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Bacterial leaf blight is caused by Xanthomonas oryzae pv. oryzae. On nutrient agar, bacterial colonies are circular, light yellow, and convex, and produce a yellow pigment (Webster and Gunnell 1992; Fig. 1).
The disease is observed on both seedlings and older plants. On seedlings, infected leaves turn grayish green and roll up. As disease progresses, leaves turn yellow to straw-colored and wilt, leading whole seedlings to dry up and die. This expression of the disease is known as “kresek” (Ou 1985, Mew et al 1993). This form of the disease on seedlings may sometimes be confused with early rice stem borer damage.
On older plants, lesions usually develop as water-soaked to yellow-orange stripes on leaf blades or leaf tips or on mechanically injured parts of leaves (Fig. 2). Lesions have a wavy margin and progress toward the leaf base. On young lesions, bacterial ooze resembling a milky dew drop can be observed early in the morning. The bacterial ooze later on dries up and becomes small yellowish beads underneath the leaf. Old lesions turn yellow to grayish white with black dots due to the growth of various saprophytic fungi. On severely infected leaves, lesions may extend to the leaf sheath.
To quickly diagnose bacterial leaf blight, a young lesion is cut across and placed in a transparent glass container with clear water. After a few minutes, the container (held against light) will show a turbid liquid exuding from the cut end of the leaf. The turbid exudates consist of bacteria emitted from the cut end of the infected leaf.
The disease occurs in both tropical and temperate environments, particularly in irrigated and rainfed lowland areas. It is commonly observed when strong winds and continuous heavy rains occur (Ou 1985, Mew et al 1993). The disease is severe in susceptible rice varieties under high nitrogen fertilization (Reddy et al 1979).
Bacterial blight is among the major diseases of rice, and has historically been associated with major epidemics (Ou 1985, Mew et al 1993). The advent of rice varieties bearing genes with resistance to the disease has changed the perception of the disease: the incorporation of host-plant resistance genes in rice varieties, their adoption, and their deployment in the world’s main rice-producing environments is probably one of the most significant evidences of the role of plant pathology in agricultural development.
Bacterial blight nevertheless remains an important concern nowadays because many countries do not endorse the release of new rice varieties unless they carry resistance to the disease.
Whenever susceptible rice varieties are grown in environments that favor bacterial blight, very high yield losses (over 70%; Mew et al 1993, Mew and Vera Cruz 2001) may be caused by bacterial blight. Nowadays, however, yield losses of 1% or less are the norm, as resistant varieties have been deployed in the main rice-producing zones of Asia (Savary et al 2000). Breeding for resistant varieties that carry resistance genes therefore remains a continuing challenge (Bonman et al 1992).
By and large, host-plant resistance has proven to be the most efficient, most reliable, and cheapest tool to control bacterial blight. Varieties resistant to bacterial blight may be considered as one of the best examples of the translation of phytopathological science into successful pro-poor and sustainable disease management (Bonman et al 1992). Twenty-eight genes conferring resistance to bacterial blight have been reported in rice (Jena and Mackill 2008). Most of these are dominant genes, and some of them have been cloned. Marker-assisted selection for resistance to bacterial blight is becoming the norm in breeding programs, and this renders the accumulation of different resistance genes in the same variety easier. Bacterial blight disease control options therefore include
Bonman JM, Khush GS, Nelson RJ. 1992. Breeding for resistance to rice pests. Annu. Rev. Phytopathol. 30:507-528.
Jena KK, Mackill DJ. 2008. Molecular markers and their use in marker-assisted selection in rice. Crop Sci. 48:1266-1276.
Mew TW, Vera Cruz CM. 2001. Bacterial blight of rice. In: Maloy OC, Murray TD, editors. Encyclopaedia of plant pathology. John Wiley and Sons, New York. p 71-74.
Mew TW, Alvarez AM, Leach JE, Swings J. 1993. Focus on bacterial blight of rice. Plant Dis. 77:5-12.
Ou SH. 1985. Rice diseases. Second edition. Commonwealth Mycological Institute, Kew, Surrey. 380 p.
Reddy APK, Katyal JC, Rouse DI, MacKenzie DR. 1979. Relationship between nitrogen fertilization, bacterial leaf blight severity, and yield of rice. Phytopathology 69:970-973.
Savary S, Willocquet L, Elazegui FA, Teng PS, Du PV, Zhu D, Tang Q, Lin X, Singh HM, Srivastava RK. 2000. Rice pest constraints in tropical Asia: characterization of injury profiles in relation to production situations. Plant Dis. 84:341-356.
Webster RK, Gunnell PS. 1992. Compendium of rice diseases. American Phytopathology Society, St. Paul, Minnesota. 62 p.
Developed with input from: I Ona, NP Castilla, S Savary, and CM Vera Cruz.