Lettuce mosaic virus

young leaves show a patchy light-green and dark-green mottle (a "mosaic") instead of one even green — hold a leaf up to the light and the pale and dark blotches stand out. This is the clearest early sign.

the small veins go pale and glassy ("clearing"), leaves pucker, crinkle and curl, and the whole plant is stunted — clearly smaller than its healthy neighbours in the same row. Badly hit plants never close into a proper head.

turn young leaves over and check for aphids — tiny soft green (sometimes pink or grey) insects the size of a pinhead, clustered on the underside and on the growing tip, often with sticky shine and ants walking on them.

with fertiliser hunger (nitrogen shortage yellows the WHOLE older leaf evenly, not a green-on-green patchy mosaic), with downy mildew (pale patches on top with a grey furry mould underneath — mosaic has no mould), or with weedkiller drift damage (twisting spread evenly across the whole bed from one side, not scattered plant-by-plant). Mosaic is patchy mottling + stunting on scattered single plants.

check hardest while plants are young — from the seedling and nursery stage through the first few weeks of growth. This is when infection does the most damage and when you can still act, so walk the crop at least once a week.

cross the field in a big "W" shape so you cover all four corners and the middle, not just the edge nearest the path. Make about 10 stops along the way.

look down at 10 young plants — tip a few leaves up to the light for the green-on-green mottle, feel for puckering and curl, note any plant that is oddly small, and turn leaves over to look for aphid colonies on the underside and growing tip.

this virus has zero tolerance — even 1 mottled or stunted seedling means act at once: mark it and pull it that same day (see control). If you find aphids building colonies on young lettuce, treat that as your warning that spread is starting and act before it runs through the bed.

(a yellow card or board smeared with grease/oil) at plant height around the bed to catch flying aphids early — rising catches mean aphids are moving in and the risk is climbing.

the moment you find a mottled or stunted plant, pull it out that same day, carry it out of the field in a bag (don't drop leaves between healthy plants), and bury or burn it. Every sick plant left standing is a source the aphids keep spreading from — removing them early is the single most useful thing you can do in-season.

an insecticide for the aphid carrier only slows aphids down; it cannot cure a sick plant and cannot fully stop the spread, because aphids pass the virus in seconds — faster than the spray kills them. Lean on clean seed, resistant varieties and roguing first; use aphid spray only as a back-up on top of those.

cover your skin, eyes, nose and mouth; spray in the cool of morning or evening with the wind at your back; keep it off food, water sources, children and bees; respect the pre-harvest interval (the minimum wait printed on the label between spraying and picking lettuce to eat); wash yourself and your clothes afterwards. Rotate to a different chemical group each time so the aphids don't build up resistance (stop being killed by it).

tighten the basics — rogue more often (every few days), knock back the aphids, and clear every volunteer and weed host around the bed. If most of the bed is already mottled and stunted, it is too late to save this crop: cut your losses, clear and destroy the whole planting so it can't seed aphids into your next one, then start fresh with certified clean seed and a resistant variety on a clean piece of ground, away from the old bed.