Lettuce

Slugs and snails are nocturnal mollusks that rasp ragged holes in soft leaves, shear off seedlings at ground level, and chew into ripe fruit that touches the soil. They feed at night and in the early morning and spend the day hidden under boards, pots, dense mulch, leaf litter, and low foliage, becoming active in exactly the cool, damp, shaded conditions that suit young plants. Damage is heaviest in wet spells and on tender spring growth: a single wet week can erase a row of seedlings while an established, tough-leaved plant of the same species is barely marked. They lay clusters of small pearly eggs in damp soil and under debris, so a garden with plenty of cover carries a resident population year after year rather than being invaded from outside.

Watch for: Irregular ragged holes chewed out of the middle of leaves and along the edges - not the clean-edged cuts a caterpillar leaves. Seedlings that vanish overnight, or that are cut off at soil level with nothing left behind. Silvery dried slime trails across leaves, soil, pots, and paving in the morning: this is the giveaway that tells you it was mollusks rather than an insect. Chewed pits in ripe fruit, especially where it rests on the ground. Damage that shows up on the lowest, dampest leaves first and gets worse after rain or evening watering. Turn over a board, an upturned pot, or a clump of dense mulch during the day and look for the animals themselves sheltering in the cool underneath.

Prevent: Take away the daytime shelter first: lift boards and upturned pots, clear piles of debris, keep dense mulch pulled back from crowns and stems, and give crowded plantings enough airflow to dry out. Water in the morning rather than the evening so the surface is dry by the time they come out. Copper tape around containers and raised beds gives them a shock they will not cross, and it is the one barrier that reliably works. Get plants past the vulnerable stage quickly - start seedlings in pots and set them out at a decent size rather than direct-sowing into a cold wet bed, since most plants stop being worth defending once the leaves toughen. Leave the ground beetles, frogs, toads, and ground-feeding birds alone, because they eat mollusks all night for free and broad-spectrum slug pellets kill them too. Where thick-leaved or resistant varieties exist for what you are growing, they take far less damage.

Treat: Hand-picking works and costs nothing: go out an hour after dark with a headlamp, drop what you find into soapy water, and repeat nightly for a week or two - persistence is what wins, not any single product. Shallow beer traps sunk to soil level catch them too, but need emptying and refilling daily to be worth anything. Iron-phosphate bait is the standard control and is far lower-risk around pets and wildlife than the old metaldehyde products, but it is not harmless: many formulations include EDTA, and a dog that eats a quantity of pellets can develop iron toxicosis. Scatter it thinly per the label, never in piles, and keep dogs off a baited bed. Clear fallen foliage and debris in autumn, where the eggs are laid. In a wet climate this is a population you manage each season rather than one you eradicate - and on most established plants light grazing is cosmetic and needs nothing at all.

The lettuce grower's primary enemy - they can erase a whole seedling planting overnight in damp weather. Copper tape around containers and beds plus nightly patrols through the seedling weeks is the reliable approach.

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.

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.

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.

Watch for: 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.

Prevent: 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.

Treat: 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.

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.

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.

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

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

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

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

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

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

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