Taro

Every part of taro - corm, cormels, leaves, stalks and sap - carries bundled needles of calcium oxalate that fire into soft tissue on chewing, causing burning, blistering and swelling of the lips, mouth, tongue and throat. Severe swelling can compromise the airway. Thorough cooking destroys them, which is why taro has been eaten safely by billions of people for thousands of years; the harm comes from eating it raw, from cutting cooking short, and from treating taro leaf like spinach. Handling the raw plant also causes an itching, prickling contact dermatitis on the hands and forearms.

Watch for: Immediate burning, prickling or scratchiness in the mouth and throat during or just after eating, heavy drooling, a hoarse or changed voice, difficulty swallowing, visible swelling of the lips or tongue, and any trouble breathing. On the skin, intense itching and prickling on the hands and forearms hours after peeling or digging.

Prevent: Cook every part, every time - boil, steam or bake corms until completely tender (20 minutes minimum, longer for large pieces) and simmer leaves for around 45 minutes in an uncovered pot, discarding the cooking water. Never taste raw taro to identify it or to check a variety. Buy food-grade planting and eating stock: ornamental elephant ears sold for foliage, and anything sold as Alocasia, are not food. Wear gloves to dig, wash and peel, or peel under running water, and rinse arms afterward.

Treat: For a scratchy mouth after a cooked meal, stop eating that batch and cook it longer - the food is underdone. For a genuine raw exposure, call poison control or emergency services immediately rather than waiting for symptoms to peak. Rinse the mouth with cool water; cold milk, ice or ice cream soothes the tissue. Do not induce vomiting. Any airway symptom - drooling, a changed voice, trouble swallowing or breathing - is an emergency call. For skin contact, wash with soap and water; the itching fades over hours and a cool compress helps.

Cultivars differ enormously in acridity - eddoe and Japanese sato-imo types are generally mildest, some wild and ornamental Colocasia are ferocious. If a fully cooked mouthful still prickles, that batch needed longer in the pot, not a bigger bite.

A water mold (Phytophthora colocasiae) that is the most damaging taro disease worldwide - it destroyed most of Samoa's taro crop in the 1990s and is established in Hawaii. It begins as small dark water-soaked spots on the leaf blade, which expand into large brown-to-purple blotches ringed with yellow, often with a fine white fuzzy margin visible in the early morning. Leaves are destroyed from the outside in, and a plant that cannot hold a canopy cannot fill a corm, so severe blight cuts yields drastically even though the corm itself is not directly attacked. It moves fastest in warm, wet, humid, still conditions and is spread by rain splash and on planting material.

Watch for: Small dark water-soaked spots appearing on the leaf blade, often near the edge or where water sits. Spots expanding into large brown or purplish blotches with a yellow halo and, at dawn, a fine white fringe of sporulation at the margin. Yellow droplets of fluid on the lesions. Whole leaves collapsing and rotting away within days in wet weather, with the plant pushing out replacements that are smaller each time. Damage worst in crowded, still, humid plantings and after long wet spells.

Prevent: Start with clean planting material - the disease travels on huli and corms, so never take stock from a blighted patch. Space plants for real airflow rather than packing them, site the bed where morning sun dries the leaves, and water at the base rather than over the canopy. Remove and destroy infected leaves as they appear instead of leaving them to sporulate, and clear all crop debris at the end of the season. Where blight is established, resistant cultivars developed in Hawaii, Samoa and the Pacific breeding programs are the only durable answer.

Treat: There is no cure for an infected leaf. Cut off and destroy affected foliage promptly - do not compost it - and open the planting up so air moves through. Copper fungicides labeled for the crop give some protection on healthy tissue in wet seasons, but they must go on before infection and be reapplied; follow the label and observe its pre-harvest interval. If a planting is overwhelmed, harvest what corm there is, destroy the tops, and start the next season with clean stock in a different bed.

Established in Hawaii and a constant management concern there; mainland gardens see it mainly in wet Gulf Coast summers. The corm stays edible - blight is a yield problem, not a food-safety one.

Aphids are small soft-bodied sap-suckers - green, black, gray, pink, or waxy white, and mostly wingless - that gather in dense colonies on the tenderest new growth: shoot tips, buds, and the undersides of young leaves. They breed extremely fast in mild weather, so a few can become hundreds inside a week. Their feeding curls and distorts soft growth, and the sticky honeydew they excrete coats everything below and turns black with sooty mold. Ants often farm and defend the colonies for that honeydew. A few aphids are normal on almost any plant and a healthy plant shrugs them off; only dense, persistent colonies do real damage, and some aphids also spread plant viruses as they feed.

Watch for: Dense clusters of tiny insects packed onto shoot tips, flower buds, and the undersides of young leaves. Curled, cupped, or distorted new growth that never opens properly. A sticky shine on the leaves - and on whatever sits under the plant - that later goes black with sooty mold. Tiny white cast skins left behind where a colony has been feeding. Ants running steadily up and down the stems, which is often the first thing you notice. Mottled, streaked, or misshapen leaves that do not grow out can mean a virus the aphids brought with them rather than the feeding itself.

Prevent: Go easy on nitrogen: the soft, lush growth that heavy feeding produces is exactly what aphids breed on. Check new growth once a week through spring and early summer so you catch colonies while they are still small, and rub the first few out with your fingers. Leave flowering plants nearby and skip broad-spectrum sprays - ladybugs, lacewings, hoverflies, and parasitic wasps do most of this work for free, and they only stay if there is something for them to eat. Keep ants off the plant where you can, since they protect aphids from those predators. Water steadily so plants are not stressed, and check any new plant over before you bring it home or move it indoors, since aphids usually arrive on something else.

