Sheath blight (rice)
Greenish-grey, snake-skin-patterned lesions on the leaf sheaths that climb the plant — worst in a dense, over-fertilised, closed canopy.
- Pathogen
- Fungus (Rhizoctonia solani)
- Spreads by
- Soil-borne sclerotia; debris; plant contact
- Favoured by
- Dense stands, excess N, warm humid canopy
- Main risk
- Blighted sheaths, lodging, poor grain fill
Overview
Sheath blight is one of the most important diseases of rice, especially in intensively grown, heavily-fertilised crops. It starts as oval, greenish-grey, water-soaked lesions on the leaf sheath near the water line, which enlarge into the classic "snake-skin" or banded pattern — grey-white centres with brown, irregular margins. Under humid conditions white-to-brown fungal threads and small brown sclerotia (resting bodies) form on the lesions.
From the lower sheaths the disease climbs up the plant to the upper sheaths and leaves. A severe attack blights the flag leaf, weakens the stem so the crop lodges, and leaves the panicles poorly filled. It is a soil-borne fungus that survives as sclerotia in the soil and on debris, and it is strongly favoured by close spacing, dense high-tillering crops, excess nitrogen and warm, humid weather with a closed canopy. Management focuses on opening up the crop (spacing, balanced nitrogen), destroying sclerotia-laden debris, and a well-timed fungicide from tillering to booting.
How to identify it
- Oval, greenish-grey, water-soaked lesions on the leaf sheaths near the water line, first on the lower plant.
- Lesions enlarging into a grey-white centred, brown-margined "snake-skin" or banded pattern.
- White-to-brown fungal threads and small brown sclerotia on the lesions in humid weather.
- Disease climbing to the upper sheaths and flag leaf, with lodging and poorly-filled panicles in a severe attack.
What causes it and when
- The soil-borne fungus Rhizoctonia solani, which survives as sclerotia in the soil and on crop debris and floats to infect sheaths at the water line.
- Strongly favoured by close spacing, dense high-tillering crops, excess nitrogen and a warm, humid, closed canopy.
- Builds from tillering onward and is worst from maximum tillering to booting/heading.
- Spreads plant-to-plant by contact in a dense stand once it establishes on the lower sheaths.
Cultural & organic control
- Avoid over-dense planting — use recommended spacing so the canopy is open and dries.
- Apply nitrogen in balanced, correctly-timed splits; avoid the heavy dose that drives dense, susceptible growth.
- Collect and destroy sclerotia-laden debris and stubble, and level the field for even water.
- Use bio-agents (e.g. Trichoderma/Pseudomonas) where recommended as part of the package.
Chemical control
- Spray a CIB&RC-registered fungicide (e.g. validamycin, hexaconazole or other recommended products) when lesions appear on the lower sheaths, before the disease climbs.
- Direct the spray at the base and lower canopy where the lesions are, not just the top leaves.
- Repeat as recommended through tillering to booting, and rotate groups; observe the pre-harvest interval.
- Confirm the current product, dose and timing for your area with your local KVK.
Prevent it coming back
- Make wider recommended spacing and balanced, split nitrogen the routine to keep an open, less-favourable canopy.
- Destroy sclerotia-bearing debris and stubble after harvest to run down the soil inoculum.
- Scout the lower sheaths from tillering and treat early, targeting the base of the crop.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
My rice sheaths have greenish-grey patches with a snake-skin pattern near the water. What is it?
Those oval greenish-grey, water-soaked lesions near the water line that grow into a grey-centred, brown-margined snake-skin pattern are sheath blight, a soil-borne fungal disease. It climbs from the lower sheaths up to the flag leaf and can lodge the crop and empty the grain. Open up the canopy (spacing, balanced nitrogen), destroy debris, and spray a registered fungicide at the base when lesions first appear.
Why is sheath blight worse in a dense, heavily-fertilised crop?
Because the fungus thrives in a warm, humid, closed canopy and spreads plant-to-plant by contact. Close spacing and heavy nitrogen produce exactly that — a dense, lush, high-tillering stand that stays wet and lets the disease climb from plant to plant. Using recommended spacing and balanced, split nitrogen keeps the canopy open and drier, which is one of the most effective ways to hold sheath blight down.
Where should I aim the fungicide spray for sheath blight?
At the base and lower canopy, where the lesions actually are — sheath blight starts on the lower sheaths near the water line and climbs up, so a spray that only wets the top leaves misses it. Spray when lesions first appear on the lower sheaths, before the disease reaches the flag leaf, direct the nozzle into the lower canopy, repeat as recommended, and rotate fungicide groups. Confirm the product and timing with your KVK.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.