Disease of Groundnut | Diseases of Field & Horticultural Crops and their Management-I Notes

1. Symptoms

Early Leaf Spot (Cercospora arachidicola)

image source - pestoscope

Early leaf spot manifests as circular spots on both leaf surfaces, measuring 1-10 mm in diameter. Initially, lesions appear as small brown spots on the upper leaf surface with yellow halos. As the disease progresses, spots become dark brown to black with well-defined margins. The lower leaf surface shows lighter brown coloration. Heavy infections cause premature defoliation, reducing photosynthetic capacity and yield.

Late Leaf Spot (Cercosporidium personatum)

image source - barmac

Late leaf spot produces darker, smaller circular to irregular spots without distinct yellow halos. Lesions are typically black on the upper leaf surface and darker brown on the lower surface compared to early leaf spot. The spots are generally smaller (1-6 mm) and more numerous. Coalescence of lesions leads to extensive blighting and severe defoliation in susceptible cultivars.

2. Etiology

Early Leaf Spot

Causal organism: Cercospora arachidicola Hori

The pathogen produces hyaline, septate mycelium and conidiophores that emerge through stomata in fascicles. Conidia are cylindrical, hyaline to pale olivaceous, straight or slightly curved, measuring 40-110 × 3-5 μm with 4-12 septa. The fungus thrives in warm, humid conditions with optimal temperature of 25-30°C.

Late Leaf Spot

Causal organism: Cercosporidium personatum (Berk. & M.A. Curtis) Deighton

This fungus produces dark brown to black conidiophores emerging through stomata. Conidia are clavate to cylindrical, golden brown to dark brown, measuring 30-75 × 4-8 μm with 3-8 septa. The organism prefers slightly higher temperatures (28-32°C) and thrives under extended leaf wetness periods.

3. Disease Cycle

Both pathogens survive primarily on infected crop residues in soil and on volunteer plants. Conidia serve as primary inoculum and are disseminated by wind, rain splash, and insects. Under favorable conditions (high humidity >85%, leaf wetness duration >10 hours, and temperatures 25-32°C), conidia germinate on leaf surfaces within 6-8 hours.

Infection occurs through stomatal penetration, with early leaf spot showing symptoms within 10-14 days and late leaf spot requiring 14-21 days of incubation. The fungi establish intercellular colonization, causing cellular necrosis and chlorosis. Secondary sporulation occurs on lesions, producing abundant conidia for polycyclic infections throughout the growing season.

Disease severity intensifies during flowering and pod development stages when canopy closure creates a humid microenvironment. Multiple infection cycles occur within a single season, particularly during monsoon periods in tropical regions. Late leaf spot typically becomes predominant during late season due to its preference for warmer conditions.

4. Management

Cultural Practices

• Crop rotation: Implement 2-3 year rotation with non-host crops to reduce inoculum buildup
• Residue management: Deep plowing or burning of infected crop debris to eliminate primary inoculum sources
• Planting density: Maintain optimal spacing to improve air circulation and reduce leaf wetness duration
• Sanitation: Remove and destroy volunteer plants and infected plant material
• Timing: Adjust sowing dates to avoid peak disease periods during excessive rainfall

Host Resistance

Cultivate resistant or moderately resistant varieties such as ICGV 91114, GPBD 4, Kadiri-6, and TMV-2. Deploy varieties with quantitative resistance genes for sustainable long-term management. Resistance breeding programs focus on introgression of wild Arachis species resistance into cultivated backgrounds.

Chemical Control

• Fungicide applications: Apply protective fungicides (mancozeb 75% WP @ 2.5 g/L, chlorothalonil 75% WP @ 2 g/L) at 15-day intervals starting from 30-35 days after sowing
• Systemic fungicides: Use tebuconazole (250 EC @ 0.5 mL/L) or azoxystrobin + difenoconazole alternately with contact fungicides to prevent resistance development
• Timing: Initiate sprays before disease onset or at first symptom appearance, with 4-6 applications per season depending on disease pressure

Biological Control

Apply Trichoderma viride or Pseudomonas fluorescens as seed treatment (10 g/kg seed) and soil application (2.5 kg/ha mixed with farmyard manure). Bacillus subtilis formulations can reduce disease incidence by 30-40% when integrated with reduced fungicide schedules.

Integrated Disease Management

Combine resistant varieties, cultural practices, biocontrol agents, and judicious fungicide use based on disease monitoring. Weather-based forecasting systems help optimize fungicide application timing. This integrated approach minimizes chemical inputs while maintaining economic yield levels and reducing environmental impact.

Economic Impact: Leaf spot diseases can cause yield losses of 50-70% in susceptible cultivars under favorable conditions, making them the most economically important foliar diseases of groundnut globally.