Angular leaf spot on beans — symptoms and treatment

Severity: medium

What is Angular leaf spot

Angular leaf spot is a fungal disease of beans caused by Pseudocercospora griseola, among the most economically significant bean diseases worldwide, particularly under cool, humid conditions. It takes its name from the shape of its lesions: the fungus cannot cross the leaf veins, so each spot is boxed in by them and comes out angular rather than round. Severity is rated medium — it does not usually kill plants, but by stripping leaves during pod fill it cuts directly into yield.

Symptoms

Look for angular brown to grey lesions on the leaves, bounded by the veins — this vein-limited, straight-edged shape is the diagnostic feature and separates it from nearly every other bean leaf disease. A yellow halo often surrounds each lesion. As infection progresses the spots merge into large dead patches, the leaf yellows, and premature defoliation follows. Loss of canopy while pods are filling leaves the crop small and poorly finished. Lesions may also appear on pods and stems.

• Early signs: small angular grey-brown spots with straight, vein-limited edges, sometimes with a yellow halo
• Advanced signs: merging lesions, extensive leaf yellowing, premature defoliation, poorly filled pods
• Confusable with: bean rust, which makes raised reddish-brown pustules you can feel; angular leaf spot lesions are flat and vein-boxed

Causes and conditions

The fungus survives on infected crop debris and on the seed, which is how it reaches a clean plot in the first place. Spores are spread by rain splash, wind and by people and tools moving through a wet crop. Cool, humid conditions favour it, and infection needs the leaves to stay wet — prolonged dew, rain and overhead irrigation all extend leaf wetness and drive the disease. Dense stands that hold moisture in the canopy, and plots where beans follow beans, give the pathogen everything it needs.

Treatment

Crop rotation + clean seed — cultural

Timing: pre-planting. Rotate beans with non-host crops (cereals) for 2-3 years. Use certified disease-free seed. Avoid overhead irrigation. Rotation starves out the debris-borne inoculum while clean seed shuts the other entry route — together they do more than any spray.

Copper-based preventive spray — chemical

Timing: first symptoms, humid weather. Bordeaux mixture 0.5% or copper hydroxide at first sign of disease, repeated every 10-14 days during humid weather. Pre-harvest interval: 7 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Rotate beans with cereals or other non-host crops rather than replanting beans in the same ground
• Sow certified disease-free seed; saved seed from an affected crop carries the fungus forward
• Water at the base and avoid overhead irrigation, which wets leaves and splashes spores
• Space plants for airflow so the canopy dries quickly after rain or dew
• Remove and destroy crop debris after harvest, and stay out of the plot while foliage is wet

Frequently asked questions

Are the beans still edible? Yes. The disease damages leaves and reduces yield rather than making the crop unsafe — pods from affected plants are fine to eat. Observe the pre-harvest interval of 7 days if you have sprayed copper.

When should I treat? Copper is preventive, so apply at the first sign of disease rather than waiting for lesions to spread, and repeat through humid weather. Once a canopy is heavily spotted, spraying will not restore it.

Can I save seed from an infected crop? No. The fungus is carried on seed, so saving it plants the disease into next season's crop. Use certified disease-free seed instead and rotate the ground.