Cucurbit mosaic virus

on the youngest leaves at the shoot tips, a patchy bright yellow-green mosaic — light and dark green mixed together like a patchwork quilt, not an even yellowing. The leaf often puckers and blisters, with raised bumpy areas between the veins.

new leaves come out narrow and strap-like (fern-like, twisted or shoe-string thin), the vine stunts with short, bunched-up growth, and the fruit turns mottled, warty and lumpy, distorted in shape with poor fruit set. A plant this far gone will not recover.

a feeding shortage (like nitrogen) yellows the older, lower leaves evenly and follows the veins — mosaic is a green-on-green patchwork on the young top leaves with distorted leaf shape. Powdery or downy mildew sits on the surface as white dust or dry angular patches you can see or wipe — mosaic colour is inside the leaf and cannot be wiped off. And unlike a spray burn, mosaic spreads from plant to plant, usually starting at the field edge nearest weeds or an older cucurbit plot.

start at emergence / young seedlings and keep going hard through vine run and early fruiting — these are the stages when the virus does the most damage. Check twice a week, and step it up in warm, dry spells (about 20–32°C) when aphids take flight.

walk a big "W" across the field so you cover all four corners and the middle, not just the row by the path. Make about 10 stops.

look at the youngest leaves at the shoot tips of a few plants for mosaic mottling and narrowed leaves, then turn the tender leaves over and look for aphids — soft, pear-shaped insects about the size of a pinhead, green or black, clustered on the growing tips. Also glance at the field margins for winged aphids arriving.

the threshold is rogue at the very first mosaic symptom — so even 1 plant showing mosaic means act that same day: pull it out. Don't wait for a count. And if you see winged aphids building up on the young growth, act on the aphids before their numbers swell, because each one can seed many new infections.

at the first mosaic sign, uproot the whole plant — roots and all — bag it, and carry it out of the field; do not drop it between the rows or compost it there, or it stays a source. Handle healthy plants first and suspect ones last, and wash your hands and tools afterwards so you don't carry sap to clean plants.

there is no chemical that cures mosaic virus once a plant is infected, and spraying insecticide at the aphids does not reliably stop it, because they pass the virus in seconds — long before the poison kills them — and broad poisons also kill the ladybirds and wasps that keep aphids down. IF you still choose to treat an aphid outbreak, protect yourself: 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; wait out the pre-harvest interval (the safe number of days between spraying and picking) before you harvest; wash well afterwards; and rotate to a different chemical group each time so aphids don't build resistance. But put your effort into prevention — it pays far better here.

if most plants are already showing mosaic early in the season, the field will not recover its yield — pull out the crop, clear all weed and volunteer hosts, and start fresh with a resistant variety, planted early and well away from the old, infected ground next time.