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Rice Blast
Rice Blast – Page 1 of 3
Nature and disease symptoms
Rice blast is one of the most important diseases of rice, caused by the fungus Magnaporthe oryzae B.C.
Couch (Couch and Kohn 2002). The pathogen may infect all the aboveground parts of a rice plant at
different growth stages: leaf, collar, node, internode, base, or neck, and other parts of the panicle, and
sometimes the leaf sheath. A typical blast lesion on a rice leaf is gray at the center, has a dark border,
and is spindle-shaped (large in the middle and tapering toward the end; Fig. 1a).
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Under favorable conditions, leaf lesions enlarge and coalesce, eventually blighting the entire leaf. Leaf
blast lesions on some varieties are sometimes similar to brown spot lesions. Collar blast causes reddish
brown to brown collar lesions (Fig. 1b) and may kill the entire attached leaf. In the case of node blast,
the node turns blackish (Fig. 1c) and breaks easily. Neck blast results in a girdled neck with grayish
brown lesions (Fig. 1d). These different symptoms have very different consequences for rice yield
(Pinnschmidt et al 1994), and neck blast is potentially the most destructive. Neck blast may be
confused with “whiteheads” caused by stem borer injuries. Both injuries result in empty, erect, whitegray,
and conspicuously injured panicles. However, unlike injury caused by stem borer, for which the
entire stem can be pulled out readily, neck blast causes only injury at the neck, and normally does not
extend further into the leaf sheath.
a b c d
Fig. 1. Blast lesions on the leaf (a), collar (b), node (c) and neck (d) of a rice plant.
Occurrence of blast
Rice blast is present wherever rice is cultivated, but the disease occurs with highly variable intensities,
depending on climate and cropping system. Environments with frequent and prolonged dew periods and
with cool temperature in daytime are more favorable to blast. This applies particularly to upland and
rainfed environments in the tropics and subtropics, as well as irrigated areas in temperate ecosystems.
Factors favoring the disease
The literature concerning factors that may influence blast is extensive, and can be only briefly
summarized here (Teng 1994). The early literature emphasized the many physical and micro-climatic
factors that may influence the life cycle of the pathogen (e.g., Hashimoto 1981), including spore
liberation, transport, deposition, infection, latency, and sporulation. For each phase of the life cycle, an
optimum of environmental factors often exists for blast. Thus, subtropical or temperate environments,
where canopy wetness is frequent along with moderate temperature, are particularly inducive to blast.
A considerable amount of work has been devoted to studying pathogen-host-environment
interrelationships in blast. Excessive nitrogen fertilizer promotes the disease. On the other hand,
moderate water stress also favors the disease, especially the sporulation of the pathogen. Blast can be
a major disease of both lowland and upland rice, under favorable conditions—for example, extended
duration of leaf wetness, a high amount of nitrogen, and cool temperature.
For more information, visit the Rice Knowledge Bank: http://www.knowledgebank.irri.org
To diagnose problems in the field, visit www.knowledgebank.irri.org/ricedoctor.
Developed with input from: N. Castilla, S. Savary, C.M. Vera Cruz, and H. Leung
Produced by the International Rice Research Institute (IRRI) • © 2010 IRRI, All rights reserved • Mar 2010

Rice Blast – Page 2 of 3
Detailed quantitative knowledge of the life cycle, aided by simulation modeling studies (Teng 1994),
provides an entry point for devising strategies for disease control, including the use of host-plant
resistance. In general, the severity of leaf blast epidemics is dependent on two key phases of the
disease cycle: infection (a deposited pathogen spore infects a healthy leaf site) and sporulation (the
amount of spores produced by a blast lesion over an infectious period). Achieving control over these two
stages is known to effectively control the disease. Resistant rice varieties probably confer resistance
through interference of the infection and sporulation processes.
Another critical factor that determines the likelihood of a blast epidemic is related to the genotype of the
rice variety that is cultivated, to the diversity of the pathogen that is present, and their interaction.
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The rice blast pathosystem is a model system for basic studies by biologists. The blast fungus, with its
high degree of genetic variability, has attracted interest in pathogen genetics and evolution. The genome
of the blast fungus has been sequenced. The available genomics tools in rice and in the fungus have
offered many opportunities for investigation of host-pathogen interaction, disease resistance, pathogen
population genetics, and evolution.
Control of rice blast
Host plant resistance
Host-plant resistance is, by far, the primary control option for blast, in spite of the difficulties this disease
represents in developing durable and efficient resistances.
The molecular genetics of blast resistance has been extensively studied (Jena and Mackill 2008),
leading to many DNA markers corresponding to major resistance genes identified. Some 40 genes for
major resistance to blast are known. Reliance on major resistance genes, however, is risky because
new genotypes of the pathogen can evolve rapidly and overcome host resistance (Zeigler et al 1994).
