Abstract: The pathogenicity of 36 isolates of Xanthomonas oryzae pv. oryzae (Xoo), which were collected from japonica rices in the Yunnan Plateau, was evaluated on 29 rice varieties including a set of 7 varieties to identify pathogenicity, ie. Haonuoyang, TN1, Kogyoku , Zhengzhuai, IR26, Nanjing 33 and Kinmaze, which may be considered as a set of differential varieties for Xoo races from Yunnan japonica rice. The efficiency of the 7 varieties was further confirmed. The results showed reversible and specific reactions between isolates and varieties. The isolates were classified into 9 pathotypes from pathotype I to Ⅸ according to their pathogenic reactions on the 7 rice varieties. The pathotype Ⅴ was the epidemic while pathogen Ⅶ was the most pathogenic. Most japonica varieties being grown in the Yunnan plateau were susceptible to Xoo. The rice lines IRBB21 (Xa-21), Zhachanglong (Xa-22t，Xa-24t)and IR1545-339 (xa-5), which were resistant to all the isolates tested, can be used as donors of resistant genes for bacterial blight in japonica rice breeding in the Yunnan Plateau.

Key words: Yunnan, Resistance, Pathotype, Xanthomonas oryzae pv. oryzae, Japonica rice