1. Downy Mildew
Causal Organism
Sclerospora graminicola (Sacc.) Schroet. - an obligate oomycete pathogen
Economic Importance
Downy mildew is the most destructive disease of pearl millet worldwide, causing yield losses of 10-40% annually and up to 80% in severe epidemics. It significantly impacts grain quality and fodder value.
Symptoms
Systemic Infection: Plants infected at seedling stage show severe stunting, excessive tillering, and pale green to chlorotic leaves. Infected plants fail to produce panicles or produce malformed, leafy panicles.
Leaf Symptoms: Narrow, longitudinal chlorotic streaks appear on leaves, often covered with white downy growth (sporangia) on the lower surface, especially during early morning hours with high humidity.
Green Ear Disease: Panicles transform into leafy structures with partial or complete phyllody. Grains, if formed, are small, shriveled, and non-viable.
Disease Cycle
The pathogen survives as oospores in soil and infected seeds. Primary infection occurs through soilborne oospores infecting seedling roots or through seed transmission. Secondary spread occurs via airborne sporangia during humid conditions. Optimal conditions: 20-25°C temperature and high relative humidity (>85%).
Management Strategies
Host Resistance: Use resistant/tolerant varieties and hybrids (most effective strategy). Resistance breeding has been successful in developing varieties with durable resistance.
Seed Treatment: Treat seeds with metalaxyl (6g/kg seed) or mefenoxam to eliminate seed-borne inoculum.
Cultural Practices: Rogue out infected plants in early stages, practice crop rotation with non-host crops, avoid excessive irrigation, and maintain proper plant spacing for air circulation.
Chemical Control: Foliar spray of metalaxyl-mancozeb (0.25%) at 15-day intervals if disease appears, though resistant varieties are more economical.
2. Ergot
Causal Organism
Claviceps fusiformis Loveless - an ascomycete fungus
Economic Importance
Ergot causes significant grain yield and quality losses (15-40%) in pearl millet. The disease is particularly severe in hybrid seed production fields and in areas with high humidity during flowering. Ergot sclerotia are toxic to humans and livestock, contaminating grain.
Symptoms
Honeydew Stage: During flowering, infected florets exude sticky, sweet, yellowish honeydew containing conidia. This gives panicles a shiny, wet appearance and attracts insects.
Sclerotial Stage: Honeydew dries up and is replaced by hard, dark brown to black, horn-shaped sclerotia (ergot bodies) that replace grains. Sclerotia protrude from glumes and are 2-15mm long.
Panicle Appearance: Infected panicles show a mixture of healthy grains and ergot sclerotia, giving a characteristic spotted appearance.
Disease Cycle
Sclerotia survive in soil or mixed with seeds. Under favorable conditions (cool, moist weather), sclerotia germinate to produce stromata bearing perithecia with ascospores. Ascospores infect unfertilized flowers. Conidia in honeydew cause secondary spread via wind, rain splash, and insects. Male-sterile lines in hybrid production are highly susceptible due to prolonged stigma receptivity.
Management Strategies
Cultural Practices: Deep plowing to bury sclerotia (>5cm depth prevents germination), crop rotation, removal of ergot bodies from harvested grain, and use of clean, ergot-free seeds.
Pollen Management: In hybrid seed production, maintain optimal male:female ratio (2:6 or 2:8) and ensure synchronous flowering to minimize stigma exposure period.
Host Resistance: Plant varieties/hybrids with shorter stigma receptivity and rapid pollination. No complete resistance is available.
Chemical Control: Foliar application of copper oxychloride (0.3%) or mancozeb (0.2%) at boot leaf stage and repeat at 50% flowering. Carbendazim (0.1%) spray at flowering stage is also effective.
Biological Control: Application of Trichoderma viride or Pseudomonas fluorescens reduces sclerotial germination.
Integrated Disease Management
Effective control of pearl millet diseases requires an integrated approach combining resistant varieties, cultural practices, seed treatment, and judicious use of chemicals. Regular field monitoring, timely intervention, and maintaining crop hygiene are essential for minimizing disease losses and ensuring sustainable pearl millet production.