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

1. Introduction & Importance

Downy mildew is a devastating disease of maize (Zea mays) causing 10-100% yield losses worldwide, particularly severe in tropical and subtropical regions. The disease affects all growth stages and can lead to complete crop failure under favorable conditions. Economic losses exceed $500 million annually in major Asian maize-producing regions.

2. Symptoms

Image Source - Eagri

Systemic Infection

Chlorotic Streaking: Narrow, parallel yellowish-white stripes along leaf veins from base to tip
Stunting: Severe plant dwarfing with shortened internodes and bunchy appearance
Downy Growth: White to grayish cottony fungal growth on lower leaf surfaces, visible in early morning
Leaf Malformation: Narrow, stiff, upright leaves with excessive tillering
Phyllody: Transformation of floral structures into leaf-like organs
Sterility: Complete or partial failure of tassel and ear development, resulting in no grain production

Local Infection

Pale green to yellow lesions on mature leaves, rectangular and bounded by veins
White sporulation on lower leaf surface
Lesions turn brown and necrotic with age

Critical Period: Seedlings at 2-4 leaf stage are most susceptible to systemic infection. Early infections result in zero grain yield from infected plants.

3. Etiology (Causal Organisms)

Major Pathogens

Peronosclerospora sorghi - Sorghum downy mildew (Asia, Australia, Africa)
Peronosclerospora maydis - Java downy mildew (Indonesia, Philippines, Southeast Asia)
Peronosclerospora philippinensis - Philippine downy mildew (Philippines, Southeast Asia)
Sclerophthora macrospora - Crazy top disease (worldwide in waterlogged areas)
Sclerospora graminicola - Green ear disease (India, Southeast Asia)
Peronosclerospora heteropogoni - Queensland downy mildew (Australia, Southeast Asia)

Taxonomic Classification

Kingdom: Chromista | Phylum: Oomycota | Class: Oomycetes | Order: Peronosporales | Family: Peronosporaceae

Key Characteristics

Mycelium: Coenocytic (aseptate), hyaline, intercellular with haustoria penetrating plant cells
Sporangiophores: Branched structures emerging through stomata, bearing sporangia
Sporangia: Asexual spores dispersed by wind and rain, germinate to produce zoospores or germ tubes
Oospores: Thick-walled sexual resting spores surviving 7-10 years in soil, primary inoculum source
Zoospores: Motile spores with two flagella, require free water for infection

4. Disease Cycle

Primary Inoculum

Oospores: Survive in soil and plant debris for 7-10 years, germinate under favorable conditions (moisture >70%, temperature 20-30°C)
Systemically Infected Volunteer Plants: Serve as sources of secondary inoculum
Alternative Hosts: Sorghum, pearl millet, wild grasses harbor the pathogen

Infection Process

Seedling Infection: Oospores germinate, producing sporangia/zoospores that infect emerging seedlings through coleoptile, young shoots, or stomata
Systemic Colonization: Mycelium grows through vascular tissues, colonizing entire plant in 7-10 days
Sporulation: After 10-15 days, sporangiophores emerge through stomata under high humidity (>90%), producing white downy growth
Secondary Spread: Sporangia dispersed by wind (long-distance), rain splash, and irrigation water to healthy plants
Oospore Formation: Sexual reproduction produces thick-walled oospores in infected tissues, released into soil upon plant decomposition

Favorable Conditions

Temperature: 20-30°C (optimal 22-25°C)
Relative Humidity: >90%
Leaf Wetness: 4-6 hours continuous
Frequent rainfall during early crop growth
High soil moisture (>70%)
Young plants (2-4 leaf stage) most vulnerable

5. Management Strategies

5.1 Cultural Management

Crop Rotation: 2-3 years with non-host crops (legumes, vegetables) to reduce oospore populations
Sanitation: Remove and destroy infected plants immediately; burn crop debris; deep plow (20-25 cm) to bury oospores
Field Selection: Avoid fields with disease history; improve drainage in waterlogged areas
Planting Date: Early planting to escape peak disease periods
Certified Seed: Use disease-free seeds from reputable sources
Plant Spacing: 60-75 cm between rows, 20-25 cm within rows for air circulation
Weed Management: Control grass weeds (wild sorghum, Johnson grass) that serve as alternative hosts
Water Management: Use drip/furrow irrigation; avoid overhead sprinklers; irrigate early in day
Balanced Nutrition: Adequate potassium and silicon enhance resistance; avoid excessive nitrogen

5.2 Chemical Management

Seed Treatment (Most Effective)

Metalaxyl/Mefenoxam: 4-6 g a.i./kg seed (25-30 days protection)
Metalaxyl-M: 2-3 g a.i./kg seed (enhanced efficacy)
Azoxystrobin: 25-30 g a.i./kg seed
Combinations: Metalaxyl + Carbendazim or Thiram for broader protection

Foliar Sprays

Metalaxyl: 0.2% (2 g/L) at 7-10 day intervals
Cymoxanil + Mancozeb: 0.25% combination
Dimethomorph: 0.1% (1 g/L)
Phosphorous acid: 0.3% as metalaxyl alternative

Application Guidelines: Apply at first symptom appearance; repeat at 10-14 day intervals; rotate fungicide chemistries to prevent resistance; focus on seedling to early vegetative stages.

