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Cowpea mosaic virus
Cowpea aphid-borne mosaic virus · Viral
Photo: BigB716$ (Wikimedia Commons) · CC0
Cowpea mosaic (cowpea aphid-borne mosaic virus) is a virus that lives inside the cowpea plant itself. It arrives two ways: hidden in infected seed, and carried plant-to-plant by aphids (small soft insects) that feed briefly on a sick plant, then on a healthy one. Once a plant is infected there is no spray that can cure it, so the whole fight is about clean seed, early removal, and keeping the aphid carriers down.
Why it matters
Cowpea mosaic can wipe out up to 70% of your harvest when infected seed and heavy aphid activity come together. It hits hardest early: young plants stay stunted, flower poorly, and set few or empty pods. And here is the hard part — the virus lives inside the whole plant (it is systemic), so there is no medicine or spray that cures a plant once it is sick. Your only wins are stopping it getting in and pulling out the sick plants before they infect the rest.
When to act
First mosaic/mottled seedlings appearing, or high aphid vector activity on a susceptible crop; rogue immediately.
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 Cowpea mosaic virus field guide
Identify it
What you see first
on the youngest leaves, a patchy mix of light-green and dark-green blotches — like a light-and-dark quilt — instead of one even green. This mottled pattern is called mosaic.
As it grows
those leaves pucker, blister and curl, looking crinkled and warped rather than flat and smooth. The plant stays short and bunched (the joints between leaves are squeezed close together) while healthy neighbours grow tall.
At flowering and podding
sick plants make few flowers and set few pods, or thin, poorly-filled pods. Whatever seed they do make can carry the virus into next season.
Don't confuse it with look-alikes
a hunger sign (like nitrogen or zinc shortage) gives an even, all-over yellowing or pale colour — not the light-and-dark patches of mosaic. Herbicide drift makes leaves narrow, strappy and cupped across a whole patch of the field at once, while mosaic shows up plant by plant, scattered. And check for aphids (tiny soft green-to-black insects clustered on the growing tips): where you see mosaic AND aphids together, mosaic is the likely cause.
Scout for it
When to look
start at the seedling stage and keep checking right through early growth (the initial and development stages) — this is when the crop is most easily infected and when early removal saves the most plants.
How to walk
walk a big "W" across the field so you cross all four corners and the middle, not just the edge by the path. Make about 10 stops along the W.
What to check at each stop
look at the youngest leaves at the growing tips of a handful of plants for the light-and-dark mosaic mottle and any puckering. At the same time, turn over the tips and check for aphids — the insects that carry the virus (the vector, the carrier that moves it from plant to plant).
The number that triggers action
you do NOT wait for many plants. Because the virus spreads from every sick plant, act as soon as you find the first mosaic/mottled seedling — even 1 plant — or when aphids are building up fast on the growing tips. First sick plant or a rush of aphids = rogue (pull and destroy) now, don't wait.
Also check your seed source
because this virus rides inside the seed, walk any field you plan to save seed from. If it shows mosaic, do not keep its seed.
Control it
Prevent
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start with clean seed (your strongest move):
plant certified virus-free or known-clean seed, and never save seed from a field that had mosaic. Most outbreaks start from infected seed, so clean seed is the single most powerful thing you can do.
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grow a resistant variety:
where a virus-resistant or tolerant cowpea variety is available, plant it. Ask your extension officer or seed dealer which resistant varieties are sold near you.
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clear the reservoirs:
pull out wild legume weeds in and around the field — they shelter both the virus and the aphids that carry it between seasons.
Act early
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rogue the sick plants:
the moment you spot a mosaic plant, uproot it whole and remove it from the field (burn or bury it, don't drop it on the path). This is called roguing. Do it while plants are young and few are infected — every sick plant left standing is a source aphids feed on and carry onward.
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protect the aphid-eaters:
ladybirds, hoverfly grubs and tiny wasps naturally eat aphids. Avoid blanket poison sprays that kill these helpers, because losing them lets aphid numbers explode.
Chemical is a weak last resort here — know its limit:
there is no spray that cures the virus. A targeted aphicide (an insecticide aimed only at aphids) can trim aphid numbers, but it gives only partial help: an aphid can pass the virus in the few seconds of a test-bite before the poison kills it (this is called non-persistent spread — passed on almost instantly, not held for long). So use it only to slow a heavy aphid build-up, never as your main plan.
If you do spray, spray safely
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, off drinking water, and away from children and bees; rotate to a different chemical group each time so aphids don't build resistance; respect the pre-harvest interval (the minimum wait printed on the label between spraying and picking pods); and wash yourself and your clothes after.
If it keeps spreading
rogue harder and more often, keep aphids down, and accept you cannot cure what is already infected. When most of the field is stunted and mosaic-mottled, cut your losses on this crop — do NOT save any seed from it — and reset for next season with certified clean seed and a resistant variety, which is the only reliable way to break the cycle.
Catch Cowpea mosaic virus before it costs you the harvest
This guide is the general answer. Divisi works out yours — the week to scout your own field for Cowpea mosaic virus, 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 Cowpea mosaic virus?
What you see first. on the youngest leaves, a patchy mix of light-green and dark-green blotches — like a light-and-dark quilt — instead of one even green. This mottled pattern is called mosaic.
When should I take action against Cowpea mosaic virus?
First mosaic/mottled seedlings appearing, or high aphid vector activity on a susceptible crop; rogue immediately.
How do I control Cowpea mosaic virus?
Prevent — start with clean seed (your strongest move):. plant certified virus-free or known-clean seed, and never save seed from a field that had mosaic. Most outbreaks start from infected seed, so clean seed is the single most powerful thing you can do.
Which crops are attacked by Cowpea mosaic virus?
Among the crops covered here: Cowpea.