a bitter taste and an off, turpentine-like odor, ruining them for eating. The adult is a slender, ant-like weevil, about a quarter inch long, with a dark head and snout, reddish legs, and a metallic blue-black body; you may see chewing on stems and leaves. This is the single most serious pest of sweet potato, worst in the Deep South and Gulf. Cause A small weevil called the sweet potato weevil. Its grubs tunnel through the sweet potatoes and stems, and the plant reacts by turning bitter, so even lightly damaged roots are ruined. It rides in on infested slips and survives in leftover sweet potatoes in the ground, and it keeps damaging the crop even after harvest in storage.The sweet potato weevil (Cylas formicarius, family Brentidae). Larvae do the principal damage by mining tunnels through the storage roots (and stems), leaving them riddled with cavities, spongy, and darkened; the feeding induces bitter terpenes even at low levels. It breeds continuously in warm climates, spreads mainly on infested slips and in leftover roots, and continues to damage the crop in storage. Organic treatment You cannot fix sweet potatoes that are already tunneled, so throw them out. The whole game is prevention: plant only certified weevil-free slips, and clean up every leftover or volunteer sweet potato in the ground after harvest, because the weevil lives on in them. Dig your crop on time instead of leaving it in the soil, and do not plant next to last year's bed.There is no rescue for tunneled roots; discard them. Control is prevention: start only with certified weevil-free slips, and practice strict sanitation, removing and destroying every leftover, cull, and volunteer root that can harbor the weevil between crops. Harvest promptly once the crop matures rather than leaving roots in infested soil, and keep new plantings away from last year's beds and from stored roots. Prevention Start with certified weevil-free slips, not saved sweet potatoes. Move the patch to a new spot each year, well away from where sweet potatoes grew before and away from where you store them. After you dig, clean up every leftover and volunteer root, because the weevil survives in them. Harvest on time rather than leaving the crop in the ground.Buy certified, weevil-free slips rather than saving your own or using field-run roots. Rotate sweet potatoes well away from last year's beds and from any sweet potato storage, since the weevil moves only short distances. Remove all volunteer and cull roots after harvest, harvest promptly, and in badly infested areas grow on a strict rotation with a fallow break. Certified slips plus sanitation carry nearly all of the control.
Symptoms Narrow holes and shallow tracks drilled into the sweet potatoes, found when you dig them. They are made by hard, shiny soil worms (wireworms) and the grubs of cucumber and flea beetles. You do not see anything above ground; you find the little tunnels and pits at harvest, and damaged roots do not store as well.Narrow holes and shallow, winding grooves bored straight into the storage roots, leaving pinhole punctures and tracks that downgrade quality and open the door to storage rots. The culprits are slender, shiny, hard-bodied wireworms (click beetle larvae) plus the larvae of cucumber beetles and flea beetles (the wireworm, Diabrotica, Systena complex). There are no useful aboveground signs; the damage is found at harvest. Cause Wireworms are the larvae of click beetles; they live in the soil for years and are worst in beds that were lawn, grass, or corn not long ago. The larvae of cucumber beetles and flea beetles do similar damage, moving into the soil to nibble the sweet potatoes as they grow.Wireworms are click beetle larvae that live in the soil for several years and are most numerous in ground recently in sod, grass, or corn. The same root-scarring complex includes larvae of spotted and banded cucumber beetles (Diabrotica) and flea beetles (Systena), which move into the soil to feed on the developing roots through the season. Organic treatment You cannot fix roots that are already tunneled, so cut out the bad spots and eat those first instead of storing them. Dig the crop on time rather than leaving it in the ground, where the worms keep boring, and keep grassy weeds down.There is no rescue for scarred roots; trim damaged areas and use those roots first rather than storing them. Harvest promptly once the crop matures rather than leaving roots in infested soil, where feeding accumulates. Keeping the ground worked and free of grassy weeds reduces the habitat these larvae favor. Prevention Do not plant sweet potatoes in a bed that was grass, lawn, or corn in the last year or two, where wireworms are worst. Rotate your crops, work the soil, pull grassy weeds, and dig on time. If wireworms keep ruining a bed, use a raised bed with clean soil.Avoid planting sweet potatoes in ground that was recently pasture, lawn, or corn, where wireworm numbers peak; extend the interval between grass and a root crop. Rotate, keep the soil well worked, control grassy weeds, and harvest on time. Where wireworm history is bad, grow elsewhere or in clean raised-bed soil.
