Several leaf spot diseases affect this crop but the leaf spots caused by Cercospora species are important which under high humidity conditions cause considerable yield losses.
Symptoms
Cercospora melongenae:
- The symptoms of the disease appear as circular to irregular leaf spots, which are usually large and brown to grayish brown in colour.
- Later spots coalesce together and affected leaves fall down prematurely and sometimes fruits may rot.
Cercospora egenulae:
- Early symptoms start as minute water-soaked specks, which gradually enlarge to become circular to angular leaf spots of 2-6 mm diameter.
- Early infection noticed on the lower leaves which gradually spreads upwards with high humidity and rainfall.
- These spots often coalesce to form bigger spots having a papery pale yellow necrotic region in the center.
- The necrotic region in the center disintegrates bearing a shot hole.
- Infected leaves turn yellow, droop and fall prematurely resulting in quick defoliation of the entire plant.
Cercospora solanigena:
- Leaf spots amphigenous circular to irregular later coalescing, necrotic, dark brown in center and light brown in margin, 0.5-2 mm wide.
Pathogen(s): The disease is caused by several species of Cercospora like C. melongenae Wells., C. solanigena Bartiya, Dubey and Singh and C. egenulae (Syd.) Chupp and Doidge.
C. melongenae:
- Conidiophores are simple, dark septate and 30-60 x 4-6 µm in size.
- Conidia are clavate, tapering to a blunt distal tip, sub-hyaline, 3-12 septate and 38- 119 x 4-8 µm in size.
C. egenulae :
- Fruiting both epi- and hypophyllous, stromata distinct dark brown, globose to irregular, fascicles dense to olivacious brown, paler and narrower towards the tip, a septate, not branched, notgeniculate, undulate with rounded apex and 2.5-4.5 x 3 - 31µm in diameter.
- Conidia hyaline to subhyaline, shorter ones distinctly cylindric and very long ones obclavate or almost acicular, straight or nearly so, 1-5 septate, base subtruncate to rounded, tip obtuse, 3-6 x 20-85 µm in diameter.
C. solanigena:
- Mycelium internal, hyphae branched, septate, dark olivaceous. Stromata well developed, pseudoparenchymatous, dark, olivaceous, 10-30 µm in diameter.
- Conidiophores arising in fascicles (1-4) from stromata, macronematous to fasciculate, erect, straight to flexuous, unbranched, smooth, 1-6 transversely septate, geniculate, light olivaceous to dark brown, 16-100 x 3-5 µm in size.
- Conidiogenous cells integrated, terminal, polyblastic, cicatrized, scars conspicuously thickened paler in colour.
- Conidia acropleurogenous, holoblastic, dry, solitary, unbranched, 1-5 transversely septate, smooth walled, hyaline, acicular, straight to curved, base truncate, apex sub acute to sub obtuse with thickened hilum.
Disease cycle and epidemiology:
- The fungus survives from one season to another in infected plant debris in soil and also in infected seeds wherever fruit rot occurs.
- Warm days and cool nights are ideal for the infection.
- The disease is favoured by high humidity and heavy persistent dews.
- Water droplets must be present for spore germination and germ-tube penetration.
- Moist wind, irrigation water and insects help in local transmission of the pathogen.
Management:
- Cultural practices like destruction of crop debris, crop rotation, use of disease free seeds and wider plant spacing should be followed to reduce the primary inoculum of the pathogen in field.
- With the initiation of the disease spray the crop with zineb (0.25%), carbendazim (0.1%) or thiophanate methyl (0.1%) and repeat at 10 to 14 days interval.
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