Mango

The annual post-harvest pruning a mango needs anyway removes most of a buildup; a horticultural oil spray afterward reaches what is left.

Rows of small brown, tan, black, white, or waxy bumps along stems, twigs, leaf midribs, and leaf undersides that do not brush off but can be scraped loose with a fingernail. A sticky shine on the leaves - and on the floor, table, or path beneath the plant - that later turns black with sooty mold. Ants running steadily up and down the stems, since they farm and defend scale for that honeydew; often the first sign anything is wrong. Yellowing leaves, weak thin growth, and dieback of individual branches or canes that are heavily encrusted. On fruit crops, scale covers and sooty mold directly on the fruit. Check the leaf undersides and the shaded inner wood, which is where colonies build up unseen.

Horticultural oil is the main control: it works by smothering, so it has to coat the insects thoroughly - stems, twigs, and leaf undersides - and it usually needs repeating, because the waxy covering shields them and no single pass reaches everything. On deciduous woody plants a dormant-season oil smothers the overwintering stage while the leaves are out of the way, and timing a growing-season spray to the crawler stage, when the young are mobile and uncovered, is far more effective than spraying settled adults. On a small plant or a light infestation, scrub or wipe the scales off by hand, or dab them with a cotton swab of rubbing alcohol. Prune out and destroy badly encrusted wood rather than trying to save it. Wash off the sooty mold once the scale is gone - it lives on the honeydew, not on the plant, and stops once the supply does. Anything you spray must be labeled for the plant, and oils can burn delicate foliage, so test first.

Inspect. Look over leaf undersides and stems regularly - on a crop you pick, every harvest is a scouting chance - and check every new plant thoroughly before you buy it, plant it, or set it beside anything else, because scale almost always arrives on a plant rather than flying in. Quarantine new houseplants for a few weeks. Keep plants unstressed and not over-fertilized, since soft, forced growth is what populations build on. Prune for airflow and light to open up the shaded inner growth where scale hides, and prune out heavily infested wood as routine maintenance - on anything that gets a hard annual pruning, that alone removes most of any buildup. Leave the lady beetles, lacewings, and parasitic wasps alone, since they normally keep scale down to nothing, and keep ants off the plant where you can, because ants protect the colonies from those predators. Indoors, avoid hot, dry, still winter air.

Scale insects are sap-suckers that spend most of their lives immobile, fixed to a stem, twig, or leaf underside beneath a waxy or shell-like cover. They look far more like bumps, blisters, or crusty encrustations than like insects, which is why an infestation is usually well established before anyone notices it. Soft scales excrete large amounts of sticky honeydew that drips onto everything below and grows a black sooty mold - and those blackened leaves are often the first thing a gardener actually sees. Armored scales produce no honeydew but sit under a hard shield that protects them from most sprays. They spread as tiny mobile crawlers in the weeks after hatching, which is the only stage that moves and the only stage that is easy to kill. Populations build on stressed plants, on crowded shaded growth, and above all on plants kept indoors in dry, still air over winter.

The part people do not expect is the family relationship to poison ivy, and the fact that a reaction shows up a day or two later, long after they have forgotten handling the fruit.

Dark brown or black streaks and hardened patches burned into the skin of fruit shortly after picking, usually running from the stem end. On skin: itching, redness, small blisters, and swelling appearing 12-72 hours after handling fruit or pruning, most often on the hands, forearms, and around the lips and eyes. Swelling of the lips and mouth after eating unpeeled fruit. A rash that keeps recurring each harvest season is the sign of established sensitization rather than an unlucky accident.

Sap burn on fruit is cosmetic and does not affect the flesh underneath - cut the blemish away and eat the fruit. For a skin reaction, wash the area promptly with soap and water or an alcohol wipe, since removing the residue early limits the rash; then treat as you would poison ivy, with cool compresses, calamine, and an over-the-counter hydrocortisone or oral antihistamine. See a doctor for a widespread rash, for facial or eye involvement, for any swelling of the lips, mouth, or throat, or for any difficulty breathing, which is an emergency. Anyone with a serious reaction should discuss mango, cashew, and pistachio with an allergist rather than experimenting.

Wear gloves and long sleeves to prune and to harvest, and keep the sap off your face. Cut or snap fruit leaving a short stem stub rather than pulling it off flush, then stand picked fruit stem-down for several minutes so the sap drains away from the skin before it burns. Wash fruit, hands, and tools afterward. Always peel a mango before eating it, and have someone else peel it if you are sensitive to poison ivy. Never burn mango prunings, wood, or leaves - inhaling the smoke can cause a severe reaction affecting the airway. People with cashew or pistachio allergy should be cautious, since the family relationship makes cross-reaction possible.

When a mango is pulled from the tree the stem spurts a caustic latex sap under pressure. On the fruit it burns dark, hard, streaky blemishes into the skin within minutes - the defect packers call sap burn. On people it does something more serious: mango is in the poison ivy and cashew family, and the sap, peel, and leaves carry urushiol-related compounds that cause a delayed itchy, blistering contact dermatitis in sensitized individuals, typically appearing a day or two after handling and lasting one to two weeks. Reactions around the mouth after eating an unpeeled mango are common, and people who react to poison ivy are the most likely to react here.

Since mango is picked firm rather than soft, the best defense is simply to harvest at maturity rather than waiting - plus bagging individual fruit on a tree small enough to reach.