Symptoms Lots of tiny holes in the leaves, as if peppered with shot, made by little dark beetles that hop away when you come near. Young slips are hurt most; big, healthy vines shrug it off. The grubs in the soil can also scar the sweet potatoes.Tiny dark beetles that jump when disturbed and chew many small round shot-holes in the leaves. Heavy early feeding on young transplants stunts growth; the soil-dwelling larvae also scar the developing storage roots. Established, vigorously running plants usually outgrow light foliage feeding. Cause Small beetles called flea beetles that spend winter nearby and jump onto young sweet potato plants. They do the most harm to freshly set slips and in warm, dry spells, and their grubs can scar the roots underground.Several species of small flea beetles that overwinter nearby and move onto young sweet potato foliage in spring and early summer. They are worst on small, slow transplants and in warm, dry weather; their larvae contribute to the root-scarring complex below ground. Organic treatment Keep young slips growing strong with steady water and they usually outgrow the damage as the vines take off. A row cover protects them early on, and clearing weeds and old plant debris takes away their winter hiding spots.Healthy, well-watered transplants usually outgrow flea beetle feeding once the vines run. Floating row cover protects young plants during the vulnerable first weeks; remove it as the vines begin to sprawl. Clearing weeds and old crop debris reduces the shelter they overwinter in. Prevention Move the patch to a new spot, pull weeds, and cover the young slips until they are established and the vines start to run. Plants that are well watered handle flea beetles much better than struggling ones.Rotate away from last year's beds, keep the area weed-free, and protect freshly set slips with row cover until they are established and running. Vigorous, evenly watered transplants tolerate feeding far better than stressed ones.
Symptoms Tiny white flying insects that scatter in a cloud when you brush the leaves, clustered underneath with a sticky film. Their feeding is minor, but they can pass on plant viruses that mottle and curl the leaves and cut your harvest. A plant that catches a virus will not get better.Clouds of tiny white, moth-like insects that lift off the foliage when disturbed, clustered on leaf undersides with sticky honeydew and sooty mold. The bigger problem is not their feeding but the viruses they transmit (such as sweet potato feathery mottle virus and sweet potato leaf curl virus), which mottle, curl, and stunt the foliage and erode yield over time; virus-infected plants do not recover. Cause Whiteflies are tiny sap-sucking insects, and as they move from plant to plant they carry sweet potato viruses. The viruses, more than the bugs, are the real concern, which is why it is best to plant certified, disease-tested slips instead of saving your own.Whiteflies colonize sweet potato foliage and, as they move plant to plant, transmit sweet potato viruses. Virus pressure, not the feeding itself, is the main reason to manage them, and it is the reason growers are advised to start from certified, virus-tested slips rather than saved or field-run material. Organic treatment Plant certified, disease-tested slips so the patch starts clean. A few whiteflies are fine, because helpful bugs usually keep them in check; for heavy clusters, spray them off with water or use insecticidal soap. Pull and throw away any plant that clearly has a virus.Start with certified, virus-tested slips to keep virus out of the patch. Tolerate light whitefly numbers, since beneficial insects often control them; a strong spray of water or insecticidal soap knocks down heavy colonies on the leaf undersides. Remove and destroy any plant showing clear virus symptoms. Prevention Buy certified slips, avoid harsh sprays that kill the helpful bugs, keep weeds down, and pull out any virus-looking plant right away. Do not save slips from a patch that had virus, because it comes back with them.Use certified slips, encourage beneficial insects by avoiding broad-spectrum sprays, control nearby weeds that host whiteflies and virus, and rogue out virus-symptomatic plants promptly. Do not save and replant slips from a patch that showed virus, since it carries over.
Diseases
Symptoms Lower leaves yellow and wilt, often on one side of the plant, and the vine is stunted; if you slit a stem near the base you see brown streaks inside. Badly affected plants wilt and die before making sweet potatoes. It usually comes in on infected slips or from the soil.Yellowing and wilting that starts on the lower leaves and progresses up the vine, often on one side, with stunting; a lengthwise cut of the stem near the base shows a brown, discolored streak in the water-conducting tissue. Plants may collapse before making a crop. It moves up from infected slips or through the soil. Cause A soil fungus (Fusarium oxysporum) that gets into the roots and stem and clogs the plant's plumbing. It lives in the soil for years and rides in on infected slips, and it builds up if you grow sweet potatoes in the same spot again and again.The soilborne fungus Fusarium oxysporum, which enters through the roots and stem base and plugs the vascular tissue. It survives for years in the soil and is carried on infected slips, so it builds up where sweet potatoes are grown repeatedly in the same ground. Organic treatment You cannot cure an infected plant, so pull it and throw it away (do not compost it). There is no spray fix; you prevent it with clean slips, resistant varieties, and rotating the bed.There is no cure for an infected plant; remove and destroy it. Management is preventive and variety-based: it is controlled by clean slips, resistant cultivars, and rotation, not by a spray. Prevention Plant certified, disease-free slips and choose a wilt-resistant variety if this has been a problem. Do not grow sweet potatoes in the same bed year after year, and never save slips from a patch that wilted.Start with certified, disease-free slips and grow Fusarium-resistant varieties (such as Sumor or other wilt-resistant releases) where the disease occurs. Rotate sweet potatoes out of an infected bed for several years, and do not save slips from a patch that showed wilt.
