Anthracnose on beans — symptoms and treatment

Severity: high

What is Anthracnose

Bean anthracnose is caused by Colletotrichum lindemuthianum. It causes dark sunken lesions on pods, stems, and leaves. Its high severity rating comes from the fact that it attacks the pods themselves rather than just the foliage — the harvest is the target, not a side casualty. It is also seed-borne, so an infected crop plants next year's outbreak unless the cycle is deliberately broken.

Symptoms

The most recognisable symptom is on the pods: dark, sunken lesions with sharply defined edges, and in humid weather pink spore masses well up in the centre of each one. That pink ooze is the confirming sign — no other common bean pod problem produces it. On the undersides of leaves, the veins turn reddish-brown, giving the leaf a distinctive dark-veined look while the surrounding tissue still appears healthy. Stems and petioles develop elongated dark cankers, and badly infected seedlings may collapse. Seed from infected pods is often discoloured.

• Early signs: reddish-brown veins on leaf undersides; small dark specks on young pods
• Advanced signs: dark sunken pod lesions with pink spore masses, stem cankers, seedling collapse, discoloured seed
• Confusable with: bacterial blight, which gives water-soaked spots with yellow borders and never the sunken lesion with pink spores

Causes and conditions

The fungus is carried on infected seed and survives on crop debris left in the field. Spores are sticky and spread mainly by rain splash, and equally by anything brushing through a wet crop — hands, clothing, tools and machinery all carry them plant to plant. This is why working in wet fields is so damaging: a single pass through a dew-soaked plot can distribute the pathogen down every row. Cool, wet weather and extended leaf wetness favour infection, and dense plantings that hold moisture in the canopy prolong exactly the conditions it needs.

Treatment

Certified seed and rotation — cultural

Timing: pre-planting. Use certified disease-free seed. Rotate with non-leguminous crops for 2-3 years. Remove crop debris. This is where the disease is actually beaten — clean seed closes the main entry route, and rotation plus debris removal clears what survives in the ground.

Mancozeb — chemical

Timing: during wet weather. Apply contact fungicide preventively during wet weather. Do not work in wet fields to avoid spreading. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Sow certified disease-free seed and never save seed from an affected crop
• Rotate with non-leguminous crops; do not follow beans with beans
• Stay out of the plot while the foliage is wet — you spread the spores yourself
• Space plants and support them for airflow so the canopy dries quickly
• Water at the base, not overhead, and remove crop debris promptly after harvest

Frequently asked questions

Can I eat beans from infected plants? Discard pods with sunken lesions. Sound pods from the same plot are edible, but pick them dry and inspect closely. If you sprayed mancozeb, observe the pre-harvest interval of 14 days.

Why does it keep coming back? Because it is seed-borne. Saving seed from an affected crop replants the fungus directly, and crop debris left in the ground carries it over. Clean seed plus rotation and debris removal is the only durable fix.

Is it contagious to other plants? It spreads readily to other beans. Rotate with non-leguminous crops — cereals and other unrelated plants are not hosts and break the cycle.