The International Rice Research Institute (IRRI) in the Philippines developed near-isogenic lines (NILs) in the background of IR 24 to possess single genes for bacterial leaf blight resistance or multiple genes in various combinations (Huang et al., 1997). In limited tests at Cuttack, Sridhar et al. (1999) found NILs containing individual resistance genes xa8 or Xa21 to be resistant. Although NILs with one resistance gene like Xa4, xa5 or xa13 were susceptible, those carrying these genes in combination (2-3 genes) were resistant to leaf blight disease at Cuttack. In field inoculation tests, Shanti et al. (2001) found effectiveness of some individual genes, with the broadest spectrum of resistance in xa5 and Xa7.

Every year, bacterial leaf blight causes damage to rice crop in several districts in India (DRR, 1975-2002). Reddy and Reddy (1992) collected 150 isolates from 25 locations in India and classified them into two pathotypes: pathotype I was avirulent on DV85 but virulent on Cemposelak and Java 14, and pathotype II was virulent on DV85 but avirulent on Cemposelak and Java 14. Pathotype I was further divided into two sub-groups (i.e., pathotype Ia and Ib, respectively) based on avirulence or virulence on another differential, IR20. We present here the results on the performance of rice genotypes carrying gene(s) for bacterial leaf blight resistance in multi-environment tests (METs) organized at 15 locations under different rice ecosystems in the all-India coordinated rice improvement project. The test locations varied in latitude, longitude and altitude. Twenty-nine rice genotypes including NILs, international differentials and checks were coded as entries (Xo-0 to Xo-28), in the sense that the evaluators at different locations knew only the entry numbers.

In this study, IR 20 with one resistance gene (Xa4) showed resistance at 10 locations (Table 1). Only at these locations, pathotype Ib could be assigned by the method of Reddy and Reddy (1992). The reaction of NIL IRBB4 with Xa4 gene for blight resistance also matched the reaction pattern observed on IR 20 at many test sites. Only at Aduthurai, Bankura, Chinsurah and Patna, IRBB4 was susceptible but IR20 was resistant to bacterial leaf blight disease. The partial resistance in IR 20 in addition to the presence of Xa4 gene (Ahmed et al., 1997) might have led to such differences in reactions from IR 20 and IRBB4. The resistant reaction on Java 14 at Ludhiana where DV 85 was found to be susceptible would indicate the presence of pathotype II of Reddy and Reddy (1992) in addition to pathotype Ib at this location. At the remaining 14 test locations, identical reactions, either resistant or susceptible, were recorded on DV 85 and Java 14. According to Reddy and Reddy (1992), these reaction patterns cannot be assigned to pathotype Ia, Ib or II. Our study demonstrates the inadequacy of the methods of

pathotyping Xanthomonas oryzae pv. oryzae using such differential cultivars.

This multi-environment blind-test clearly demonstrated the expression of a high degree of resistance in differentials (Java 14 and DV 85) and resistant checks (IET 8320 and IET 8585). Both Java 14 (Xa1 + Xa3 + Xa12) and NH24-10-1-3 (Xa4 + xa5 + Xa21) exhibited a broad spectrum of resistance across the country; they showed a susceptible reaction at only three of the 15 test locations. NILs carrying different single genes for resistance to bacterial leaf blight viz., Xa1, Xa3, Xa4, xa5, Xa7, xa8, Xa10, Xa11, xa13, Xa14, Xa18, and Xa21, and two genes viz., Xa4 + xa5, and Xa4 + xa13 were shown to be susceptible at 7-12 of the 15 locations tested in the country. Yet, resistance genes Xa4, xa5, xa13, and Xa21 in two-, three-or four-gene combinations were found to exhibit a higher level of field resistance in diverse ecosystems, barring a few exceptions (3-6 locations). This study clearly shows the potential for gene pyramiding to develop cultivars with durable resistance to bacterial leaf blight disease.

Acknowledgement

We express our sincere thanks to staff of IRRI, Philippines, and CRRI, Cuttack, for providing us with the required pure seeds, and to Dr. B. Mishra, DRR, Hyderabad, for encouragement and facilities. We thank all the scientists responsible for the conduct of the all-India coordinated rice pathology experiments at different locations in the country.

References

Ahmed, H.U., M.R. Finckh, R.F. Alfonso and C.C. Mundt. 1997. Epidemiological effect of gene deployment strategies in bacterial leaf blight of rice. Phytopathology 87: 66-70.

DRR. 1975-2002. Production Oriented Survey. Directorate of Rice Research, Hyderabad, India.

Huang, N., E.R. Angeles, J. Domingo, G. Magpanty, S. Singh, G. Zhang, N. Kumarvadivel, J. Bennett and G.S. Khush. 1997. Pyramiding of bacterial leaf blight resistance genes in rice marker assisted selection using RFLP and PCR. Theor. Appl. Genet. 95: 313-320.

Reddy, M.T.S. and A.P.K. Reddy. 1992. Occurrence of pathotypes of Xanthomonas campestris pv. oryzae in India. Indian J. Mycol. Pl. Pathol. 22: 205-206.

Shanti, M.L., M.L.C. George, V.C.M. Cruz, Ma Bernando, R.J. Nelson, H. Leung, J.N. Reddy and R. Sridhar. 2001. Identification of resistance genes effective against rice bacterial leaf blight pathogen. Plant Dis. 85: 506-512.

Sridhar, R., J.N. Reddy, U.D. Singh and P.K. Agarwal. 1999. Usefulness of combinations of bacterial leaf blight genes at Cuttack, Orissa, India. Int. Rice Res. Notes 24: 24-25.