Sheath Blight in Paddy: How to Identify and Control it
Sheath blight doesn't announce itself the way some paddy diseases do; what makes it particularly worth understanding is that it’s almost a byproduct of modern, intensive paddy farming. Dense planting and high nitrogen use are exactly what this disease thrives on. The irony is these practices are practised for high yield, and this disease is capable of reducing crop yield by up to 50% in difficult, unmanaged situations.
This guide covers what causes it, how to identify it and how to control it afterwards.
What is Sheath Blight?
Sheath blight is a soil-borne fungal disease, affecting cereal crops, primarily rice. It is caused by a fungus called Rhizoctonia solani,
It is called “sheath blight” because the fungus attacks leaf sheaths. The fungus survives as hard‚ black or brown structures called sclerotia in the soil or on crop debris‚ which floats when the field is flooded․ They also infect leaf sheaths near the waterline
Favourable conditions that trigger Blight Outbreak
Well, we discussed what causes sheath blight in paddy, the fungus called Rhizoctonia solani, but some favourable conditions trigger blight outbreaks:
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High Humidity(85% – 95%): high humidity favours infection and lesion expansion.
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Warm Temperature (28°C – 32°C): fungal infection peaks at warm and humid temperatures.
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Excess nitrogen usage: Over nitrogen application produces lush foliage and soft cell walls, which gives fungus perfect comfort room to enter.
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Dense Planting: close seedling spacing (less than 15 cm apart) blocks wind movement and sunlight penetration.
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Standing Water: continuous deep flooding allows fungal sclerotia to float directly against plant stems.
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Semi-Dwarf Varieties: high-yielding, short plants have their leaf sheaths closer to standing water, making it easier for the fungus to reach upward.
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Dew-Heavy Weather: consecutive cloudy, windless days with long-lasting dew keep paddy wet longer.
How to Identify Sheath Blight
Early Symptoms
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Oval or irregular greenish-grey spots appear on the leaf sheath.
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Its size(spots) is about 0.5-0.3 cm long (often 1–2 cm).
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Lesions might look slightly sunken.
Advanced Symptoms
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Lesions enlarge and merge together, forming bigger patches on sheath.
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The centre of lesions becomes straw-white and dry, with a dark reddish-brown border around them.
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On mature lesions, there are tiny, hard, spherical bodies resembling mustard seeds.
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Under humid conditions, there is white, cottony fungal growth on infected sheaths.
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Infection spreads from leaf sheath to leaf blades, upper leaves, panicles, and tillers, causing drying of leaves.
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Presence of several lesions of leaf sheath can cause death of the entire leaf. In severe cases, all the leaves of the plant can become damaged and die.
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Older plants and five- to six-week-old leaf sheaths are highly susceptible.
Distinguishing Sheath Blight from other Look-Alike Diseases -
| Disease | Primary Location | Key Visual Feature | Physical Signs |
|---|---|---|---|
| Sheath Blight | Lower sheath spreading upward | Bleached centres with reddish-brown borders; "snake-skin" pattern | Brown, mustard-seed-like sclerotia on stem surface |
| Sheath Rot | Flag leaf sheath (top of plant) | Oblong spots with grey centre; encloses rotted, unemerged panicles | White powdery mould inside the flag sheath |
| Stem Rot | Base of stem at water line | Dark black lesions at water line; hollowed stem collapse | Tiny, pinhead-sized black dots inside the stem cavity |
| Rice Blast | Leaves, nodes, panicle neck | Spindle/diamond-shaped lesions with pointed, tapered ends | Fuzzy, greyish fungal growth under high humidity |
How to Control Sheath Blight
Management is directed towards reducing the soil sclerotia population, maintaining crop canopy climate, and the use of specific chemical or biological treatments during the crop's growing season.
1. Soil Preparation & Sanitation
(Pre-Planting Stage)
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Deep-plough fields during summer to bring buried fungal sclerotia to the surface, where heat and birds can eat them.
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Remove alternate weed hosts (Echinochloa, Cyperus, and wild grasses) along field borders.
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Mix Trichoderma viride or Pseudomonas fluorescens (2.5 kg/ha with 50 kg farmyard manure) and apply it into moist soil during land preparation.
2. Planting Density & Seed Treatment
(Nursery and Transplanting Stage)
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Treat seed with Pseudomonas fluorescens 10 g/kg or Carbendazim 50% WP 2 g/kg before sowing․
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Plant crops at a spacing of 20 cm × 15 cm or 20 cm × 20 cm․ Do not plant too densely to avoid excess humidity within the canopy․
3. Water and Nutrient Management
(Vegetative to Booting Stage)
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Apply nitrogenous fertilizer in 3 to 4 split applications. A properly split basal and top-dressing schedule keeps plant tissue from becoming excessively soft and susceptible.
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Always ensure that recommended potassium dosages are applied.
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Practice alternate wetting and Drying (AWD) or drain the field for 2–3 days during late tillering.
4. Targeted Spray Application
(At first lesion appearance)
Spray fungicide towards the lower leaf sheath and water line.
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Validamycin 3% L - 2.7 mL per litre of water.
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Thifluzamide 24% SC - 150 mL per litre of water.
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Azoxystrobin 18.2% + Difenoconazole 11.4% SC - 1.0 mL per litre of water.
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Hexaconazole 5% EC - 2.0 mL per litre of water.
Bottom line…
Balance your nitrogen, give your plants room to breathe, and check the waterline before the disease reaches the flag leaf.