Diagnose and Respond to Sheath Blight in Rice

Identify sheath blight from oval greenish-grey lesions on lower sheaths spreading upward in dense canopies, and control it with lower plant density, balanced N, and fungicide at booting.

Sheath blight, caused by Rhizoctonia solani, is the second most damaging rice disease after blast. It thrives in dense high-N canopies with high humidity at the base. It survives between seasons as sclerotia in the soil.

When rice is most vulnerable

Most damage from panicle initiation through grain filling. High plant density, high N, and continuous flooding are the classic risk factors.

How sheath blight shows in satellite data

• NDVI decline in the densest, highest-N zones during booting to grain filling - the opposite pattern from N deficiency (which shows lower NDVI on the poor zones).
• Lodging patches may appear late season as stems weaken.
• Poor separation from other basal diseases in imagery alone; confirm on ground.

Sheath blight is a canopy-base disease. It hides under the top leaves and is easily missed by both satellites and quick walk-throughs. Scout by parting the canopy.

Field symptoms

• Oval, greenish-grey water-soaked lesions on lower leaf sheaths near the waterline
• Lesions enlarge and turn straw-colored with dark-brown margins
• Disease climbs the plant to upper sheaths and leaves in dense canopies
• White fluffy mycelium and small brown sclerotia (like mustard seeds) on infected tissue
• Weakened stems, lodging, and poor grain fill

Reduce the damage by

• Reduce plant density and avoid excessive N - the two most effective preventive controls.
• Balance N with K - adequate K reduces severity.
• Grow moderately resistant cultivars and avoid highly susceptible high-yielding hybrids on chronic sheath-blight fields.
• Deep plowing and stubble incorporation to bury sclerotia.
• Crop rotation with non-host crops where practical.
• Fungicide (validamycin, hexaconazole, or azoxystrobin) at maximum tillering to booting - only if scouting confirms an active outbreak.

Set an alert for early warning

Turn the diagnosis into a trigger. Configure alerts on the high-density high-N zones - the disease consistently starts there.

Recommended settings for sheath blight on rice:

• NDVI zone alert - flag high-NDVI zones that suddenly drop by 0.08 or more during booting to grain filling.
• Weather alert - sustained relative humidity above 95% for 3+ days during booting.

Related

• Rice Diagnosis
• Potassium - K deficiency worsens sheath blight.
• Set up automated crop alerts