Rice blast, rice rotten neck, rice seedling blight, and blast of rice
Magnaporthe oryzae and Magnaporthe grisea
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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.
The name of the 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.
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.
Caution and careful notice should be taken when using any plant protection products (insecticides, fungicides, and herbicides). It is the grower’s sole responsibility to keep track of the legal uses and permissions with respect to the laws in their country and destination markets. Always read the instructions written on labels, and in a case of contradiction, work in accordance to the product label. Keep in mind that information written on the label usually applies to local markets. Pest control products intended for organic farming are generally considered to be less effective in comparison to conventional products. When dealing with organic, biologic, and to some extent a small number of conventional chemical products, a complete eradication of a pest or disease will often require several iterations of a specific treatment or combination of treatments.