Why is it a problem? Severe epidemics of this fungus have resulted in large scale losses, including total loss by some farmers. However, it may not be as severe after widespread adoption of more resistant rice varieties following the epidemics during the 1990s.
Where and when is it a problem? It occurs in all rice-growing areas, but is particularly prevalent in moist and shady areas. Epidemics occur under large scale monoculture of susceptible varieties and during prevalence of favourable environmental conditions. The last widespread epidemic occurred during 1995-96 rice seasons.
Blast can affect leaf blades, stem nodes, panicle and grains. Leaf blast attacks especially between the seedling stage and tillering stage. Early leaf lesions are rounded, white to grey-green with darker green borders. Older lesions become spindle-shaped with grey centre and brown margin. At heading stage blast again increases attacking nodes and panicles. Infected nodes start rotting and eventually the culm can break at the node. Nodal blast can also result in barren panicles (white heads). In early Neck Blast, lesions at the neck appear greyish-green and later turn black. Infected necks can rot and break. Early Neck Blast leads to chaffy grains while late Neck Blast gives partly filled grains with kernels that are chalky, brittle and often useless.
Confusion with other pests: Rice blast is not easily confused with other rice diseases.
Life cycle: The fungus infects the plant by the spore germinating on the plant surface and then exerting a haustoria into the plant cells. A minimum of 8 hours of moisture is needed for infection to occur. At optimum temperatures and high relative humidity new lesions can appear in 4-5 days but in colder parts of Bhutan it takes around ten days. The fungus produces new spores in the spring that re-infects rice. The pathogen survives as mycelia and conidia on diseased rice straw, seed stubble and possibly on weed hosts.
Dispersal: The sexual stage is rare in nature so the blast disease is spread almost exclusively by conidia (asexual spores) from the anamorphic state. Spores are carried by wind and splashing rain. Movement can be over long distances.
When can damage be expected? Multiple rice cropping systems with susceptible varieties can increase the chance of blast outbreaks, especially in moist micro-climates.
Hosts: Blast can also attack crop such as millets and other grasses such as Echinochloa species.
Rice blast outbreaks are rare but severe. Constant monitoring and rapid response is therefore needed. Preventative measures are also needed in areas already known to be susceptible to rice blast.
Regular scouting for Leaf, Nodal and Neck Blast, especially in blast prone areas, is essential in order to come to a timely decision on the need for spraying. Give attention to areas near water bodies or forest or shady areas. Although outbreaks are rare, they be devastating when they do occur, causing up to 100% loss. Monitoring is therefore important, and should start at the nursery stage of rice and continue through until tillering and flowering stages.
Effect of variety
Rice cultivars differ considerably in their resistance to rice blast. Resistance levels are relatively well understood for the cultivars available in Bhutan. Resistant varieties can be used in areas known to be susceptible to rice blast.
Specific management is only needed on farms where there is a known history of problems.
- Cultivate rice varieties resistant to blast such as Chumroo, No. 11 and IR64, especially in blast-prone areas such as near rivers, forest or in the shade. Susceptible varieties such as the traditional Janam, Dumja and Themja can be grown in open and wide valleys where accumulation of moisture and dew is low.
- Use disease-free seed.
- Never use seed from a blast-infected field since the fungus can be transmitted through the seed.
- Raise seedlings on a wet bed, as dry nurseries generally favor blast.
- Plant as early as possible within the recommended planting period.
- For leaf blast, re-flood if the field has been drained. Maintain flood at 4 -6 inches to ensure soil is covered.
- Do not over fertilize with manure or fertilizer as too much nitrogen will increase susceptibility of paddy to blast.
- Avoid high density planting as blast incidence increases with increasing plant density.
- Farmers in the same area should plant at the same time.
- Do not leave the field dry after transplanting as paddy is more resistant to blast when grown under proper water management.
- Remove or burn infected straw and stubbles in the field.
- Treat pre-germinated rice seed with fungicide for high blast disease risk areas.
- Soak seeds in water for 24 hours.
- Treat seed with a solution of tricyclazole at the rate of 3g/kg of seed.
- Allow seed to germinate for 24-48 hours before sowing.
- Leaf blast can be controlled effectively if detected at an early stage. Spraying is not normally effective when plants are already infected by Nodal Blast or Neck/Panicle Blast.
- The need for follow-on sprays for leaf blast control depends on monitoring of disease development.
- Fungicide spray against foliar blast early in the season will not help against collar or neck blast. Therefore, monitor fields for development of collar or neck blast at later stages, and separate sprays are required to manage these late stage blast infections.
- Fungicide spray:
- Spray Tricyclazole 75 WP (100gm per 100 litre of water).
- Follow up with a second spray 14 days after the first spray.
Image acknowledgements: Rice knowledge bank, IRRI