Treat: Knock the colonies off with a hard jet of water and repeat every few days - on most plants that alone ends it. Pinch out or prune off the worst-infested tips rather than treating the whole plant. If colonies keep rebuilding, spray insecticidal soap or horticultural oil directly onto the insects, covering leaf undersides, and repeat weekly until they are gone; treat early or late in the day, and never spray a wilted or heat-stressed plant. Avoid broad-spectrum insecticides - they kill the ladybugs, lacewings, and parasitic wasps that were about to solve the problem for you, and aphid numbers usually come back worse afterward. Anything you spray must be labeled for the plant you are treating; follow that label and observe its pre-harvest interval. On anything you eat, wash leaves, heads, and buds well before use, since aphids hide down in crowns, folds, and tight growth.

The feeding itself barely troubles a big taro plant - the reason to care is dasheen mosaic virus, which aphids move from plant to plant and which then rides in the corms and huli you save. Streaked, feathery, distorted new leaves mean the virus; remove that plant and never propagate from it.

A complex of soil water molds and fungi (chiefly Pythium species) that rot the corm from the inside, turning firm white flesh into soft, sour, discolored mush. It is a disease of stagnation and injury rather than of wetness as such - taro is perfectly happy standing in moving or regularly refreshed water, but sits vulnerable in dead, airless, warm water or in heavy waterlogged soil. Wounds from digging, from cultivating too close, or from insect feeding are the usual doorway, and infected planting corms carry it into a clean bed.

Watch for: A plant that stops growing, yellows, and then collapses at the crown while its neighbors look fine. A sour or fermented smell at the base. Soft, dark, water-soaked patches on a lifted corm, sometimes with firm tissue right beside them. Corms that feel light for their size. Rot spreading in storage from a single wounded corm to those touching it.

Prevent: Keep water moving or refresh it rather than letting a flooded bed go stagnant and hot, and on upland plantings make sure the ground drains even though it stays moist. Plant only firm, sound, disease-free corms and huli. Handle the crop gently at digging - every fork wound is an entry point - and cull damaged corms out of the storage batch rather than hoping. Rotate the bed rather than growing taro in the same ground year after year.

Treat: There is no rescue for a rotted corm. Pull affected plants with the surrounding soil, bin them rather than composting, and improve water movement or drainage before replanting. Cut well back into firm, sound, sweet-smelling flesh if you are salvaging part of a corm for the kitchen, and discard anything soft, discolored or sour-smelling entirely - do not trim around a rot and cook the rest.

The commonest way a mainland taro crop is lost in a tub or barrel, where the water goes stagnant and warm in midsummer. Top the tub up with fresh water rather than letting it sit and evaporate.

Spider mites are not insects but tiny sap-feeding arachnids, smaller than a grain of salt, and you almost always notice the damage before you notice them. They live on the undersides of leaves and drain the cells one at a time, which is why the leaf goes finely stippled with pale dots before it bronzes and dries. They breed explosively in hot, dry, dusty, still air - a generation can pass in a week at high temperature - so a barely visible population becomes a webbed, bronzed plant within a fortnight. Drought-stressed plants and plants wintered indoors in dry heated air are the classic hosts. Broad-spectrum insecticides make matters worse rather than better, because they kill the predatory mites and lady beetles that normally hold mites in check.

Watch for: Fine pale stippling or speckling across the upper leaf surface, as though the color had been sandblasted out of it, and later a dull bronze or rusty cast over the whole leaf. Delicate webbing across leaf joints, shoot tips, and undersides - that comes with a heavy infestation and means you caught it late. Leaves that dry, curl, and drop early while the plant looks thirsty no matter how much you water it. Hold a leaf over a sheet of white paper and tap it: the specks that fall and then start walking are mites. With a hand lens, look on the undersides for the mites themselves, pale or reddish, and their round translucent eggs. Damage that shows up first on the hottest, dustiest, most sheltered side of a plant, or on a pot sitting near a radiator indoors.

Prevent: Do not let plants get drought-stressed - that is the single strongest trigger, and a plant watered steadily through heat is far less likely to have a mite problem at all. Hose the foliage down, undersides included, during hot dry spells: it knocks mites off, washes away the dust they thrive in, and raises the humidity around the plant for a while. Indoors and under cover, keep the air from going hot, dry, and stagnant, wipe or rinse the leaves occasionally, and keep pots away from radiators and heating vents. Inspect any plant carefully before you bring it indoors for winter or set it among others, since mites almost always arrive on something else. Avoid broad-spectrum insecticides: they wipe out the predatory mites, lacewings, and lady beetles that do most of this work for you, and reliably leave you with a worse outbreak than you started with.

Treat: Start with water. A strong jet aimed at the leaf undersides, repeated every few days for a couple of weeks, knocks populations down and breaks the cycle - on many plants that alone ends it. On anything that regrows quickly, shearing back badly infested growth resets the plant to clean foliage. If it keeps building, spray insecticidal soap or horticultural oil directly onto the undersides where the mites actually live, and repeat every five to seven days for three rounds: contact sprays do not kill the eggs, so a single application always fails. Anything you spray must be labeled for the plant you are treating - follow the label and observe its pre-harvest interval on anything you eat. Predatory mites are an effective release in a greenhouse or on an indoor plant. Above all, correct the heat, dust, and drought stress that caused it, or the mites simply come back.

Almost never a problem on a properly watered taro, which is exactly the point - mites on taro are a symptom that the plant has been allowed to dry out. They are far more common on tub-grown and overwintered indoor plants than in a wet bed.