📈 Diseases of Rice — Blast, Brown Spot, Sheath Blight

Rice (Oryza sativa) is the staple food for over half the world's population. It is affected by more than 70 diseases, of which blast, brown spot, and sheath blight are among the most destructive.

Rice Blast

Causal Organism

Magnaporthe oryzae (anamorph: Pyricularia oryzae)

Symptoms

• Leaf blast: Diamond-shaped or spindle-shaped spots with grey centre and dark brown margins on leaves
• Node blast: Black lesions at nodes causing breakage
• Neck blast: Infection at the panicle base causing chaffy grains — the most damaging form
• Finger blast: Infection of panicle branches

Disease Cycle

The fungus survives on infected crop debris and collateral hosts. Conidia are wind-dispersed and germinate under high humidity. Appressoria penetrate the cuticle directly.

Favorable Conditions

• Temperature: 20-25 degrees C
• High relative humidity (>90%)
• Prolonged leaf wetness, excessive nitrogen fertilization

Management

• Grow resistant varieties (e.g., Tetep, CO 39)
• Avoid excessive nitrogen application
• Seed treatment with Tricyclazole 75 WP (0.4 g/kg seed)
• Foliar spray of Isoprothiolane or Kasugamycin

Brown Spot

Causal Organism

Bipolaris oryzae (syn. Helminthosporium oryzae)

Symptoms

• Oval to circular brown spots with yellow halo on leaves
• Dark brown lesions on glumes, causing discoloured grains
• This disease caused the infamous Bengal Famine of 1943

Favorable Conditions

• Nutrient-deficient soils (especially low nitrogen and potassium)
• Temperature: 25-30 degrees C with high humidity

Management

• Apply balanced fertilization, especially potassium and zinc
• Seed treatment with Mancozeb or Carbendazim
• Foliar spray of Propiconazole 25 EC

Sheath Blight

Causal Organism

Rhizoctonia solani (Teleomorph: Thanatephorus cucumeris)

Symptoms

• Irregular, oval, greenish-grey lesions on leaf sheaths near the water line
• Lesions enlarge and coalesce, causing banding pattern
• Severe infection leads to lodging and chaffy grains

Disease Cycle

The pathogen survives as sclerotia in soil and on plant debris. Sclerotia float on water and infect sheaths at the water line.

Management

• Avoid dense planting; maintain proper spacing
• Remove and destroy sclerotia from field water
• Apply Validamycin 3L or Hexaconazole 5 EC
• Biocontrol with Trichoderma viride

Summary Cheat Sheet