Nonetheless, some resistance genes are found to confer broad-spectrum resistance against pathogen
strains tested. Partial resistance, on the other hand, is usually controlled by multiple genes, and it may
offer a more stable form of resistance. Combining broad-spectrum resistance genes with multiple
quantitative resistance genes may be a promising approach to develop durable resistance (Jena and
Mackill 2008, Manosalva et al 2009).
In some situations, blast can be managed through the use of diverse varieties with different levels of
resistance and modified cultural practices. Good control of panicle blast can be achieved through
interplanting rice varieties (Zhu et al 2000). Multilines, comprising several near-isogenic lines each
carrying different resistance genes, have been successfully used to control blast in Japan (Koizumi
2001).
Crop management
Split applications of nitrogen based on actual requirements of the crop are recommended to reduce
disease intensity. The excessive use of nitrogen fertilizer promotes luxuriant crop growth, which
increases the relative humidity and leaf wetness of the crop canopy, and so favors blast. Flooding the
soil as often as possible can be effective, particularly in tropical areas where conditions are not very
favorable to blast.
The application of silicon fertilizers (e.g., calcium silicate) to soils that are deficient in this element has
reduced blast. Because of its high cost, silicon should be applied efficiently. Cheap sources of silicon,
for example, straw of rice genotypes with high silicon content, can be considered to make this approach
economically viable.
Chemical control
Many fungicides have been developed to control blast, which represents the second fungicide market
worldwide. Systemic fungicides are often used to control blast in many rice-growing areas. The use of
fungicides with similar modes of action over extensive periods is not recommended because it has
resulted in the emergence of resistant populations of the pathogen (Kim et al 2008).
For more information, visit the Rice Knowledge Bank: http://www.knowledgebank.irri.org
To diagnose problems in the field, visit www.knowledgebank.irri.org/ricedoctor.
Developed with input from: N. Castilla, S. Savary, C.M. Vera Cruz, and H. Leung
Produced by the International Rice Research Institute (IRRI) • © 2010 IRRI, All rights reserved • Mar 2010
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Rice Blast – Page 3 of 3
References
Couch BC, Kohn LM. 2002. A multilocus gene genealogy concordant with host preference indicates
segregation of a new species, Magnaporthe oryzae, from M. grisea. Mycologia 94:683-693.
Hashimoto A. 1981. Water droplets on rice leaves in relation to the incidence of leaf blast: use of the
dew balance for forecasting the disease. Rev. Plant Prot. Res. 14:112-126.
rice fact sheets
Jena KK, Mackill DJ. 2008. Molecular markers and their use in marker-assisted selection in rice. Crop
Sci. 48:1266-1276.
Kim YS, Oh JY, Hwang BK, Kim KD. 2008. Variation in sensitivity of Magnaporthe oryzae isolates from
Korea to edifenphos and iprobenfos. Crop Prot. 27:1464-1470.
Koizumi S. 2001. Rice blast control with multilines in Japan. In: Mew TW, Borromeo E, Hardy B, editor.
Exploiting biodiversity for sustainable pest management. Los Baños (Philippines): International Rice
Research Institute. p 143-157.
Manosalva, PM, Davidson RM, Liu Bin, Zhu XY, Hulbert SH, Leung H, Leach JE. 2009. A germin-like
protein gene family functions as a complex quantitative trait locus conferring broad-spectrum disease
resistance in rice. Plant Physiol. 149:286-296.
Pinnschmidt HO, Teng PS, Luo, Y. 1994. Methodology for quantifying rice yield effects of blast. In:
Zeigler RS, Leong SA, Teng PS, editors. Rice blast disease. Wallingford, Oxon (United Kingdom): CAB
International, Los Baños (Philippines): International Rice Research Institute. p 381-408.
Teng PS. 1994. The epidemiological basis for blast management. In: Zeigler RS, Leong SA, Teng PS,
editors. Rice blast disease. Wallingford, Oxon (United Kingdom): CAB International, Los Baños
(Philippines): International Rice Research Institute. p 409-434.
Zeigler RS, Leong SA, Teng PS, editors. 1994. Rice blast disease. In: Zeigler RS, Leong SA, Teng PS,
editors. Rice blast disease. Wallingford, Oxon (United Kingdom): CAB International, Los Baños
(Philippines): International Rice Research Institute. 626 p.
Zhu Y, Chen H, Fan JH, Wang Y, Li Y, Chen J, Fan JX, Yang S, Hu L, Leung H, Mew TW, Teng PS,
Wang Z, Mundt CC. 2000. Genetic diversity and disease control in rice. Nature 406:718-722.
For more information, visit the Rice Knowledge Bank: http://www.knowledgebank.irri.org
To diagnose problems in the field, visit www.knowledgebank.irri.org/ricedoctor.
Developed with input from: N. Castilla, S. Savary, C.M. Vera Cruz, and H. Leung
Produced by the International Rice Research Institute (IRRI) • © 2010 IRRI, All rights reserved • Mar 2010
3