Identification of QTLs underlying resistance to a virulent strain of Xanthomonas oryzae pv. oryzae in rice cultivar DV85

Bacterial blight (BB), caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the most devastating diseases of rice in China. A strongly virulent Xoo strain, designated Z-173, is widely distributed across China and Southeast Asia. Indica rice DV85 is known to carry the two resistance genes, xa5 and Xa7. However, their effectiveness against Z-173 is unknown. Using a recombinant inbred line (RIL) population derived from a cross between DV85 and the susceptible cultivar Kinmaze, we have identified the quantitative trait loci (QTLs) responsible for the resistance of DV85 to Z-173. Following 2 years of phenotyping, three QTLs associated with the resistance were detected. These were linked to RFLP markers X362, X292 and G1091 on chromosomes 3, 5, and 6, respectively. Qxa-5 and Qxa-6 probably correspond to xa5 and Xa7, respectively. Both the xa5 and Xa7 resistances are stable over different years, and act independently of one another in determining resistance. The effect of xa5 was larger than that of Xa7. Efficient ways to improve the resistance to Z-173 are discussed.