Mungbean yellow mosaic virus

scattered bright yellow flecks or spots on the youngest leaves at the top of the plant. They are the colour of a ripe lemon and stand out clearly against the healthy green.

the yellow flecks join up into a bright yellow-and-green patchy pattern (a "mosaic") over the whole leaf. Badly hit leaves turn almost fully yellow, and the plant stays short and stunted with very little flowering.

infected plants set few small, twisted pods with shrivelled, half-empty seed — plants that got sick young may give no usable pods at all.

turn a young leaf over and look underneath for tiny white flies the size of a pinhead. Brush the plant and a little cloud of them flies up like white dust, then settles back. These whiteflies are what carry the virus from plant to plant.

yellowing from hunger (poor soil / lack of nitrogen), which is an even, all-over pale yellow starting on the OLD lower leaves — not a bright patchy mosaic on the young top leaves. Virus yellow is bright, blotchy, and worst at the growing tip; hunger-yellow is dull, even, and worst at the bottom. Yellowing from waterlogging affects whole patches of low ground, not scattered single plants.

start checking from the moment seedlings come up and keep it up hard through the first half of the season (seedling, branching and early-podding stages) — this is when the plant is most easily infected and when early infection does the most damage.

at least twice a week, walk a big "W" shape across the field so you cover all four corners and the middle — not just the row nearest the path where you always look.

at about 10 stops look at the youngest top leaves of 10 plants for bright yellow flecks or mosaic, and turn a few leaves over to check for whiteflies underneath.

hang a few yellow sticky cards (or a yellow tin smeared with grease/oil) at plant height around the field. Whiteflies are drawn to yellow and stick to it — a card that is filling up fast with tiny white flies is your warning that the fly is building up.

act the moment you find even one plant with bright yellow mosaic on its young leaves, OR whitefly numbers clearly rising on your leaf checks and sticky traps. There is no "wait and see" — because the disease has no cure, the trigger is the first sign, not a big count. One sick plant left standing is a factory feeding the virus to the rest.

as soon as you spot a plant with yellow mosaic, pull it out whole and remove it from the field (bury or burn it). Do this the same day you see it. Removing one infected plant early stops it feeding the virus to a hundred healthy neighbours — this is the most important thing your own hands can do.

remember, no spray cures a virus-sick plant. Sprays only reduce the whitefly that spreads it. If the fly is building up badly, use a systemic insecticide — a "systemic" means the chemical is taken up inside the plant and poisons the flies as they feed (unlike a "contact" spray that only kills what it lands on). A seed treatment at planting plus a careful early foliar spray protects the young crop when it matters most.

cover your skin, eyes, nose and mouth; spray in the cool of the morning or evening with the wind at your back, never towards yourself. Keep the spray away from food, drinking water, children, animals and bees. Respect the pre-harvest interval — the number of days the label says you MUST wait between the last spray and picking pods, so no poison is left on the food. Wash your body and clothes afterwards.

don't use the same insecticide over and over. Rotate to a different chemical group each time you spray so the whitefly doesn't become immune to it and the product keeps working next season.

if new yellow plants keep showing up despite roguing, step up whitefly control, remove sick plants daily, and note that a crop badly hit early may not be worth saving. When most plants are already yellow and stunted, cut your losses — harvest whatever healthy pods remain, clear and destroy the crop residue so it can't seed next season's outbreak, and next time plant a resistant variety, sow early, and start on clean weed-free ground.