Sheath blight of rice

Sheath blight is a destructive fungal disease caused by Corticium sasakii (a synonym of Rhizoctonia solani). This pathogen acts as a soil-borne fungus, surviving in the form of sclerotia or mycelium within the soil and crop debris between seasons. It is one of the most serious threats to global rice production, particularly in humid tropical and subtropical regions.

The disease primarily targets rice plants (Oryza sativa), although it possesses a broad host range. The pathogen is particularly devastating in high-input agriculture systems where rice is grown under continuous flooding, which facilitates the horizontal spread of fungal propagules across the water surface.

Symptoms typically manifest on the leaf sheaths near the water level as elliptical, greenish-gray lesions. As the infection progresses, these lesions enlarge, develop a tan or white center with a dark brown border, and may eventually merge, covering large parts of the leaf. Under high humidity, dense mycelial growth is clearly visible on the lesion surfaces.

Environmental conditions are the primary drivers of disease development. The fungus thrives in temperatures between 25°C and 30°C coupled with high humidity. Canopy microclimate is critical; dense planting and excessive nitrogen fertilization create a lush, humid environment that promotes the rapid transition of the fungus from initial infection sites to the flag leaves.

The economic impact of Corticium sasakii is significant, as it leads to reduced photosynthesis, poor grain filling, and increased lodging, often resulting in severe yield losses. Management strategies focus on an integrated approach:

• Deploying resistant or tolerant rice cultivars.
• Managing nitrogen applications to avoid excessive vegetative growth.
• Improving field sanitation to reduce sclerotial inoculum.
• Applying systemic fungicides at the early stages of disease appearance.
• Ensuring proper plant spacing to improve canopy airflow.