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Nature and disease symptoms
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.
Onseedlings, infected leaves turn grayish green and roll up.
Asdisease progresses, leaves turn yellow to straw-colored and
wilt,leading whole seedlings to dry up and die. This expression of
thedisease is known as kresek (Ou 1985, Mew et al 1993). This
formof the disease on seedlings may sometimes be confused with
earlyrice stem borer damage.
On older plants, lesions usuallydevelop as water-soaked
toyellow-orange stripes on leafblades or leaf tips or on
mechanically injured parts ofleaves (Fig. 2). Lesions have a
wavy margin and progress toward theleaf base. On young lesions,
bacterial ooze resembling a milky dewdrop can be observed early in
the morning. The bacterial ooze later ondries up and becomes small
yellowish beads underneath the leaf. Oldlesions turn yellow to
grayish white with black dots due to the growthof various
saprophytic fungi. On severely infected leaves, lesions mayextend
to the leaf sheath.
To quickly diagnose bacterial leaf blight, a young lesion is cut
acrossand placed in a transparent glass container with clear water.
After a
few minutes, the container (held against light) will show a
turbid liquidexuding from the cut end of the leaf. The turbid
exudates consist ofbacteria emitted from the cut end of the
infected leaf.
Occurrence of bacterial leaf blight
The disease occurs in both tropical and temperate environments,
particularly in irrigated and rainfedlowland 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).
Importance of the disease
Bacterial blight is among the major diseases of rice, and has
historically been associated with majorepidemics (Ou 1985, Mew et
al 1993). The advent of rice varieties bearing genes with
resistance to thedisease has changed the perception of the disease:
the incorporation of host-plant resistance genes inrice varieties,
their adoption, and their deployment in the worlds 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 notendorse the release of new
rice varieties unless they carry resistance to the disease.
Whenever susceptible rice varieties are grown in environments
that favor bacterial bl ight, very high yieldlosses (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 deployedin the main rice-producing
zones of Asia (Savary et al 2000). Breeding for resistant varieties
that carryresistance genes therefore remains a continuing challenge
(Bonman et al 1992).
ricef
actshe
ets
Bacterial Blight
For more information, visit the Rice Knowledge Bank:
http://www.knowledgebank.irri.orgDeveloped with input from: I. Ona,
N. Castilla, S. Savary, C.M. Vera Cruz
Produced by the International Rice Research Institute (IRRI)
2010, IRRI, All rights reserved Mar 2010
Fig. 1. Colonies of Xanthomonasoryzae pv. oryzae.
Fig. 2. Typical symptoms ofbacterial blight on leaves.
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Control of Bacterial Leaf Blight
By and large, host-plant resistance has proven to be the most
efficient, most reliable, and cheapest toolto control bacterial
blight. Varieties resistant to bacterial blight may be considered
as one of the bestexamples of the translation of phytopathological
science into successful pro-poor and sustainabledisease management
(Bonman et al 1992). Twenty-eight genes conferring resistance to
bacterial blighthave been reported in rice (Jena and Mackill 2008).
Most of these are dominant genes, and some ofthem have been cloned.
Marker-assisted selection for resistance to bacterial blight is
becoming thenorm in breeding programs, and this renders the
accumulation of different resistance genes in the samevariety
easier. Bacterial blight disease control options therefore
include
The use of resistant varieties. Sources of single resistance
genes and multiple resistancegenes are now available for breeding
purposes.
The use of balanced amounts of plant nutrients, especially
nitrogen.
Ensuring good drainage of fields (in conventionally flooded
crops) and nurseries.
Keep fields clean - removing weed hosts and plowing under rice
stubble, straw, rice ratoonsand volunteer seedlings, which may
constitute sources of inoculum.
Allowing fallow fields to dry to suppress inoculum in the soil
and plant residues.
References
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 plantpathology. 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 leafblight 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. Ricepest constraints in
tropical Asia: characterization of injury profiles in relation to
productionsituations. Plant Dis. 84:341-356.
Webster RK, Gunnell PS. 1992. Compendium of rice diseases.
American Phytopathology Society, St. Paul,Minnesota. 62 p.r
ice
factsheets
Bacterial Blight Page 2 of 2
For more information, visit the Rice Knowledge Bank:
http://www.knowledgebank.irri.org
Developed with input from: I. Ona, N. Castilla, S. Savary, C.M.
Vera Cruz
Produced by the International Rice Research Institute (IRRI)
2010, IRRI, All rights reserved Mar 2010
