Lettuce

On heading types the colonies sit deep inside the head where you will not see them until you cut it open - check the crown, and soak garden lettuce before serving.

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.

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.

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.

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.

Yellow angular blotches bounded by leaf veins, white down on the underside in morning humidity.

Remove affected leaves promptly; harvest ahead of a wet forecast; copper protects new growth if pressure is high.

Morning watering at the base, generous spacing, resistant varieties where falls are wet.

Cool, damp-weather disease: pale angular patches on top of leaves with white fluffy growth beneath, disfiguring and opening rot pathways.

A felled lettuce transplant is a total loss, and a single cutworm will work its way down a row night after night. Collar the transplants at planting rather than waiting to find out whether you have a problem.

Seedlings and transplants toppled at the soil line overnight, or shoots chewed and notched near the base, with the severed tops left lying beside the stumps. Dig an inch down within a few inches of the damage and you will usually find the culprit: a plump, dull gray-brown caterpillar that curls into a C when touched.

Dig around each fallen plant and destroy the caterpillar - left alone, one cutworm will fell another plant every night. Replant the gaps with collars fitted. Hand-picking after dark with a flashlight works well on a small bed, and a fast-growing crop soon outgrows the vulnerable stage.

Push a cardboard, foil or tin-can collar an inch into the soil around each seedling and transplant so the stem cannot be reached. Cultivate the bed a couple of weeks before planting to expose the larvae to birds and drying, and clear grassy weeds and debris, which is where the moths lay their eggs.

The soil-dwelling larvae of night-flying moths. They spend the day curled just under the surface and come out after dark to chew through young stems at soil level, felling seedlings and transplants outright - a toppled plant lying beside its own stump is the signature. Some species also climb to gouge and notch emerging shoots and spears. They are worst in freshly worked ground, in beds that were weedy or grassy the year before, and on the first plantings of spring.

Brown, dry edges on the newest inner leaves, especially during warm spells after rain.

Trim affected edges (the rest is fine to eat) and smooth out the watering rhythm - new growth will be clean.

Steady, even watering and mulch; avoid the feast-famine moisture cycle; some varieties resist it markedly.

Not a disease: browning, papery leaf edges caused by a calcium-delivery hiccup during fast growth and moisture swings - lettuce's version of blossom end rot.