Pests
Symptoms Leaves chewed ragged, sometimes stripped to the stems. The beetles are oval, about 3/8 inch, with yellow and black stripes; the larvae are fat, pinkish-orange grubs with black heads; and the eggs are clusters of bright orange ovals on the undersides of leaves. The big grubs do most of the eating, and damage hurts most once the potatoes are forming underneath.Ragged, chewed foliage that can be stripped to the stems in a bad outbreak. Look for oval, 3/8-inch adults with yellow and black lengthwise stripes, plump pink-to-salmon larvae with black heads and two rows of dark spots, and bright yellowish-orange egg clusters on the undersides of leaves. The last-stage larvae do most of the damage. Potatoes tolerate up to about 30 percent defoliation before flowering but only about 10 percent once tubers are sizing. Cause The Colorado potato beetle, a striped beetle that spends winter in the soil near old potato patches and comes out in spring to feed and lay eggs. Each female lays hundreds of eggs and there can be two rounds in a summer, so numbers build fast. It also goes after eggplant and other nightshades.Larvae and adults of the Colorado potato beetle (Leptinotarsa decemlineata), which overwinter 5 to 10 inches deep in soil in and around last year's potatoes and emerge in spring to feed and lay eggs. Each female lays up to 350 eggs, and there is often a second generation by midsummer in central and southern areas. It also attacks eggplant and other nightshades. Organic treatment Pick the beetles, grubs, and orange egg clusters off by hand and drop them in soapy water; check the plants every couple of days. There is no easy spray fix, because the beetles have grown resistant to many of them, so hand-removal is the most reliable home control.Handpicking is effective in a home garden: knock adults and larvae into a pail of soapy water and crush the orange egg clusters on the leaf undersides; check every few days. There is no quick spray cure, and the beetle has developed resistance to many insecticides, so persistence with handpicking and removal does most of the work. Prevention Move potatoes to a new spot each year, away from where they (or eggplant) grew last year, because the beetles wake up right there in spring. A row cover over the bed early keeps them off, straw mulch slows them down, and crushing the first egg clusters stops a generation before it starts.Rotate potatoes well away from last year's bed, since the overwintered adults emerge where potatoes grew before. Floating row cover laid over the bed at emergence keeps the beetles off until you need to remove it for the developing plants, and a thick straw mulch can slow the adults walking in. Crush early egg clusters before they hatch to break the first generation.
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 plants are hurt most; big healthy plants shrug it off. The grubs can also nibble the tubers.Tiny dark beetles that jump when disturbed and chew many small round shot-holes in the leaves. Heavy early feeding on young plants stunts growth; the larvae feed on roots and can leave small tracks or pits on tubers. Established, vigorous plants usually outgrow light feeding. Cause Small beetles called flea beetles that spend winter nearby and jump onto young potato plants in spring. They do the most harm to little plants and in warm, dry spells.Several species of small flea beetles, including the tuber flea beetle, that overwinter nearby and move onto young potato foliage in spring. They are worst on small, slow plants and in warm, dry weather. Organic treatment Keep young plants growing strong with steady water and they usually outgrow the damage. A row cover protects seedlings early on, and clearing weeds and old plant debris takes away their winter hiding spots.Healthy, well-watered plants usually outgrow flea beetle damage. Floating row cover protects young plants during the vulnerable early weeks; remove it once the plants are established. Clearing weeds and old crop debris reduces the shelter they overwinter in. Prevention Move the potatoes to a new spot, pull weeds, and cover the young plants until they are big and strong. Plants that are well fed and watered handle flea beetles much better than struggling ones.Rotate away from last year's nightshade beds, keep the area weed-free, and protect young plants with row cover until they are large enough to tolerate feeding. Vigorous, evenly watered plants are far more tolerant than stressed ones.
Symptoms Tiny soft insects, usually green or pink, crowded on the new growth and the undersides of leaves, sometimes leaving a sticky film. Their feeding is minor, but they can pass on plant viruses that make the leaves mottle, curl, or roll and cut your harvest. A plant that catches a virus will not get better.Small soft-bodied insects, often green or pink, clustered on growing tips and leaf undersides, sometimes with sticky honeydew and sooty mold. The bigger problem is not their feeding but the viruses they transmit (such as potato virus Y and leafroll), which cause mottled, curled, or rolled leaves and reduce yield; virus-infected plants do not recover. Cause Aphids are small sap-sucking insects, and as they move from plant to plant they carry potato viruses. The viruses, more than the bugs, are the real concern, which is why it is best to plant certified disease-free seed potatoes instead of saving your own.Several aphid species colonize potato foliage, and as they move plant to plant they spread potato viruses. Virus pressure, not the feeding itself, is the main reason to manage them, and it is the reason gardeners are advised to start from certified disease-free seed potatoes rather than saved tubers. Organic treatment Plant certified disease-free seed potatoes so the patch starts clean. A little aphid activity is fine, because ladybugs and other helpers 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 seed potatoes to keep virus out of the patch. Tolerate light aphid numbers, since beneficial insects (ladybugs, lacewings, parasitic wasps) often control them; a strong spray of water or insecticidal soap knocks down heavy colonies. Remove and destroy any plant showing clear virus symptoms. Prevention Buy certified seed potatoes, avoid harsh sprays that kill the helpful bugs, keep weeds down, and pull out any virus-looking plant right away so it does not spread.Use certified seed potatoes, encourage beneficial insects by avoiding broad-spectrum sprays, control nearby weeds that host aphids and virus, and rogue out virus-symptomatic plants promptly so they do not seed an outbreak.
