Divisi
Cercospora leaf blight
Cercospora kikuchii · Fungal
Photo: Francisco Sautua · CC BY-SA 4.0
Cercospora leaf blight is a fungal disease of soybean caused by Cercospora kikuchii. The fungus survives on infected seed and on old crop residue, then infects during warm, humid weather (about 25–32°C) with wet leaves, attacking mainly the upper, sun-exposed leaves and the seed. It shows up late in the season as a bronze-purple blight of the top canopy and as 'purple seed stain' on the beans — cutting seed quality and, through early loss of the top leaves, yield.
Francisco Sautua · CC BY-SA 4.0 
Francisco Sautua · CC BY-SA 4.0
Why it matters
Cercospora leaf blight hits your soybean from two sides. In the field it browns and drops the top leaves during pod-fill — the very leaves that should be filling your pods — and a bad attack can cost you up to about a third (30%) of your yield. On the beans themselves it leaves ugly pink-to-purple stains that buyers reject and that make saved seed rot in the ground and carry the disease into next year. It is hard to beat because the fungus hides on your seed and last year's stubble, and once the blight is spreading in warm, wet weather a spray only slows it — it cannot bring back a leaf that has already browned and fallen.
When to act
Scout from pod-fill onward; consider a fungicide when purple-bronze upper-canopy blighting begins during warm, humid weather in a crop with yield potential.
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 cercospora leaf blight field guide
- Identify it
- Scout for it
- Control it
13 field-guide sections in the app
The full guide to cercospora leaf blight — every symptom to check, how and when to walk your field looking for it, and the controls that actually work, ranked. Built into your crop plan, so you are told to look before it costs you a harvest.
Crops it affects
Frequently asked questions
How do I identify cercospora leaf blight?
The first thing you see: the leaves at the very top of the plant — the ones in full sun — lose their fresh green and take on a dull reddish-purple to bronze cast, as if lightly scorched. This colour comes on late, during pod-fill, and shows up in patches across the field, not evenly everywhere.
When should I take action against cercospora leaf blight?
Scout from pod-fill onward; consider a fungicide when purple-bronze upper-canopy blighting begins during warm, humid weather in a crop with yield potential.
How do I control cercospora leaf blight?
Prevent it — start with clean seed: the fungus rides into your field on the seed and on last year's soybean stubble. Plant clean, treated, disease-free seed, never seed from a purple-stained lot, and rotate — give the land a season or two in maize or another non-soybean crop and bury or clear the old soybean trash so the fungus has nothing left to survive on.
Which crops does cercospora leaf blight affect?
Among the crops covered here: Soybean.
Catch it before it costs you a harvest
Divisi builds scouting into your crop plan — it tells you when to walk your field, what to look for at that stage, and what to do when you find it. Free for farmers, works offline.
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