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Yam nematode
Scutellonema bradys · Nematode
Photo: Airypedia, via Wikimedia Commons · CC BY-SA 4.0
Yam nematode (Scutellonema bradys) is a tiny, thread-like soil worm that burrows into the outer flesh of yam tubers and causes dry rot. Too small to see with the eye, it is carried into each new season inside infected planting setts and in the soil, and it keeps multiplying in store — leaving a crumbly brown layer under the skin that ruins tubers for eating, market and seed.
Flixtey, via Wikimedia Commons · CC BY-SA 4.0
Why it matters
Yam nematode causes dry rot — a crumbly, light-brown decay just under the skin of your tubers. A bad attack can spoil up to 60% of your harvest (more than half), turning yams you meant to eat, sell or plant back into waste. It is hard to beat because the worm is too small to see, it hides deep inside the tuber and in the soil, and it rides into next season inside the setts you replant — so once it is in your seed yams and your land, it comes back year after year.
When to act
Dry-rot cracking and brown flesh found in cut-tests of tubers at or after harvest, or a history of dry rot in the plot; select clean setts before replanting.
This guidance is general, compiled from published agronomic references including FAO and CABI. Thresholds and timing shift with your crop, your variety and the season.
In much of the region a spray is simply called mushonga — medicine. But a chemical group you cannot name is one you cannot rotate, and a product you did not write down has no pre-harvest interval you can observe. Mushonga, and other farming words →
The full Yam nematode field guide
Identify it
The damage (what you see first)
cut a tuber across and look just under the skin — a thin light-brown, dry, crumbly layer, like dry corky sawdust, is the earliest sign. Over time it darkens, deepens, and the skin cracks, flakes and peels.
It gets worse in store, not better
in a warm store the brown layer spreads deeper into the tuber, and other rots, moulds and bacteria often move in behind it.
The worm itself you cannot see
it is thinner than a hair and lives inside the tuber flesh. You will never spot it by eye — you know it only by the dry-rot pattern it leaves.
Don't confuse it
with soft rot (a wet, slimy, bad-smelling mush — that is bacteria, not this) or with yam beetle damage (clean, gouged holes on the outside). Nematode dry rot is dry, crumbly and corky, sits just under the skin, and does not smell foul.
Scout for it
When to check
from mid-season onward, and above all at harvest — this is the tuber-filling stretch when the nematode has had time to build up inside the tubers. Also cut-test any tubers still sitting in store.
How to check (the cut-test)
slice across a tuber — a thin slice off one end is enough — and look under the skin for the tell-tale light-brown, crumbly dry-rot layer.
Where to sample
take your test tubers from different spots, not one corner. Walk a "W" across the plot and lift one plant at each turn of the W, cutting 10 tubers in all.
Act on this number
if even 1 tuber in 10 shows the brown crumbly layer — or your plot simply had dry rot last season — treat the whole plot as infested and select only clean setts before you replant.
Control it
Plant clean setts (most important)
cut your seed setts only from healthy tubers with no brown layer under the skin. Never replant a tuber that showed dry rot — recycling infected setts is the single fastest way the worm spreads onto fresh land.
Rotate and rest the land
do not grow yam on the same plot year after year. Move yam to a fresh plot and grow cereals or legumes (maize, beans) for 2–3 years, or leave badly infested land fallow, so the worms left in the soil starve out.
Treat setts and store cool and dry
where you can, dip setts in hot water before planting to kill worms hiding inside (take care — hot water scalds; let the setts cool before you handle them). Then store only sound, unblemished tubers in a cool, dry, airy place so any survivors cannot multiply in the heat.
Feed the soil to fight back
work in organic matter — well-rotted compost, poultry manure or neem cake before planting, or use a bionematicide (a natural, worm-killing product) where you can get one. These build up soil life that preys on the nematodes and lowers their numbers. When applying any such product, cover your skin, nose and eyes, work with the wind behind you, keep it off food, water, children and bees, and wash well afterwards.
If dry rot keeps coming back
understand that there is no safe spray that will rescue an already-infected crop — the only real fix is clean seed plus rest. Take a longer break from yam (3 or more years) on that land, source fresh, clean seed yams from a trusted, dry-rot-free supplier instead of saving your own, and cut your losses on badly rotted tubers: eat or sell the sound parts quickly and never keep rotten ones for seed.
Catch Yam nematode before it costs you the harvest
This guide is the general answer. Divisi works out yours — the week to scout your own field for Yam nematode, and what to do the day you find it.
Crops it affects
Plan the crops it attacks
All planting calendars, country by country
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Frequently asked questions
How do I identify Yam nematode?
The damage (what you see first). cut a tuber across and look just under the skin — a thin light-brown, dry, crumbly layer, like dry corky sawdust, is the earliest sign. Over time it darkens, deepens, and the skin cracks, flakes and peels.
When should I take action against Yam nematode?
Dry-rot cracking and brown flesh found in cut-tests of tubers at or after harvest, or a history of dry rot in the plot; select clean setts before replanting.
How do I control Yam nematode?
Plant clean setts (most important). cut your seed setts only from healthy tubers with no brown layer under the skin. Never replant a tuber that showed dry rot — recycling infected setts is the single fastest way the worm spreads onto fresh land.
Which crops are attacked by Yam nematode?
Among the crops covered here: Yam.