Blast of Rice: Causes, Symptoms, Yield Loss and Control

Rice (Oryza sativa) is the staple food crop for more than half of the world’s population, particularly in Asia and sub-Saharan Africa. Among the many diseases that affect rice production, rice blast disease is one of the most destructive. Caused by the fungal pathogen Magnaporthe oryzae, it can attack the rice plant at any growth stage and affect almost all aerial parts of the plant, causing significant yield losses.

This disease is found in almost all rice-growing regions of the world and is a major constraint to rice production and food security. It is estimated that rice blast destroys rice enough to feed more than 60 million people each year. In countries like Bangladesh, India, China, the Philippines, and several African nations, rice blast poses a recurring threat to farmers, especially under conducive weather conditions.
The Pathogen: Magnaporthe oryzae

This fungus was formerly known as Pyricularia oryzae, and it's a hemibiotrophic pathogen, meaning it has both biotrophic (living tissue) and necrotrophic (dead tissue) stages during infection.

Life Cycle
· Spores (conidia) are spread by wind, rain splash, and insects.
· Spores germinate in the presence of moisture and infect plant tissues through appressoria (specialized infection structures).
· The fungus penetrates the host cell wall and colonizes the plant, forming lesions.
· It completes its life cycle in 5-7 days, allowing for rapid epidemic development under favorable conditions.

Favorable Conditions
Several environmental and agronomic factors promote the development and severity of rice blast:

Climatic Conditions
· Temperature: Optimal range between 20°C to 30°C.
· Relative Humidity: Over 90% for at least 10-12 hours favors infection.
· Rainfall: Frequent rains and overcast weather support spore spread.
· Dew: Morning dew plays a crucial role in spore germination.

Crop Management Factors
· Dense planting restricts airflow and increases humidity.
· Excessive nitrogen fertilization creates lush green canopies, favorable for fungal growth.
· Lack of crop rotation and monoculture increase pathogen build-up.
· Use of susceptible varieties increases the risk of epidemic outbreaks.

Symptoms
Rice blast can infect leaf, collar, neck, node, panicle, and even seeds. The severity depends on the plant growth stage and environmental conditions.

Leaf Blast
· Initial symptoms appear as small, grayish-green, water-soaked lesions.
· Lesions enlarge into spindle-shaped spots with gray centers and brown margins.
· Severe infection may lead to drying and premature death of leaves.

Collar Blast
· Affects the junction between leaf blade and sheath.
· Causes wilting and drying of leaves.
· Significant because it indicates systemic infection.

Node Blast
· Appears as dark brown to black lesions on the nodes.
· Infected nodes may break or cause lodging of plants.

Neck Blast (Panicle Blast)
· One of the most economically damaging stages.
· Infection of the neck (base of panicle) causes panicles to turn white and dry out.
· Often results in empty grains or grain sterility.

Economic Impact
· Estimated yield losses range between 10% and 30%, but under epidemic conditions, losses can reach up to 80% or total crop failure.
· It leads to increased cost of production due to fungicide application and seed replacement.
· Indirect effects include food insecurity, especially in developing countries dependent on rice.

Diagnosis and Detection
Field Diagnosis
· Observation of typical spindle-shaped lesions on leaves.
· White, dried panicles in the case of neck blast.

Laboratory Diagnosis
· Microscopic observation of fungal spores and conidiophores.
· Culture on Potato Dextrose Agar (PDA) medium to observe colony morphology.
· Molecular techniques like PCR and ELISA for precise identification.

Management of Rice Blast
A successful blast management strategy requires an Integrated Disease Management (IDM) approach, combining cultural, biological, chemical, and resistant varieties.

Use of Resistant Varieties
· One of the most economical and environmentally friendly control methods.
· Resistance may break down due to pathogen variability, so gene pyramiding (multiple resistance genes) is preferred.

Examples of blast-tolerant varieties:

· IR64, BRRI dhan50, BRRI dhan83, Samba Mahsuri, GSR IR1-5-S14-S3-Y1-Y2

Cultural Practices
· Proper spacing between plants to ensure good air flow.
· Balanced fertilizer use—especially limiting excess nitrogen.
· Avoid late planting, which may coincide with favorable disease conditions.
· Drain fields periodically to reduce humidity.
· Field sanitation—removal and destruction of crop residues.

Chemical Control (Fungicides)

Fungicides are effective but must be used judiciously.
Fungicide
Active Ingredient
Dose
Time of Application
Tricyclazole
Tricyclazole 75% WP
0.6–1 g/liter
Leaf/neck blast stage
Azoxystrobin
Azoxystrobin 250 SC
1 ml/liter

Before panicle emergence
Propiconazole
Propiconazole 25 EC
1 ml/liter
At heading
Combination products
Tricyclazole + Propiconazole
As per label
Better efficacy

Best Practices:
· Use preventive sprays when weather is conducive.
· Follow label recommendations and avoid overuse.
· Rotate fungicides to prevent resistance development.

Biological Control
· Use of biocontrol agents like:
o Trichoderma harzianum
o Pseudomonas fluorescens
o Bacillus subtilis

These agents compete with or inhibit the pathogen in the soil or on plant surfaces.
Seed Treatment
· Hot water treatment: 53–54°C for 10 minutes.
· Use of fungicide-treated seed before sowing.
· Helps reduce primary inoculum.
Climate-Smart Agriculture
· Adoption of disease forecasting tools to predict blast outbreaks.
· Use of mobile apps and SMS alerts for farmer awareness.
· Crop models that integrate weather data to suggest management actions.
Integrated Disease Management (IDM)

An effective IDM package for blast includes:
1. Use of resistant varieties
2. Certified, treated seed
3. Proper spacing and planting time
4. Balanced fertilizer application
5. Timely fungicide sprays
6. Crop residue management
7. Adoption of weather-based advisory systems
Future Research
· Genetic improvement: More focus on gene pyramiding and durable resistance.
· Pathogen surveillance: Understanding M. oryzae populations to guide breeding.
· Precision agriculture: Drone-based monitoring for early detection.
· Biofungicide development: Eco-friendly alternatives to chemicals.

Conclusion

Rice blast is a formidable enemy to rice productivity worldwide. However, it is manageable with an integrated approach involving resistant varieties, good agronomic practices, and timely chemical and biological interventions. Education and awareness among farmers, along with support from governments and institutions, are crucial for effective disease control.

With the challenges of climate change and rising food demand, rice blast management will remain a priority in achieving global food security.

 
Dr. Md. Mahfuz Alam is Principal Scientific
Officer at Crops Division, Bangladesh
Agricultural Research Council (BARC), Dhaka.