Groundnut leaf spot

small round spots on the lower, older leaves — the leaflets near the ground yellow, then dry up and drop off, so the plant goes bare from the bottom upward. In a bad year the vines look half-stripped by pod-fill.

roughly circular spots the size of a pin-head to a small seed, brown to nearly black, often ringed by a yellow halo (a soft yellow glow around the spot). Turn the leaflet over: late leaf spot shows dark, rough spots with a dusty grey-black mould on the underside (that dust is the spores); early leaf spot spots are a bit lighter, more reddish-brown, with the halo more obvious on top.

always starts low and old — the bottom leaves — and climbs upward as it spreads. Worst in warm, damp, dewy weather and in fields where you grew groundnut last season too.

groundnut rust, which makes orange-brown powdery bumps (pustules) that burst on the underside and rub off orange on your finger — leaf spot spots do not rub off orange. Also not the same as plain leaf yellowing from hunger (nutrient shortage), which yellows evenly with no distinct dark round spots. If you see clear dark rings with a yellow halo, and lower leaves dropping, it is leaf spot.

start walking the field from about 4 to 6 weeks after the plants come up, through the flowering, pegging and pod-filling stretch — that is when the disease bites hardest and defoliation costs you pods. Check once a week, and always after a wet or dewy spell, when the fungus spreads fastest.

cross the field in a big "W" so you cover all four corners and the middle — not just the row by the path. Make about 10 stops along the W.

bend down and look at the lower, older leaves of a few plants. Turn a leaflet over to check the underside for the dusty grey-black mould of late leaf spot. Note how many leaves carry spots and whether lower leaves are already yellowing or dropping.

the trigger is the very first spots showing up on the lower leaves during pod-fill — as a plain count, if you find spots on even 1 in 10 of the plants you check, especially after a wet spell in a field that grew groundnut last season, treat that as your signal to act now. Don't wait for the whole field to spot up — by then the leaves are already falling.

Rotate — grow a cereal like maize, sorghum or millet on that land for at least two seasons before groundnut returns, so the fungus in the old trash starves out (rotation = not growing the same crop on the same land year after year). Never plant groundnut straight after groundnut.

after harvest, plough down or gather and burn the old groundnut haulms (the leafy vines) where the spores overwinter, and pull out any volunteer groundnut (self-sown plants from last year) that carries the fungus into the new crop.

plant certified clean seed and choose a tolerant/resistant variety bred to slow leaf spot where you can get one — this alone cuts a big share of the damage. Avoid over-thick stands: a crowded canopy stays wet longer and the fungus loves that, so keep to the recommended spacing so leaves dry off after dew and rain.

these fungal controls don't cure dead leaves — they protect the healthy ones still on the plant, so start at the very first spots, not when the plant is half bare. Prioritise the non-chemical steps above; on their own the tolerant variety plus good rotation and sanitation carry most of the load.

on a high-value crop, from the first spots, spray a fungicide (a plant-disease spray — the recommended one is a chlorothalonil-type contact/protectant that coats the leaf surface, or a triazole systemic that moves inside the leaf), repeating every 2 to 3 weeks through pod-fill. Safety: cover your skin, eyes, nose and mouth; spray in the cool of morning or evening with the wind at your back; keep it off food, drinking water, children and grazing animals; respect the pre-harvest interval (the number of days you must wait between the last spray and lifting the crop) on the label; and wash yourself and your clothes after. Resistance: don't spray the same product every time — rotate between different fungicide groups so the fungus doesn't learn to survive it.

re-check that you started early enough and covered the undersides of the lower leaves; a wet spell can outpace one spray, so tighten to the shorter end of the spray interval and rotate to a different fungicide group. When it's too late — the plant is already stripped bare from the base up → stop spending on sprays, because bare leaves won't come back this season. Harvest promptly before the weakened pegs rot and pods drop in the soil, keep the least-diseased vines for haulm, and plan next season now: a clean rotation, a tolerant variety and buried trash so you don't carry the disease forward.