Bean

Cercospora Leaf Spot of Legumes

Cercospora canescens

Fungus

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In a Nutshell

  • Small pale-brown ring spots, surrounded by reddish brown margins on leaves.
  • Spots on branches and green pods.
  • Heavy defoliation.
  • Yield reduction.

Can also be found in

4 Crops
Bean
Black & Green Gram
Pigeon Pea & Red Gram
Soybean

Bean

Symptoms

Symptoms vary slightly depending on the strength of the pathogen and the plant type. Small water-soaked ring spots with a brown center and a yellow halo first appear on leaves 3-5 weeks after sowing the crop. At later stages of the disease, the spots become increasingly numerous and turn necrotic (dark brown) with reddish brown margins that appear slightly depressed. They can also develop on all other plant parts, especially on green pods. Under favorable environmental conditions, severe leaf spotting can lead to heavy defoliation at the time of flowering and pod formation. The fungus grows on the surface and inside the pods, damaging them completely, and often leading to 100% yield loss.

Recommendations

Organic Control

Hot water treatment of seeds is possible. The application of neem oil extracts are also effective in reducing the severity of the disease (higher pod and seed numbers, healthier pods, higher weight).

Chemical Control

Always consider an integrated approach with preventive measures along with biological treatments if available. If treatment with fungicides is needed, apply products containing mancozeb, chlorothalonil @1g/l or thiophenate methyl @ 1 ml twice at 10 days interval.

What caused it?

Leaf spot disease is caused by the fungus Cercospora canescens which infects both blackgram and greengram. The fungus is seed-borne and can survive for periods of more than 2 years on plant debris in the soil. Following the root system it can travel huge distances within the soil. It also thrives in alternative hosts or volunteer crops in the field. The transmission to lower plant parts occurs via splattering water and air. Elevated day and night temperatures, moist soils, high air humidity or heavy stormy rains are favorable conditions for the spread of the fungus.

Preventive Measures

  • Make sure that soil is well drained.
  • Use seeds from healthy plants or from certified disease-free sources.
  • Plant resistant or tolerant varieties.
  • Plant late to avoid damage of floral structures.
  • Intercrop with tall growing cereals and millets to avoid transmission between rows.
  • Leave sufficient space between plants to maintain good ventilation.
  • Mulch the plants to avoid transmission of fungi to lower leaves.
  • Ensure good field hygiene by removing all plant residues and burning them.
  • Clean contaminated equipment.
  • Avoid working in the field while plants are wet.
  • Crop rotation with non-host crops is recommended.

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