5.3 Host Resistance

Resistant Cultivars: Most economical and sustainable strategy; select varieties resistant to locally prevalent pathogens
Qualitative Resistance: Major genes (R genes) providing complete race-specific resistance
Quantitative Resistance: Multiple genes providing durable, partial, race-non-specific resistance
Modern Breeding: Marker-assisted selection, genomic selection, gene pyramiding for enhanced resistance

5.4 Biological Control

Trichoderma harzianum/viride: 4-6 g/kg seed or 2.5 kg/ha soil application
Pseudomonas fluorescens: 10 g/kg seed treatment
Bacillus subtilis/amyloliquefaciens: Seed treatment or foliar spray
Organic amendments (neem cake, compost) enhance beneficial microflora

5.5 Integrated Disease Management (IDM)

Pre-Planting

Select resistant cultivars and well-drained fields
Deep plow to bury residue and oospores
Apply organic amendments and biocontrol agents
Treat seeds with systemic fungicides

Growing Season

Scout fields weekly for early detection
Remove and destroy infected plants immediately
Apply foliar fungicides when disease appears
Manage irrigation and control weeds
Maintain balanced nutrition

Post-Harvest

Remove and burn infected crop residues
Deep plow to bury debris
Plan crop rotation
Prevent volunteer maize growth

5.6 Regulatory Measures

Strict quarantine on seed imports from infected areas
Seed certification programs
Movement restrictions from infected regions
Disease surveillance and mandatory reporting
Buffer zones around infected fields

6. Differential Diagnosis

Diseases with Similar Symptoms

Maize Streak Virus: Broken discontinuous streaks, no downy growth, leafhopper-transmitted
Bacterial Leaf Stripe: Water-soaked lesions with bacterial ooze, no downy growth
Nitrogen Deficiency: V-shaped yellowing from leaf tip, affects older leaves first
Stewart's Wilt: Irregular wavy lesions, bacterial streaming, flea beetle transmission
Northern Corn Leaf Blight: Large cigar-shaped gray lesions, no systemic symptoms

Diagnostic Procedures

Field: Examine lower leaf surfaces early morning for white downy growth; check for parallel chlorotic stripes and systemic symptoms
Laboratory: Microscopic examination of sporangiophores; wet chamber incubation; tissue staining; PCR-based detection; ELISA

7. Regional Considerations

Asia-Pacific: P. maydis and P. philippinensis predominant; monsoon season critical; metalaxyl resistance reported
India/South Asia: P. sorghi and S. graminicola; sorghum as alternative host; kharif season most affected
Africa: P. sorghi spreading; limited access to inputs; focus on cultural practices
Australia: P. heteropogoni and P. maydis; strict quarantine and surveillance
Waterlogged Areas: S. macrospora causes crazy top; drainage improvement essential

8. Research & Future Directions

Emerging Technologies

Molecular Breeding: Marker-assisted selection, genomic selection, gene pyramiding, CRISPR/Cas9
Precision Agriculture: Remote sensing for early detection, AI-based image recognition, disease forecasting apps
Alternative Controls: RNA interference, resistance inducers, microbiome engineering, nano-fungicides
Climate Change: Adapting management to changing disease patterns and developing climate-resilient varieties

9. Case Studies

India: Mandatory seed treatment reduced disease from 40-60% to below 5%
Philippines: Resistant varieties reduced yield losses from 30-50% to less than 10%
Australia: Quarantine measures successfully prevented establishment of exotic species
Thailand: Integrated management reduced fungicide use by 40% while maintaining control

10. Key Takeaways for Farmers

Early Detection is Critical: Scout weekly from seedling stage; look for chlorotic streaking and downy growth
Prevention is Best: Use seed treatment (metalaxyl 4-6 g/kg) + resistant varieties as first defense
Sanitation Reduces Disease: Remove infected plants immediately; burn residues; rotate crops 2-3 years
Manage Environment: Improve drainage, optimize spacing, avoid overhead irrigation
Integrated Approach: Combine resistant varieties + seed treatment + cultural practices + timely fungicides
Act Early: Apply foliar fungicides at first symptoms; waiting reduces effectiveness
Protect Seedlings: Systemic infection at seedling stage = zero yield from infected plants
Use Certified Seed: Never save seed from infected fields

11. Economic Impact

Yield Losses: 10-100% depending on infection timing and pressure. Susceptible varieties without protection commonly lose 30-80% yield. With integrated management, losses can be reduced to below 5%.

Cost-Benefit: Resistant varieties + seed treatment provide best cost-effectiveness. Foliar fungicides reserved for high-value crops or severe pressure situations.

12. Conclusion

Downy mildew remains a major constraint to maize production worldwide, particularly in tropical and subtropical regions. Successful management requires understanding disease biology, early symptom recognition, and implementing integrated strategies. The foundation of effective management lies in using resistant cultivars combined with cultural practices that reduce inoculum and disease-favorable conditions.

Most effective strategy: Plant resistant varieties + seed treatment with metalaxyl provides excellent protection at minimal cost. Chemical control through seed treatment protects critical seedling stage, while foliar applications may be necessary under high pressure. Over-reliance on fungicides risks resistance development, making integrated approaches essential for sustainable maize production.