Symptoms Hard, shiny, tan-colored worms in the soil that drill narrow holes into the potatoes. You usually do not notice anything above ground; you find the little tunnels when you dig the crop. Tunneled potatoes do not store well.Slender, shiny, yellow-brown, hard-bodied larvae in the soil that bore narrow holes straight into the tubers, leaving clean round tunnels that downgrade quality and invite rot in storage. There are no useful aboveground symptoms; the damage is found at harvest. Cause The larvae of click beetles. They live in the soil for years and are most common in beds that were lawn or grass not long ago. You tend to find them only when you dig.The larvae of click beetles, which live in the soil for several years and are most common in ground recently in sod or grass. They feed on many roots and tubers and are hard to detect until harvest. Organic treatment You cannot fix potatoes that are already tunneled, so cut out the bad parts 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.There is no rescue for tunneled tubers; trim damaged areas and use them first rather than storing them. Harvest promptly once the crop matures rather than leaving tubers in infested soil, where damage accumulates. Prevention Do not plant potatoes in a bed that was grass or lawn in the last year or two, where wireworms are worst. Rotate your crops, work the soil, and dig on time. If wireworms keep ruining a bed, use a raised bed with clean soil.Avoid planting potatoes in ground that was recently lawn or pasture, since wireworm numbers are highest there. Rotate, keep the soil well worked, and harvest on time. Where wireworm history is bad, grow elsewhere or in clean raised-bed soil.
Diseases
Symptoms Dark, wet-looking spots on the leaves and stems that spread fast, with a fuzzy white mold underneath in cool, damp weather. The plants can collapse in just a few days, and the rot moves down into the potatoes, leaving firm reddish-brown corky patches inside. This is the famous, most dangerous potato disease, the one behind the Irish famine.Dark, water-soaked spots that appear on leaves and stems and enlarge rapidly, often with a downy white mold on the leaf undersides in cool, wet weather. The foliage can collapse within days, and the disease moves into the tubers, which develop a firm, reddish-brown, dry, corky rot beneath the skin. This is the disease behind the Irish potato famine and the most serious threat to a potato crop. Cause A fungus-like organism (Phytophthora infestans) that thrives in cool, wet, humid weather and blows in on the wind. It attacks tomatoes too, so an infected tomato nearby is a danger, and it survives in leftover infected potatoes.The water mold Phytophthora infestans, favored by cool, wet, humid weather and spread on wind and splashing water. It infects both potato and tomato, so a nearby infected tomato is a real risk, and it survives between seasons in infected tubers and cull piles. Organic treatment You cannot cure a badly infected plant, so pull and destroy infected plants fast (do not compost them) to slow it down. In a wet year a copper fungicide started early can help, but mostly it is about prevention. Throw away every infected potato so it does not bring the disease back next year.There is no home cure once a plant is badly infected. Remove and destroy infected plants quickly to slow spread, and do not compost them. In wet seasons a copper-based fungicide applied early and repeated can suppress it, but the real defenses are cultural. Destroy all infected tubers so they cannot carry it over. Prevention Plant certified disease-free seed potatoes and a resistant variety like Kennebec, water at the soil in the morning so the leaves stay dry, give plants room for air to move, and keep them away from tomatoes. Clean up any leftover or volunteer potatoes, and cut and remove infected vines before you dig so the disease does not reach the potatoes.Start with certified disease-free seed, choose resistant varieties (such as Kennebec or Sebago), keep foliage dry by watering at the base in the morning, space for airflow, and never plant near tomatoes. Remove volunteer potatoes and cull piles that harbor the pathogen, and destroy any infected vines before harvest so spores do not reach the tubers.
