Groundnut rust

turn a lower leaf over and look at the underside. Rust starts there as tiny orange to reddish-brown raised bumps (pustules) — like flecks of rust or brick dust, each smaller than a pinhead. Rub one with your thumb and an orange powder comes off on your skin — that powder is the spores.

the bumps grow more numerous and burst open, and matching yellow spots show on the top of the leaf. Heavy infection turns leaves brown and brittle; they dry out but stay hanging on the plant instead of dropping cleanly. The disease climbs from the bottom leaves toward the top.

which makes round, dark brown-to-black spots (no orange powder rubs off). Rust is the one that leaves rusty powder on your thumb. The two often sit on the same plant at once — if you see both, treat for both.

ordinary old-age yellowing near harvest colours the whole leaf evenly with no bumps underneath, and drought scorch dries the leaf edges first. Rust is defined by the raised orange pustules on the leaf underside — always check underneath before you decide.

start once the crop begins flowering and is filling pods — the development, mid-season and late-season stages are when rust does its harm. Look harder after a run of warm (about 20-28°C), muggy, humid days or heavy dews, because that wet, warm weather is what the fungus loves.

every few days walk a large "W" shape across the field so you cover all four corners and the middle, not just the edge by the path. Make about 10 stops along the way.

pick a plant and turn over its lower, older leaves — rust always shows on the bottom leaves first. Look for the orange pustules and rub one to confirm the rusty powder.

as soon as you find the first orange pustules on the lower leaves of plants that are filling pods — even a few plants — treat it as the trigger to act. Do not wait for the whole field to turn orange; by then it is too late to save much.

plant a rust-tolerant groundnut variety — many also resist leaf spot, so one good seed choice fights both. Use clean, healthy seed, and don't plant a new crop right beside an older, already-rusted groundnut field.

rotate — follow groundnut with a cereal like maize or sorghum so rust has no groundnut to survive on. After harvest, gather and destroy (burn or bury) the old rusted haulms and pull out any volunteer groundnut plants, because these carry spores over to the next crop. Avoid over-thick, damp stands — space plants so air moves and leaves dry quickly after dew or rain.

if rust appears while pods are still filling and keeps climbing, a fungicide (a triazole type or chlorothalonil) is the strongest tool (about 80% effective). Spray at the first pustules, not later, and you can tank-mix it to cover leaf spot at the same time. Note: chlorothalonil is a protectant (contact) spray — it shields leaf surfaces it lands on but does not move inside the plant, so cover the leaves well, especially the undersides; a triazole is systemic — it moves inside the leaf and gives a little back-action on early infection.

cover your skin, eyes, nose and mouth; spray in the cool of morning or evening with the wind at your back so the mist blows away from you; keep it off food, drinking water, children and bees; wash yourself and your clothes afterward; and always respect the pre-harvest interval (the minimum number of days the label says you must wait between the last spray and lifting your nuts). To stop the fungus getting used to one chemical, rotate between different fungicide groups rather than spraying the same one every time.

repeat the fungicide on the label's schedule and make sure you are covering the leaf undersides. If the field is already mostly brown and the leaves are dead and hanging, stop spending on spray — it will not bring dead leaves back. Instead lift the crop as soon as the pods are mature, then clear and destroy every rusted haulm and volunteer, rotate to a cereal next season, and switch to a rust-tolerant variety so the next crop starts clean.