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.
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.
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).
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.