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Sheath blight in paddy (poramuripu)

Sheath blight, locally known as poramuripu, is a fungal disease of rice caused by the necrotrophic basidiomycete Rhizoctonia solani Kuhn (anastomosis group AG1-IA). It is among the most economically important diseases of rice worldwide and is the principal driver of mid-season fungicide use in dense-canopy paddy systems.

Identification and symptoms

Lesions begin as oval, water-soaked greenish-grey patches on leaf sheaths near the water line and expand upward into characteristic ragged 'snake-skin' bands with dark margins. Sclerotia - small brown survival structures - form on lesions and drop into the soil. Severe infection extends to leaf blades and may kill upper leaves, exposing the panicle to additional stress.

Host crops and life cycle

The fungus survives as sclerotia in soil and on plant debris. High nitrogen rates, dense planting, prolonged flooding and warm humid canopies are the principal triggers. The pathogen has a wide host range beyond rice, including maize and weeds, which sustains inoculum across rotations.

Damage and economic impact

By killing the leaf sheath at and above the water line, the disease reduces functional leaf area, undermines stem strength and predisposes plants to lodging. Heavy infection in dense, over-fertilised paddy regularly causes substantial yield loss and reduces head rice recovery.

Management

  • Cultural: split nitrogen rather than heavy basal N, widen spacing, drain periodically to break humidity.
  • Chemical: ICAR-IIRR multi-location trials identified thifluzamide 24% SC as the most effective single fungicide; carbendazim 0.1% foliar reduced disease by ~86%, with tebuconazole + trifloxystrobin (83%), hexaconazole (74%) and propiconazole (45%) all giving significant suppression. Combination formulations such as propiconazole + difenoconazole and captan + hexaconazole are also recommended. Treatment integrates with the boot-stage spray window (Paddy Panicle Stage Management).

See also: Paddy Blast Disease, Brown Spot Paddy, False Smut Paddy, Paddy Sheath Stem Rot, Paddy Panicle Stage Management.

References

  1. Rhizoctonia solani pathophysiology: status and prospects of sheath blight management. Frontiers in Plant Science.
  2. Evaluation of new fungicide formulations for sheath blight and brown spot. Indian Phytopathology (ICAR).