It is a common disease in the rice growing countries of South Eastern Asia and also in Japan. It cause losses at three distinct phases. Firstly, poor germination of infected seeds. Secondly, leaf infection during seedling stage. Thirdly, shriveling and poor setting of seeds due to general weakening of the plants.
The fungus attacks all parts of the plant in all stages of development. In the seedling stage, the tip of the cotyledon, become dark brown and infection spreads to the hypocotyls …
It is a common disease in the rice growing countries of South Eastern Asia and also in Japan. It cause losses at three distinct phases. Firstly, poor germination of infected seeds. Secondly, leaf infection during seedling stage. Thirdly, shriveling and poor setting of seeds due to general weakening of the plants.
Symptoms :The fungus attacks all parts of the plant in all stages of development. In the seedling stage, the tip of the cotyledon, become dark brown and infection spreads to the hypocotyls and the seedling dies.
On the leaves the disease appears in the form of small minute dots to circular, oval spots (1-14 x 0.5-3 mm). The smaller spots are dark or purplish brown, while the larger spots are dark brown at the edge but towards the center, they may be pale yellow, dirty white, brown or grey, with a yellow halo around the spots. In severe infection, the entire leaves may become brown and dry out.
When heads appear they are distorted in various ways. The diseased spots on the heads first appear on or near the lowest points of the rachis and then the infection spreads all over the glumes, etc. In severe cases grain formation fails.
The stems are also affected, which turn yellow, pale brown and finally dark brown. Surface of stem shows a velvety appearance due to the dense growth of conidiophores
Casual Organism :Drechslera oryzae Perfect Stage:Cochliobolus miyabeanus
Disease Cycle:The fungus is soil or seed borne but it does not remain viable. Mostly, the primary infection is though air-borne spores from external sources (perennial grass hosts and early sown rice crops) or paddy straw heaps kept for cattle feed. The conidia after germination penetrate the host through motor cells of stomata.
Epidemiology :For dispersal of conidia for primary and secondary infection, minimum temperature of 27-28.5ºC, R.H. 90-99%, a wind velocity of 4.0-8.8 Km/hour and low to moderate rainfall (0.4-14.4 mm) are the most conducive. Susceptibility increases with age. Topmost leaves are the most resistant and second leaves are the most susceptible. Potash application decreases while excessive N application increases the susceptibility. Optimum temperature for conidial formation is 21-26ºC and R.H. 92.5% while for germination of conidia, these are 25-30ºC and 92%, respectively.
Control :
- Burn or destroy stubbles and possible alternate hosts like Setaria sp., Leersia sp., Echinochloa sp.
- Passage of water from badly infected field to healthy field should be checked.
- Seed dressing may also be helpful.
- Hot water treatment at 55ºC for 10 minutes.
