The name of the rice blast disease is derived from the visual symptoms it causes to rice plants. When conditions are favorable, plant tissues become undergo necrosis and heavily infected leaves will appear burnt from a distance.
Rice blast can cause lesions on leaves, leaf collars, nods, stems, panicles and grains. In about 90% of the time, panicles and grains exhibit visual symptoms, while leaf sheets and roots do not show symptoms.
Initially, purple spots are formed on young leaves. With time, the spots become spindle-shaped. Large lesions usually develop a diamond shape with grayish center and brown margin. Infection on the necks can be fatal; it can lead to a condition called neck blast in which the entire panicle might fall over.
Rice blast disease is the most devastating plant diseases in rice. The damages it can inflict and potential yield losses are so great that rice blast disease has been ranked among the most important plant diseases in the world. Rice blast is found throughout most of the rice growing countries.
The disease favors temperatures around 24 degrees Celsius and long periods of high moisture conditions, which are commonly found throughout flooded rice fields.
The fungus produces conidia (spores of an asexual reproduction). Under windy conditions, conidia become airborne, eventually descending on other rice plants where it will germinate and cause new infections. When warm and wet weather is present, the formation of new conidia occurs within hours. Sources of inoculum include infected seeds and overwintering of spores on volunteer plants.
Disease management must involve several different strategies
Using high quality disease-free seeds is the first step and always recommended.
Plant genetically resistant varieties.
Maintain adequate space: Avoid over density planting in order to allow light to penetrate.
Crop rotations if possible.
Adequate fertilization: Overuse of nitrogen fertilizers can increase new infections of rice blast, but often fail to significantly increase yields.
Sanitation: Infected plant debris must be removed at the end of cultivation in order to avoid the fungus from surviving and infecting newly planted rice.
Attempts to fight blast of rice results in the development of important chemicals.
The following is a list of fungicides used in one or more parts of the world: dexaconazole, tricyclazole, prochloraz, carpropamid, azoxystrobin, and carbendazim.
*Names marked in red are considered to be highly poisonous to beneficial insects.
*Names marked in green are considered to be organic and IPM (integrated pest management) compatible.