Rice blast affects all the above-ground parts of the plant: leaf, collar, node, neck, parts of panicle, and sometimes leaf sheath. Leaves exhibit yellow to light green chlorotic, eye-shaped lesions with pointed ends. The borders of these lesions are necrotic and the center is grey to white. The size of the lesions depends on the age of the plant, the variety and the time of infection. As the lesions grow, the leaves dry out gradually. If the junctions of leaves and sheaths are infected, collar rot can appear, and the leaves above the junctions die. Nodes can also be affected. This leads to brown nodes and the breaking of stems, occasionally causing the complete death of seedling or young plants. At later growth stages, a severe leaf blast infection reduces leaf area and consequently for grain fill and yield.
Symptoms of rice blast are caused by the fungus Magnaporthe grisea, which is one of the most destructive diseases of rice. It can also infect other agriculturally important cereals such as wheat, rye, barley, and pearl millet. The fungus can survive on the straw after harvest and thus be carried over to the next season. Plants usually become less susceptible to the pathogen as they mature. The disease is favored by cool temperatures, frequent rainfalls, and low soil moisture. A prolonged period of leaf moisture is also required for infection. In upland rice, sites prone to dew formation (large day to night temperature differences) are at risk. Finally, plants sown in soils with high nitrogen or low silicon levels are more likely to develop the disease.
To this day, no effective biological control of the disease is commercially available. Experiments are ongoing to test the viability of products based on Streptomyces or Pseudomonas bacteria on the fungus and the incidence/spread of the disease.
Always consider an integrated approach with preventive measures together with biological treatments if available. Seed treatment with thiram is effective against the disease. Fungicides containing azoxystrobin, or active ingredients of the family of triazoles or strobilurins can also be sprayed at nursery, tillering and panicle emergence stages to control rice blast. One or two fungicide applications at heading can be effective in controlling the disease.