Fruit disappearing from the outermost, most exposed parts of the plant first, and from the top of a tree where you cannot reach anyway. Single neat pecks and gouges in ripening fruit, with several fruits sampled rather than one eaten through - the classic sign that it was birds rather than an animal. Heads of grain or seed going visibly lighter and emptier as they ripen, with hulls and chaff scattered underneath. Freshly sown seed vanishing off the surface of a bed before it germinates. Bird traffic in and out of the plant picking up as the crop colors, and birds waiting on nearby wires and branches. Wasps arriving at the pecked and split fruit shortly afterward.

There is no treatment once a crop is being taken - only protection and timing, so the answer is to get netting on and to pick what is ready. Reflective tape, flash ribbon, spinners, and decoy predators do work, but only for a few days at a time: birds habituate fast, so move and rotate them and never rely on them as the main defense for a crop you care about. Cut and gather grain or seed heads a little early, at firm dough, and finish drying them under cover instead of leaving them standing for the flocks. Plant a little extra, or a decoy crop the birds prefer, and accept sharing anything that crops heavily. On a wild or foraged patch, fruit going to birds and small mammals is the point of the plant rather than a problem to solve.

Net, and net early. Bird netting draped over a frame or hoops so it stands off the fruit, with the edges closed at ground level, is the only thing that reliably works at harvest scale - and it has to go on before the crop starts to color, because once birds have found the food they are far harder to displace. Keep fruit trees small enough that netting them is actually feasible; that decision gets made years earlier, at planting. Bags of mesh, organza, or paper slipped over individual heads, clusters, or prize fruit protect a small crop without netting the whole plant. Then shorten the exposure: pick promptly and completely rather than leaving ripe fruit hanging as a lure, clear split and dropped fruit that draws birds and wasps alike, and where a crop can finish off the plant, harvest a little early and ripen or dry it under cover. Rake in or cover broadcast seed so it is not lying in plain view.

Birds find ripening fruit and seed before you do. Robins, waxwings, starlings, catbirds, jays, sparrows, finches, blackbirds, and doves all work a garden, and the pressure arrives in a narrow window - the last days before harvest, when sugar rises, color changes, and the crop softens or the grain hardens. A flock can strip a small planting in a day or two, and birds that only peck rather than take whole fruit still leave wounds that rot or draw wasps. Squirrels, chipmunks, and other wildlife usually turn up on the same crop at the same time. This is competition rather than disease: the birds are not harming the plant, and a big tree or a heavy-cropping bush often has enough for everyone - but on a small planting it can be the difference between a harvest and none.

That makes it the main flowering disease in dry-winter California, where anthracnose is much less of a problem, and it is timed and treated with the same bloom sprays.

Round white powdery spots on the upper leaf surfaces that spread and merge until whole leaves look dusted. It usually starts on the oldest, lowest, most shaded leaves and works upward, so check there first. Look at the undersides too, and at stems, buds, and flower stalks, which get coated as it advances. Leaves that yellow, brown, curl, or go crisp and drop early under the coating. New growth that opens small, crumpled, or twisted where the fungus has reached the shoot tips. On plants where it gets onto the fruit, a dull gray film, russeted or scarred skin, and cracking. Late in the season, tiny dark specks scattered through the white felt - that is the fungus setting up to overwinter on the debris.

Start early: sprays protect healthy tissue, they do not clean up leaves that are already coated. Pick off and bin the worst leaves as soon as you see them, and open the plant up by thinning or pruning so air moves through. On anything that regrows quickly, shearing the whole plant back forces a clean flush and beats spraying. If it keeps spreading, potassium bicarbonate, horticultural oil, or neem applied weekly will hold it - cover both leaf surfaces. Sulfur also works, but it burns the foliage of some plants, should never go on in heat, and must not be mixed with or applied close behind an oil spray. Whatever you use must be labeled for the plant you are treating: follow that label and observe its pre-harvest interval on anything you eat. Do not compost badly infected material. Late-season mildew on a plant that is finishing anyway is usually cosmetic; on a crop still filling out it is worth acting on.

Give plants room. Crowding and still air are the biggest single driver, so space at the full recommended distance, thin seedlings properly, and divide or prune out congested growth until air moves through the plant. Full sun and an open position help; deep shade and a still corner against a wall are where it always starts. Water at the base rather than overhead, and water early in the day so nothing stays damp overnight. Keep plants evenly watered - drought stress at the roots leaves them far more susceptible, even though the fungus itself likes dry air. Go easy on nitrogen, since soft lush growth is the most vulnerable. Where resistant varieties exist for what you are growing, choosing one is the single most effective thing you can do on a plant that mildews every year. Clear infected leaves and stems away at the end of the season rather than composting them.

Powdery mildew is a fungal disease that coats leaves - and often stems, buds, and fruit - in a white-to-gray powder that looks like spilled talcum. It is not one fungus but a large family of them, each fairly specialised to its own group of plants, which is why mildew on one crop rarely jumps to an unrelated one beside it. Unlike most leaf diseases it does not need wet leaves: it thrives on warm dry days followed by humid nights, and it takes hold fastest where plants stand crowded in still air or in shade. The coating blocks light, so heavily covered leaves yellow, brown, and drop early. On many plants that is mostly disfiguring and arrives late enough not to matter; where it strips away the leaves a crop still needed to finish, it costs you size, sweetness, and quality.