Root-knot nematode

a patch of plants that are stunted and yellow and wilt in the midday heat even though the soil is wet and you have watered — then perk up again in the cool of evening. The patch is uneven — sick plants sitting right next to healthy ones, not the whole field failing evenly.

gently lift a sick plant and knock off the soil. Nematode roots carry knobbly swellings and knots — the 'galls' — strung along them like beads or tiny potatoes, from pinhead-size up to a marble. The root is swollen into the lump. The roots are short, forked and stunted, and older ones may already be rotting. (A gall, or knot, is a swelling the nematode makes inside the root.)

— it is smaller than a pinhead, thinner than a hair, and hidden inside the root and the soil. You judge this pest only by the galls on the roots, never by spotting an animal.

Beans and cowpeas carry helpful round nitrogen nodules on their roots — but those sit on the outside on a little stalk, rub off easily and leave the root smooth, and are pink or red inside when you split one open. Nematode galls are part of the root itself, cannot be rubbed off, are knobbly and irregular, and are not pink inside.

Poor soil or dry weather yellows the whole field evenly, and the plants recover once you feed or water them. Nematode plants stay stunted, wilt in a patch even though you have watered, and — the giveaway — carry galls on the roots. When you are unsure, always pull a plant and look at the roots.

from soon after transplanting or emergence right through the fast-growing and flowering weeks — the earlier you find it, the more you can still do. Watch hardest in warm weather and light, sandy ground, where the pest breeds fastest. Also dig and look when you clear an old crop and before you plant a new one in that bed.

, when the wilting sign shows most, and go straight to any patch of stunted, midday-wilting or yellow plants — that patch is where the nematodes are worst.

cross the field in a big 'W' so you cover the middle and all four corners, not just the edge by the path. Make about 5 stops, and at each stop lift 1–2 plants — around 10 plants in all — including a few from the healthy areas so you have something to compare against.

you cannot scout this pest from above — you must dig or gently pull plants and wash the soil off the roots. Sample plants at the edge of a bad patch (still sick but not yet dead) — a fully dead plant's roots have already rotted and won't show clear galls. Run your eye down every root looking for the beaded knots.

if you find galls on the roots of even 1–2 plants out of the 10 you lift — or you can already see a clear patch of stunted, midday-wilting plants — the nematode is established and confirmed. The roots already galled cannot be saved, so act now to protect the rest of this crop and, above all, the next crop you grow in that ground.

Where you can buy one, work a bionematicide into the planting hole and root zone — a living fungus or bacterium (such as Paecilomyces lilacinus or Bacillus firmus) that attacks nematodes and their eggs in the soil. Apply it to moist soil and follow the label. Because it is alive, handle it with care: avoid breathing the powder when you mix it, cover your skin, nose and eyes, keep it cool and out of the sun, keep it moist, wash your hands after, and keep it off food and away from children. It works best stacked on top of rotation and clean soil — not on its own.

Be clear-eyed: there is no safe spray you can put on to kill nematodes already around a growing crop. The powerful soil poisons (fumigants — chemicals that gas the whole soil) are dangerous, costly and not meant for smallholder use — leave them aside. Your real strength here is prevention, not a cure. And because there is no chemical to rotate, your 'anti-resistance' plan is simply to stack several methods together — resistant variety plus rotation plus clean soil plus the biological — so the pest never gets on top of any single one.

pull out and destroy — burn or bury far away — the worst plants together with their roots so they don't seed the soil with even more nematodes. Do not replant a susceptible vegetable in that bed. Switch the ground to a non-host grass or cereal, or marigold, for a season or two to starve the nematodes right down, keep feeding it compost, then begin your next susceptible crop with a resistant variety and clean seedlings. Accept this crop's loss and protect the land for next time — that is how you win against this pest.