Rice sheath spot

The pathogen Rhizoctonia oryzae-sativae is the causative agent of the rice sheath spot disease. As a soil-borne fungus, it survives in the form of sclerotia in the soil or on infected plant debris. When conditions become suitable, the mycelium grows and infects the lower parts of the rice plants, progressively moving upwards, which can severely compromise the structural integrity of the crop.

Rice (Oryza sativa) is the primary host for this fungus. The disease is particularly destructive in regions where rice is grown under continuous flooding, as the water facilitates the movement of the pathogen's propagules from one plant to another. While predominantly affecting rice, the fungus can also impact the yield potential of wild grasses within the rice ecosystem.

Symptoms initially appear on the leaf sheaths near the water level. These manifestations present as small, elliptical spots with grayish-white centers and distinct dark brown margins. As the infection progresses, these lesions expand and coalesce, leading to severe chlorosis, necrosis of the sheaths, and potentially lodging of the rice plants due to stem weakening.

The development and spread of Rhizoctonia oryzae-sativae are heavily dependent on warm temperatures (25–30°C) and high humidity levels. Excessive nitrogen fertilization creates a lush, dense canopy that traps moisture and limits airflow, thereby creating an optimal microclimate for the fungus to multiply rapidly throughout the paddy fields.

The economic impact of the disease is significant, often resulting in reduced grain filling, lower overall yields, and poor grain quality. Integrated pest management is essential to mitigate these losses:

• Selection and planting of resistant or tolerant rice varieties.
• Implementation of precise nitrogen management to avoid excessive vegetative growth.
• Effective weed control to remove alternative hosts for the pathogen.
• Controlled water management, including periodic drainage of paddy fields if feasible.
• Application of fungicides, such as strobilurins or triazoles, upon the early detection of disease symptoms.