Gadenin日本語Get the appWhy picked plants are more vulnerable
A heading lettuce closes over and protects its own crown. A loose leaf plant you pick every week never does. It carries open leaf-stalk wounds, a permanently exposed center and a base that stays damp under the remaining leaves, and that combination is what most loose leaf problems exploit.
The practical consequence is that hygiene at picking time matters more than spraying. Clean cuts, no stubs left behind, yellowed leaves removed from the bed, and watering at the base in the morning rather than over the leaves in the evening will prevent more disease than anything you apply afterwards.
Slugs and snails: the main loss
Slugs do more damage to loose leaf lettuce than every other pest combined, because the crop is picked young and even light grazing makes the leaves unsaleable to your own kitchen. They feed at night and shelter under the mulch, boards and dense low leaves during the day, so the damage appears with no visible culprit.
Ragged holes with no clear edge, and silvery trails on the soil in the morning, are the diagnosis. Deal with it by removing daytime shelter near the row, keeping mulch an inch clear of the crowns, watering in the morning so the surface is dry by dusk, and going out with a torch an hour after dark twice a week during wet spells. That last measure is by far the most effective.
| Damage seen | Likely cause | First action |
|---|---|---|
| Ragged holes, silvery trails | Slugs or snails | Night torch patrol, clear shelter |
| Seedlings vanish overnight | Slugs at emergence | Barrier around the band, patrol nightly |
| Neat round holes in leaf centers | Caterpillars | Search the undersides, pick off by hand |
| Small shot holes on young leaves | Flea beetles | Cover the band with fine mesh |
| Whole plant severed at soil level | Cutworms | Dig around the stem base and remove |
Aphids in the crown
Aphids colonise the center of a loose leaf plant, exactly where you cannot easily see them and cannot easily wash them out. They arrive on soft, heavily fed growth, so the first fix is to stop over-feeding: half-strength liquid feed after cutting, not rich nitrogen at planting.
Check the newest center leaves at every picking by tipping the plant sideways in the light. Caught at a dozen insects, a firm jet of water into the crown from a hose clears them; caught at a hundred, the plant is not worth saving and should be pulled before it seeds the rest of the bed. Fine mesh over the batch from planting keeps them out entirely, at the cost of having to lift it to pick.
- Inspect the crown weekly Tilt each plant toward the light and look into the center. Aphids are always in the newest leaves, never on the outer ones you are picking.
- Jet the crown early A firm spray of plain water directed into the center removes a small colony. Do it in the morning so the crown dries before nightfall.
- Pull heavily infested plants A plant with sticky honeydew and sooty mold in the crown will not produce clean leaves again. Remove it from the garden rather than composting in place.
- Leave the predators alone Ladybird larvae and hoverfly larvae in the bed will clear a moderate infestation in about ten days. Spraying kills them along with the aphids.
Base rots, downy mildew and gray mold
The fungal problems of loose leaf lettuce nearly all begin where a leaf meets the soil. Gray mold enters through the stubs left by careless picking. Base rot spreads through leaves lying flat on damp ground. Downy mildew shows as pale yellow patches on the upper leaf surface with a grayish felt on the underside, and it thrives in cool, humid, still air.
The controls are structural rather than chemical: raise the bed four inches (10 cm) so water leaves the root zone, space at six inches (15 cm) rather than four so air moves at the base, water in the morning at the soil, remove every yellowing leaf at each pick, and cut leaf stalks flush instead of leaving stubs. Where mildew appears, pick off affected leaves immediately and do not compost them near the bed.
| Symptom | Problem | Conditions that caused it | Response |
|---|---|---|---|
| Yellow patches above, gray felt below | Downy mildew | Cool, humid, still air | Remove leaves, widen spacing, improve airflow |
| Fuzzy gray growth on leaf stubs | Gray mold | Torn stubs left after picking | Cut flush, remove infected tissue |
| Brown slimy collapse at the base | Base rot | Wet soil, leaves lying flat | Raise the bed, pull affected plants |
| Brown scorched leaf margins | Tipburn, not a disease | Uneven watering in fast growth | Water more evenly, feed less |
Tipburn is often mistaken for disease and treated with fungicide. It is a calcium transport failure caused by erratic moisture. Steady watering fixes it; nothing sprayed on the leaves will.
Rotation and a record of what appeared
Do not grow lettuce in the same ground more than one year in three. Soil-borne base rots and lettuce root aphid build up where the crop repeats, and by the time symptoms are obvious the bed is already a problem for several seasons. Successive sowings within one season are fine, but move the whole run each year.
Keep a note of the first date each problem appeared and which end of the bed it started in. Slug pressure, mildew and aphid arrival are all fairly consistent from year to year in one garden, and knowing that mildew shows up in your third autumn week lets you widen spacing on that batch before it does, rather than after.