Rice blast is one of the most economically destructive diseases of rice farms around the world, including Nepal. The disease is caused by the fungus Pyricularia oryzae, with its sexual stage known as Magnaporthe grisea. Rice blast can attack a plant at any stage of growth, affecting leaves, nodes, collars, panicles, and even rice seeds. In Nepal, it was first reported in 1966 A.D., and since then, it has remained a major concern in both rainfed and irrigated rice systems.
Understanding the full biology of this disease helps students, researchers, and farmers make better disease predictions and implement timely management strategies.
Understanding the Symptoms of Rice Blast
Rice blast progresses in multiple forms, and each form affects a different part of the plant. The most recognizable stage is leaf blast, but severe outbreaks often show symptoms on nodes, collars, and panicles.
Leaf Blast
The earliest symptoms appear as small, water-soaked, bluish-green specks on the leaf surface. These enlarge into characteristic diamond-shaped lesions with a grey center and dark brown margins. As lesions merge, leaves dry prematurely and plant vigor declines.
Node Blast
Node blast affects the internodal region, producing irregular black lesions that completely encircle the node. When the infection becomes severe, the node breaks and all tissues above the infected portion die. This form is destructive because it directly disrupts nutrient and water flow.
Collar Blast
The collar is the junction between the leaf blade and the leaf sheath. Blast infection at this point causes necrosis, and the entire leaf may dry. Collar infections often go unnoticed but significantly reduce photosynthesis.
Neck or Panicle Blast
Neck blast is the most damaging stage of the disease. When the pathogen attacks the peduncle during panicle emergence, the neck turns brownish-black and becomes constricted. Early infection prevents grain filling, leaving the panicle erect, resembling “dead heart.” Late infection allows partial grain filling but leads to poor quality grains. Lesions may also appear on panicle branches and spikelets.
Seed Symptoms
Infected seeds may show brown spots, blotches, or even the classic diamond-shaped lesions.
Etiology: What Causes Rice Blast?
The causal organism produces gray conidiophores that bear terminal, pear-shaped, typically two-septate conidia. These conidia spread through air and water and serve as the main source of secondary infection.
The fungus also produces toxins such as pyricularin and alpha-picolinic acid, which contribute to symptom development.
Favorable Conditions for Rice Blast Development
Rice blast outbreaks are heavily influenced by environmental factors. The disease thrives under the following conditions:
- High nitrogen fertilization
- Intermittent rainfall or drizzling
- Cloudy and humid weather
- Relative humidity between 93% and 99%
- Low night temperature ranging from 15°C to 20°C
- Long durations of dew and slow wind movement
Presence of alternate hosts such as Digitaria marginata, Panicum repens, and Dinebra retroflexa, and collateral hosts like sugarcane also help the fungus survive in the environment.
Survival and Spread of the Pathogen
The pathogen survives as mycelium and conidia on soil, rice debris, infected seed, and alternate hosts. During the cropping season, spores disperse through wind and rain splash. Field-to-field spread can occur quickly under continuous cloudy weather.
Management Strategies for Rice Blast
Effective management requires an integrated approach combining host resistance, cultural practices, seed treatments, biological agents, and judicious use of fungicides.
Resistant Varieties
Growing resistant varieties is the most sustainable strategy. Recommended varieties include Barkhe 2014, Barkhe 2024, and Super 004 for leaf blast, while Barkhe 1005, Barkhe 3004, and Barkhe 1006 are effective against neck blast.
Seed Treatment
Treating seeds before sowing reduces primary infection. Common treatments include:
- Seed treatment with Pseudomonas fluorescens (Biocare-B) at 10 g per liter of water for 30 minutes.
- Seed treatment with Bavistin or carbendazim at 2–3 g per kg of seed.
- Root dip treatment of seedlings in Pseudomonas fluorescens (4 g per liter of water for 30 minutes) before transplanting.
Chemical Control
Fungicide applications are useful during high-risk periods such as tillering and neck emergence. Recommended sprays include hexaconazole 5% EC at 2 ml per liter or kasugamycin 3% SL at 1.5 g per liter of water. Hinosan at 1.5 ml per liter is also effective, especially when applied during seed bed stage, tillering, and panicle initiation.
Systemic fungicides such as triazoles and strobilurins are highly effective when used strategically, particularly at heading.
Conclusion: Building a Strong Defense Against Rice Blast
Rice blast continues to be a major constraint to rice production, but understanding its symptoms, favorable conditions, and management options allows farmers and students to make informed decisions. When resistant varieties, biological agents, proper seed treatment, and timely fungicide applications are used together, the disease can be effectively managed even in high-risk environments.
To deepen your understanding of rice diseases, explore our detailed plant pathology resources on Pedigogy.com, designed for agriculture and veterinary science students preparing for academic exams and field practice.
If you want to master plant pathology and improve your exam performance, explore more research-based content at Pedigogy.com, Nepal’s growing platform for agriculture and veterinary resources. You can find the full course here: Visit our Plant Pathology section here.
Updated on November 20, 2025
