Plant-parasitic nematodes

irregular patches of the field — not the whole field, and never neat along the rows — where plants are stunted, pale or yellow and wilt in the midday heat even when the soil is still wet. Early on they perk up again in the cool of evening; later they stop recovering at all. The give-away: these plants stay hungry-looking even after you add fertiliser, because the damaged roots can no longer drink it up. The same bad patches come back in the same spot season after season.

A single nematode is thinner than a hair and shorter than a pinhead, so you will never see one with your eyes; you read it from the roots instead. Dig up a poor plant and wash the soil off. Root-knot type (common on tomato, potato and okra): the roots are swollen into knots, beads and galls — like a string of tiny beads or swollen knuckles, from millet-grain to marble size, with pimples on potato tubers. Lesion, cyst and burrowing types: dark sunken spots, forked or stubby roots, hairy side-roots and rotting — carrots come up forked and whiskery and unsellable, and bananas grow reddish-black rotten roots and topple over in wind.

Helpful legume nodules rub off easily between your fingers and are pink inside — they feed the plant. Nematode galls are part of the root itself, swollen from the inside, and will not rub off.

Thirsty plants across the whole field recover fully after rain or watering; nematode patches stay stunted and wilt again even in wet soil. A fertiliser shortage shows up evenly over the field and greens up after feeding; nematode patches are patchy and ignore fertiliser. Termites and grubs chew the roots clean away, while nematode roots are still there but swollen, galled or lesioned — not eaten off.

start while the crop is young — from transplanting or emergence and on through the weeks of fast leafy growth and flowering, because young roots are hit hardest and that is your window to react. Whenever you spot a patch lagging behind the rest, walk a big "W" shape across the field so you cross all four corners and the middle — not just the edge by the path — and flag every short, pale or wilting patch.

In each poor patch, dig up (never yank) 3 to 5 of the worst-looking plants, and dig 2 healthy plants from a good part of the field to compare against. Yanking snaps the fine feeder roots and you lose the very evidence you need. Gently shake or wash the soil off the roots, then look closely for knots and beads, dark lesions, forking, and stubby or rotting roots.

if even 1 in every 5 poor plants you lift has knotted, galled or lesioned roots, treat the WHOLE field as infested and act now — this season to limit the loss, and above all to plan next season's rotation and clean seed. And remember the field's history: a field that has shown this damage before is infested for years, so assume it and prevent from the day you plant. Where a lab is within reach, send a soil-and-root sample for a nematode assay (a lab count of the worms in a scoop of your soil) to confirm it and grade how bad it is.

When you lift or pull any plant at the end of the season, turn the roots over and look them over — this is the clearest read of all on galling and rot, and it tells you whether this ground is safe to plant the same crop again or must be rotated to something else next season.

Work a biocontrol product into the soil exactly as its label directs — these are living nematode-eaters such as a nematode-trapping fungus or a beneficial soil bacterium — and dig in nematode-suppressing green manures (a crop like marigold grown and then turned back into the soil while still green). Safety: follow the label exactly; store it cool and out of the sun because it is a living organism that dies in heat; keep it off food and away from children; and wash your hands after handling. It is gentle on you, on bees and on your soil life, but it works slowly and best alongside the prevention above — not on its own.

Once nematodes are in the soil around a growing crop, there is no safe, simple spray a smallholder can pour on to kill them. The harsh soil fumigants the big farms once used are dangerous, restricted and not for hand use — leave them alone. Your money and effort are far better spent on the resistant seed, rotation, sanitation and biocontrol above; that really is where the win is.

Rope off the worst patches and work them last, so you don't carry worms out on your tools and boots. Pull and burn the badly galled plants together with their roots — do NOT compost them, the roots are packed with eggs. Take that ground out of tomato, potato, carrot, okra and banana for 2 to 3 seasons (put it under cereals, grass, clean fallow or solarising) and grow your prized crops on clean ground meanwhile. Keep adding organic matter every season. A badly infested field is beaten down over several seasons, not cleared in one — but with resistant seed, rotation and clean planting it does come back into use.