Cassava brown streak disease

on the middle-aged leaves, a feathery yellow pattern that follows the small side-veins — as if someone painted yellow feathers or blotches along the fine veins while the rest of the leaf stays green. The leaf keeps its normal shape (it is not twisted or crinkled), and it often shows on some leaves or branches and not others.

look for dark brown, dead streaks and dashes on the green bark of the stems and along the leaf stalks — like thin scratches of dried blood on the smooth green skin. These brown streaks are a strong sign it is brown streak and not something else.

cut a storage root in two. A sick root has dry, brown, corky rot in patches or rings running through the normally white flesh, and the skin outside may show pinched-in waists or dark pits. Remember — the roots can be badly rotten even when the plant looks almost healthy on top, so never judge by the leaves alone; always cut open a few roots.

turn over the soft young leaves at the shoot tips and look for tiny white flies smaller than a grain of rice sitting underneath; tap the tip and a small white cloud rises up. Whiteflies are how the disease moves from one plant to the next.

, the other big cassava virus: mosaic makes a patchy green-and-yellow mottle with twisted, crinkled, misshapen leaves and stunts the whole plant, while brown streak keeps the leaf shape normal, puts the yellow along the veins, and does its real harm inside the roots. Plain hunger from poor soil yellows the whole leaf or its edges evenly, not in a feathery vein pattern.

start once the crop is growing strongly (the development stage) and keep checking through mid- and late season, right up to harvest. Look more often in warm weather and in low-to-mid-altitude fields, where whiteflies breed fastest and the virus spreads quickest — and remember the root rot only worsens the longer a sick plant stands.

walk a large W across the field so you pass through all four corners and the middle, not just the row beside the path. Stop at about 10 spots along the W.

on a few plants look for the feathery yellow along the veins on the middle leaves, brown streaks on the green stems and leaf stalks, and — by turning over the top leaves — any build-up of whiteflies.

because a virus has no cure, act on the very first sign — even 1 sick plant in 20 showing vein-yellowing or brown stem streaks means pull that plant out now. A whitefly build-up (a white cloud rising off many shoot tips) is your signal to tighten sanitation and clear old stands.

dig and slice open a few roots on about 1 plant in 10, because the roots can be rotten with almost no sign on the leaves. If you find the brown corky rot starting, harvest that field promptly — every extra week in the ground lets the rot spread deeper into the roots.

the moment you spot an infected plant, uproot it and destroy it (burn or bury it) — do not leave it standing and never take cuttings from it. Roguing early, while only a few plants are sick, stops the whiteflies from carrying the virus to the rest of the field.

whiteflies bring the virus in from nearby old, infected cassava, so clear away old sick stands, do not plant new cassava right beside an old field, and avoid overlapping plantings that keep a green bridge for the flies. Harvest on time — leaving the crop standing extra-long only lets the root rot get worse.

there is no spray that cures this virus, and spraying insecticide to kill the whiteflies rarely stops the disease and is usually not worth the money or the risk for a smallholder. Put your effort and money where it works — resistant varieties, clean cuttings, and pulling out sick plants.

when many plants are sick and the roots are already rotting, harvest whatever is still usable now before the rot spreads deeper, and do not save a single cutting from this field. Next season start fresh — a resistant variety from clean cuttings, planted in a different plot away from the old stand, with the old infected plants cleared out so they can no longer feed the whiteflies and the virus.