Turmeric

On turmeric, spider mites are chiefly a problem during the long indoor pre-sprout and container phases in dry heated air - turmeric likes humidity anyway - and outdoor tropical turmeric rarely suffers them.

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.

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.

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.

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.

On turmeric the damage lands on the rhizome you are growing: galled, knotted, cracked and lumpy when dug, with the wounds inviting rhizome rot. Container growing in fresh mix largely sidesteps this pest - one more reason cold-zone growers use pots.

Stunted, yellowing, unthrifty plants that wilt in the heat even though the soil is moist, and poor yields for no obvious reason. Confirm by digging: beady swellings strung along the roots and side rootlets, and root crops that are forked, branched, cracked or knotted rather than clean.

There is no in-season cure. Pull and destroy affected plants rather than composting them, and bring that ground back over several seasons with rotation, resistant varieties and marigold cover crops. Where a crop can be grown in a container of fresh mix, that sidesteps the problem entirely.

Rotate to non-hosts, and grow resistant varieties where they exist for your crop. A French marigold cover crop grown thickly the season before and turned in genuinely lowers populations. Build organic matter and mulch heavily, which feeds the soil life that suppresses them. Plant only clean stock, never move soil from an infested bed around the garden, and solarize badly infested ground through the hottest part of summer.

Microscopic soil roundworms (Meloidogyne species) that invade roots and force them to swell into beady galls. A galled root cannot take up water and nutrients properly, so the plant stunts, yellows, wilts in heat despite watering, and yields poorly, and root and tuber crops come up forked, knobbly, hairy or cracked instead of clean. They are worst in warm, sandy soils and build up steadily wherever a susceptible crop is grown in the same ground year after year. The wounds they leave also open the way for fusarium wilt and rots.