Soybean mosaic virus

on the young trifoliate leaves (the three-part leaves near the top), a patchwork of light-green and dark-green blotches — like a shadow pattern that will not wash off. This mottling is called mosaic, and it is the clearest early sign.

infected leaves pucker, crinkle and curl downward, look narrower and more strap-like than healthy leaves, and the whole plant grows short and stunted. The mosaic pattern shows up strongest in cool weather.

infected plants set fewer pods, and when you open them the seed coat is often mottled, streaked or discoloured (dark bleeding lines running from the seed scar) instead of clean and even-coloured.

feeding damage or fertiliser shortage. Aphid or spider-mite feeding gives yellow speckles or a sticky, sooty leaf, not a green-on-green pattern. A nitrogen shortage turns the whole older leaf evenly pale-yellow — mosaic is patchy light-and-dark-green on the young leaves, and it comes with puckering and stunting that hunger does not.

start walking the field from the development stage (once plants are growing fast and putting out new trifoliate leaves) and keep it up through mid-season (flowering and early pod-set) — these are the stages when infection costs you the most, so this is when to catch it.

trace a big "W" across the field so you cross all four corners and the middle, not just the row by the path where you always look.

make about 10 stops along the W and check 10 plants at each stop. At every plant look at the youngest top leaves for the light-and-dark-green mosaic, puckering and curling.

turn a few leaves over and check the growing tips for aphids (small soft green or black insects clustered on the stem and under leaves). Hang a couple of yellow sticky traps or a yellow water pan at the field edge — heavy aphid catches mean the virus can move fast, so tighten your guard.

there is no rescue spray, so the trigger is different from an insect pest. If you find even 1 mosaic plant in your first 10 stops, or aphid numbers are climbing, act now — pull the sick plants and protect the rest. The earlier and the more infected plants you see, the more urgent it is.

as soon as scouting finds mosaic plants, pull them out (rogue them) and remove them while infected plants are still few, so aphids have less virus to pick up and carry. Clear weeds and volunteer legumes in and around the field, as these host the virus between crops.

where you can, lay reflective mulch (shiny material on the soil) or plant a tall border/barrier crop around the field — both confuse and deter aphids so fewer land and spread the virus.

because aphids spread this virus in seconds (non-persistent spread), an insecticide kills the aphid too late to stop infection and is rarely worth it for mosaic alone. If you ever do spray for another reason, protect yourself: cover your skin, eyes and nose, spray in the cool of morning or evening with the wind at your back, keep it away from food, water, children and bees, respect the pre-harvest interval (the days you must wait between spraying and harvest, printed on the label), wash well afterwards, and rotate to a different chemical group each time so aphids don't build resistance.

keep roguing infected plants and controlling weeds and aphids to protect your healthy plants. If mosaic has spread through most of the field, accept the loss for this crop — do not save any seed from it — and next season come back stronger with a resistant variety and fresh certified seed, which is the only thing that reliably breaks the cycle.