Inheritance of resistance to rice bacterial leaf blight
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In order to design appropriate control strategies for a major disease of rice Uganda, the bacterial leaf blight (BLB) disease caused by Xanthomonas oryzae pv.oryzae on rice, this study was initiated to a) assess prevalence of the disease, b) characterize the BLB pathogen and c) determine the mode of inheritance of the BLB resistance in Ugandan rice germplams. A baseline survey conducted in Eastern Uganda revealed that K5 (Kaiso), K23, K85, K98, Sebaggala and “Supa” were the most grown rice varieties. The disease incidence on these varieties varied from 51.7% to 100%, with K23 being the most infected variety while Kaiso was the least infected. The prevalence of BLB ranged from 40% to 81.7%, with the lowest infestation in Mbale and the highest in Budaka. On average, the BLB prevalence was 67.8% in farmer’s field. Pathogenicity tests of six Ugandan isolates on 21 differential lines showed diversity among isolates. The reaction of some pyramided differentials and IR24 to Ugandan isolates suggested a different pattern between Asian and Ugandan isolates. Analysis of relationships between isolates identified UX00 to be unique and thus, this isolate was selected for subsequent studies on inheritance of resistance to BLB. The performance of F2 hybrids derived from IRAT104 x NERICA4, CO39 x NERICA10 and CO39 x IRAT104 to BLB fitted the Mendelian ratio of 3:1, while those derived from crosses NERICA14 x CO39, IRAT216 x NERICA14, NERICA4 x IRAT216 and NERICA14 x IRAT216 segregated in a 13:3 ratio; and those from crosses NERICA4 x NERICA14, NERICA14 x NERICA4, NERICA4 x IRAT104, IRAT104 x NERICA10, IRAT104 x CO39 and CO39 x IRAT216 fitted the 15:1 ratio. However, crosses NERICA10 x NERICA14, NERICA10 x CO39, CO39 x NERICA14, NERICA4 x NERICA10 and NERICA14 x IRAT104 failed to fit any expected segregating ratio, suggesting that complex genetic interactions of resistance genes and isolates may be involved. Significant SCA and non-significant GCA mean squares indicated the preponderance of non- additive gene effects in controlling the BLB resistance. The genotypes NERICA14, NERICA10 and NERICA4 had negative but non-significant GCA estimates and thus contributed to BLB resistance in the crosses. The crosses CO39 x NERICA10 and NERICA14 x IRAT104 had the desirable SCA values. The observed highly significant reciprocal (P≤ 0.001) mean squares among crosses imply maternal contribution of the parents in controlling BLB resistance in rice. Estimates of narrow sense heritability (0.9%) and broad sense heritability (16.4%) also confirm the non-additive gene action in governing the resistance to BLB in rice. Therefore, this study has confirmed the importance of BLB in rice fields in eastern Uganda and also generated information that will guide the design of appropriate breeding strategies for improvement of rice against BLB in the country.