Symptoms Plants that are stunted and wilt easily even with water, and sweet potatoes that come up cracked, bumpy, and rough-skinned with little knots on them. The damage is worst in sandy soil and hot regions. You usually see it clearly only when you dig the roots.Stunted, off-color, wilt-prone vines above ground, and below ground storage roots that are cracked, bumpy, and galled, with small knots and rough, split skin that downgrade quality and storage. The southern root-knot nematode is the main species, and damage is worst in sandy soils and warm regions. Cause A microscopic soil roundworm (the southern root-knot nematode) that attacks the roots and makes them crack and knot. It lives in the soil and feeds on many crops, so it builds up when you grow the same kinds of plants in one spot over and over.The southern root-knot nematode (Meloidogyne incognita), a microscopic soil roundworm that invades the roots, causing galling and the cracked, bumpy roots that grade out. It persists in the soil and on many hosts, so it builds up where susceptible crops are grown repeatedly. Organic treatment You cannot fix roots that are already cracked and knotted. Instead, prevent it: grow a nematode-resistant variety, rotate the bed to different crops, and build up the soil with compost. In hot climates, covering moist soil with clear plastic through the summer can knock the numbers down.There is no rescue for galled, cracked roots. Management is preventive: grow nematode-resistant varieties, rotate to non-host or less-susceptible crops, and keep the soil high in organic matter. Solarizing the bed in the hot months and cover cropping can lower populations over time. Prevention Grow a nematode-resistant variety like Centennial or Covington, rotate your crops so sweet potatoes and their susceptible cousins are not in the same bed each year, and add plenty of compost. Start with certified, clean slips.Choose root-knot-resistant varieties (such as Centennial, Covington, or other resistant releases), rotate away from susceptible hosts on a multi-year cycle, and raise soil organic matter. Start with certified, clean slips so you do not introduce new populations.
Symptoms Dark, sunken, round spots on the sweet potatoes, firm and greenish-black to brown, and the affected part can taste bitter. It also shows as dark patches on the lower stems. It gets worse in storage and comes in on infected slips or from the soil.Round, dark, sunken lesions on the storage roots that are firm and greenish-black to dark brown, sometimes with a bitter taste in the affected tissue; on the lower stems it shows as dark cankers. It spreads in storage and in the field from infected slips and contaminated soil, and it is a major cause of cull and storage loss. Cause A fungus (Ceratocystis fimbriata) that gets into the roots and stems through cuts and bruises, lives in the soil and on old plant debris, and rides in on infected slips. It likes warm, damp conditions and keeps spreading on stored sweet potatoes.The fungus Ceratocystis fimbriata, which infects roots and stems through wounds, survives in soil and crop debris, and is spread on infected slips and by handling. Warmth and moisture favor it, and it continues to develop on stored roots. Organic treatment You cannot cure an infected sweet potato, so throw out affected ones and never store damaged roots. The fix is prevention: clean slips, gentle handling, curing right after harvest, and keeping tools and bins clean.There is no cure for infected roots; discard them and do not store blemished ones. Management is preventive: clean slips, careful handling to avoid wounds, prompt curing, and sanitation. Prevention Use certified, disease-free slips, rotate the bed, and handle the sweet potatoes gently so you do not bruise them, since the fungus enters through cuts. Cure the crop right after digging and clean up leftover roots.Start with certified, disease-free slips from clean stock, rotate out of infected beds, handle roots gently to avoid the wounds the fungus enters through, and cure the crop promptly after harvest. Discard cull and volunteer roots that carry the fungus over.
Symptoms Scurf is dark brown to black blotches on the skin of the sweet potato; it does not rot the inside, but it makes the roots look poor and dry out faster in storage. Soft rot turns bruised or too-cold roots into a soft, wet, smelly mush in the bin. Both are worst on damaged roots that were not cured or were stored wrong.Scurf shows as brown to blackish blotches on the skin surface that do not penetrate the flesh; it is a cosmetic, skin-quality problem that also increases moisture loss and shrinkage in storage. Bacterial and fungal soft rots turn wounded or chilled roots into a soft, wet, sometimes foul mush in storage. Both are worst on bruised, poorly cured, or improperly stored roots. Cause Scurf is a skin fungus that comes in on infected slips and lives in the soil. Soft rot is caused by bacteria and fungi that get into bruised roots, especially when the sweet potatoes were not cured or were stored too cold or too wet.Scurf is caused by the fungus Monilochaetes infuscans, carried on infected slips and in soil, and confined to the skin. Storage soft rots are caused by bacteria and fungi (including Rhizopus) that enter through wounds and are favored by bruising, poor curing, and cold or damp storage. Organic treatment You cannot fix scurf or soft rot. Scurfy sweet potatoes are still fine to eat once peeled, but soft, rotting ones should be thrown out. Prevent both with clean slips, gentle handling, curing, and proper warm storage.There is no rescue for scurfed or rotting roots; scurfed ones are safe to eat after peeling, but soft-rotted ones are discarded. Management is preventive: clean slips, careful handling, prompt curing, and correct warm storage. Prevention Plant certified, clean slips, rotate the bed, and be gentle with the sweet potatoes so you do not bruise them. Cure them warm and humid right after digging so the skins heal, then store them warm (not in the fridge), and never store damaged roots.Use certified, scurf-free slips, rotate out of infected beds, and handle roots gently to avoid wounds. Cure the crop warm and humid right after harvest to heal skins, then store warm (never cold or wet), and do not store bruised or diseased roots.
