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Rice blast is one of the most serious diseases in rice ( Oryza sativa L.) worldwide. Jin 23B is the maintainer line, a parent for a number of hybrid rice varieties used widely in China. However, Jin 23B is highly susceptible to rice blast. In this study, Pi1, Pi2, and D12 were introgressed to improve the blast resistance of Jin 23B and its derived hybrids, Jinyou 402 and Jinyou 207, by marker-assisted selection (MAS). The improved Jin 23B, which carried single, two, and three genes, were evaluated for their resistance to rice blast using natural inoculation methods in disease nursery of Xianfeng, Hubei, China. The results showed that, the greater the number of genes contained in the improved Jin 23B and hybrids, the higher the resistance to rice blast. Pi1, Pi2, and D12 showed a strong dosage effect on the resistance to blast in the hybrid background during the entire growth duration in the field condition, being very useful for breeding blast-resistant hybrids. The result of examining agronomic traits showed that the improved Jin 23B and its derived hybrid rice were taller than or similar to controls, when there was no disease stress. [ABSTRACT FROM AUTHOR]