The disease was first of all reported from China in 1637 and then in Japan in 1704. Presently, the disease is prevalent in almost all the rice growing countries of the world but is especially serious in humid areas. This has been considered as the most important disease affecting the crop and causing losses up to 70%.
Symptoms :The fungus attacks all the aerial parts of plant at all the growth stages. On leaves, it appears in the form of small bluish flecks (about 1-3 mm in dia.). On older leaves, they remain circular but on younger leaves they enlarge up to several centimeters in length and about one centimeter wide. By this time, central portion becomes pale green or dull grayish green and water soaked in appearance, while outer rim is dark brown. In older spots center becomes grey or almost straw coloured. Similar spots are also formed on leaf sheath.
Brown to black spots or rings is formed on the rachis of maturing inflorescence. Ears may also show similar spots. The most characteristic appear on culm. The neck becomes shriveled and covered with grey fluffy mycelium. If the infection takes place much earlier before grain formation, the latter are not filled and panicle remains erect. But if infection takes place after some grains have been formed, panicle hangs down. Due to necrosis of neck tissue, ear tends to break and fall off causing maximum damage.
Casual Organism : Pyricularia oryzae
Disease Cycle: The fungus survives on collateral hosts such as sugarcane and some other graminaceous plants. It also survives in the form of chlamydospores and sclerotia. Survival in the soil is extinct and seed-borne inoculum fails to initiate disease due to high soil temperature. The conidia are released from the conidiophores by dew or rain and are disseminated by wind currents mostly up to 1-2 m from the source. They germinate in 3-4 hours and penetrate the host cells directly. The incubation period is about 4 days and the next crop of conidia is ready in 6-7 days, which cause secondary infection.
Epidemiology : The disease severity depends on variety, age, host nutrition and meteorological factors such as temperature and moisture. Rice is susceptible to blast at three distinct growth stages i.e., seedling, rapid tillering (15-30 days after transplantation) and ear or neck emergence stage. Silicone content is directly proportional with resistance. P. oryzae is considered a night pathogen and all the vital processes of disease cycle (spore release, germination, infection and spore production) require free water, which is provided by night time dew. Night temperature of 20ºC alternating with day temperature of 30°C with 14 hours of light is the most favourable for infection by P. oryzae. Subsequent progress of the disease is favoured best at 15°C.
Control :
- Precautionary measures: Field sanitation, destruction of alternate hosts, early planting and seed treatment with proper seed dressing fungicides such as Benlate (0.25%).
- Foliar sprays with Benlate, Bavistin (0.1%).