Symptoms Dark spots with rings inside them, like a little target or bull's-eye, on the older lower leaves, often with a yellow edge. The spots spread and the lower leaves die off; in a bad case the plant loses its leaves and the potatoes get brown corky patches. Tired, stressed plants get it worst.Small dark brown to black lesions on the older, lower leaves, each with concentric rings that give a target or bull's-eye look, often ringed by a yellow halo. Spots multiply and merge and the lower canopy dies back; severe infection causes defoliation and brown, corky, dry spots on the tubers. Stressed and aging plants are hit hardest. Cause A fungus (Alternaria solani) that lives on old plant debris and in the soil and spreads in warm, damp weather. It comes on gradually, especially on stressed plants, rather than wiping a plant out overnight like late blight.The fungus Alternaria solani, which survives on crop debris and in the soil and spreads in warm, humid weather with wet foliage. It is more of a slow, stress-related decline than the explosive collapse of late blight. Organic treatment Pick off the worst spotted lower leaves, keep the plants healthy with even water and feeding, and water at the soil so the leaves dry fast. A copper fungicide can help if you start early in a wet spell, but keeping plants strong and the leaves dry does most of the work.Remove badly spotted lower leaves, keep plants vigorous with steady water and feeding (stress worsens it), and water at the base in the morning. A copper or other labeled fungicide can slow spread in a wet season if started early, but cultural care carries most of the load. Prevention Do not grow potatoes (or tomatoes and peppers) in the same spot for a few years, use clean seed and a resistant variety, clear away old plant debris, give plants room, water at the base, and keep them well fed so they stay strong.Rotate potatoes and other nightshades out of the bed for several years, start with clean seed and resistant varieties, clear crop debris, space and hill for airflow, avoid overhead watering, and keep plants well nourished so they resist the slow decline.
Symptoms Rough, brown, corky patches or pits on the potato skin, sometimes spreading into larger areas. The leaves look fine, so you only see it when you dig the potatoes. It does not rot them, it just roughens the skin, and they are fine to eat once peeled, but they do not store or sell as well.Rough, brown, corky scabs or pits on the tuber skin that can merge into larger patches; sometimes raised, sometimes sunken. The leaves and stems are not affected, so it is found only at harvest. It is a skin-quality problem rather than a rot, and the potatoes are fine to eat after peeling, but it downgrades appearance and storage. Cause A soil germ (Streptomyces scabies) that gets worse when the soil is not acidic enough (above about pH 5.5), when it is dry while the potatoes are forming, and when fresh manure is used. It lives in the soil for years, so just rotating crops does not get rid of it.The soil bacterium Streptomyces scabies, which is favored by less-acidic soil (pH above about 5.5), dry conditions while tubers are forming, and fresh manure. It persists for years in the soil and on many hosts, so rotation alone does not eliminate it. Organic treatment You cannot fix scabbed potatoes, but they are fine to eat after peeling. There is no spray for it; you prevent it by managing the soil instead.There is no rescue once tubers are scabbed; the potatoes are still edible after peeling. Management is preventive: it is controlled by soil conditions, not a spray. Prevention Keep the soil acidic (about pH 5.0 to 5.3 if scab is a problem) and never add lime to a potato bed. Keep the soil evenly moist while the potatoes are forming, skip fresh manure, plant a scab-resistant variety if it keeps happening, and rotate the bed every few years.Keep soil acidic, around pH 5.0 to 5.3 where scab is a problem, and never lime a potato bed. Keep the soil evenly moist for the 4 to 9 weeks after planting when tubers form, avoid fresh manure, and grow scab-resistant varieties (such as Russet Burbank or Superior) where the disease recurs. Rotate on a 3 to 4 year cycle.
Symptoms Blackleg makes the bottom of the stem turn black and slimy while the top of the plant yellows, wilts, and stays small, usually from a bad seed piece. Soft rot turns the potatoes themselves into a soft, wet, smelly mush, in the ground or in storage. Both get into the potato through cuts and bruises and spread in wet conditions.Blackleg shows as an inky-black, slimy rot at the base of the stem with yellowed, wilting, stunted tops, often from infected seed; bacterial soft rot turns tubers into a soft, wet, foul-smelling mush, in the ground or in storage, and can rot a tuber completely within days. The bacteria enter through wounds and spread in wet conditions. Cause Bacteria that thrive in wet, poorly drained, warm soil and spread through cuts. They often come in on a bad seed piece, and they are worst when the soil is waterlogged or when bruised potatoes are stored damp.Bacteria in the Pectobacterium and related groups, which are favored by wet soil, poor drainage, and warm conditions, and which spread on cutting knives and through wounds. They often arrive on infected or poorly cured seed pieces and are worst in waterlogged ground and in bruised, wet-stored tubers. Organic treatment There is no cure once a plant or potato is infected, so pull out affected plants and never store soft or damaged potatoes. The fix is prevention: be gentle with the tubers and keep them dry.There is no cure for an infected plant or tuber; remove and discard affected plants and do not store blemished or soft potatoes. Management is preventive: handle tubers gently and keep them dry. Prevention Use certified, well-cured seed pieces and plant them in warm, well-drained soil, not cold mud. Let cut seed pieces heal before planting, do not overwater, dig in dry weather, and handle the potatoes gently. Never store bruised or wet potatoes.Plant certified, well-cured seed pieces (or whole small seed potatoes) in warm, well-drained soil, let cut seed suberize before planting, avoid overwatering and waterlogged beds, harvest in dry conditions, cure the crop, and avoid bruising. Do not store damaged or wet tubers.
