Thanatephorus cucumeris (or Rhizoctonia solani)
Why is it a problem? This fungus can kill rice plants, or result in greatly reduced yields. Patches of dying rice can be very noticeable. However, the disease normally does not spread fast, and at present it is not a cause for concern.
Where and when is it a problem? At worse, the disease kills small patches of rice within a field. Extensive patches have never been seen.
Early symptoms of sheath blight include oval sheath spots at or just above the water level, often at the junction of the leaf and sheath. Early spots are pale green to off-white with a narrow purple-brown border. Lesions may join as the disease moves up the plant. Both leaves and sheaths are commonly attacked and killed as the disease grows upward. Sheath blight can infect the panicle under severe conditions, preventing all grain fill and sticking panicles together into a vertical spike. Infections often result in damaged circles or patches up to several feet in diameter in the field.
Confusion with other pests: Rice sheath is not easily confused with other rice diseases in Bhutan.
Lifecycle: Sheath blight is a modified single cycle disease. This means that the fungus infects a plant and does not produce a new generation on that plant which will attack other plants the same season. Sheath blight starts when a sclerotium or a piece of infected plant stem from a previous season floats on the surface of flood water and comes in contact with a rice stem. The fungus grows out onto the sheath at the flood line and begins growing upwards on the plant, occasionally penetrating the sheath and causing a lesion or spot. As the rice plant shifts to reproductive stage the disease becomes more aggressive causing larger lesions and growing upwards more rapidly. The fungus eventually reaches the upper leaves or panicles.
Dispersal: The fungus is soil borne and can be dispersed through water movement or through soil.
When can damage be expected? Infections are normally localised.
Hosts: Rice sheath blast has a very wide host range. In Bhutan it has been recorded from rice, cabbage, cauliflower, lettuce, maize, potato and tomato, although it has only been reported as a problem on rice.
Only small, localised patches of rice are normally affected by the disease. If management is needed this can be done without the use of chemicals.
This disease is rarely if ever severe. General paddy monitoring should be sufficient to detect outbreaks.
Effect of variety
- Plant as early as possible within the recommended planting period.
- Maintain proper spacing between and among plants.
- The disease plant stands should be roughed out.
- Do not over fertilize with nitrogen.
- Drain out the water from the field if the disease develops extensively from some parts of the field.
Fungicide application is not recommended. Please contact NPPC for advice should the disease become uncontrollably severe.
Image acknowledgements: Rice knowledge bank, IRRI