Differential Detection and Analysis of Pathotypes and Differentiation against Xanthomonas oryzae pv. oryzae in Southern China

Pathogenic isolates were collected from different rice-cropping regions in southern China to dissect the pathogenic disintegration and variation of Xanthornonas oryzae pv. oryzae （Xoo）. Two sets of rice-Xoo differential hosts, including Chinese system with such five euhivars as IR26, Java14, Nangeng15, Tetep and Jingang 30 and international system with a series of nearisogenic lines （NILs） including IRBB5, IRBB13, IRBB3, IRBB14, IRBB2 and IR24 carrying different known resistance genes,were used to detect pathogenic disintegration for the reaction between host and pathogen with leaf-clippingmethod at the rice booting stage. The results showed the type of the pathogen were divided intosix pathotypes, i.e. , Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅴ and Ⅸ based on the Chinese differential system, and seven pathogenicraces including RI, R2, R3, R4, R5, R8 and R10 based on the international differential system. The pathogenicity frequency of Xoo pathotypes V and IV and pathogenic races R8 and R5 were 27.40% , 19.30% and 44.67% , 15.34%, which were considered to be the prevailing races in southern China. Pathogenic rates of pathotypes Ⅳ, Ⅴ, Ⅸ and races R8, R5 against 500 varieties derived from southernChina were 96.40% , 95.00% , 50.40% , 62.00% , and 42.60%, respectively. Among which pathotype Ⅸ was the most virulent pathotype. The pathotype Ⅴ became preponderant pathotype and the new pathotype Ⅸ grew up quickly.

Resistance Analysis on Major Resistant Genes of Rice Bacterial Blight in Different Donors and Genetic Background

The different donors of eight rice bacterial blight( BB) major resistant( MR) genes,Xa3,Xa4,xa5,Xa7,Xa10,Xa11,Xa14 and Xa23,were inoculated with two Chinese Xoo strains,strain IV and strain V,respectively. The strain IV was a predominant pathotype in south China,while the strain V was a severe virulentstrain. Two susceptible sterile lines and two susceptible inbred rice were employed to study the effect of geneticbackground on expression of resistance to BB for Xa4,xa5,Xa7 and Xa23. The application value of twodominant resistant genes Xa7 and Xa23 on hybrid rice was also evaluated. The results indicated that the resistance inheritance behaviors of most of the MR genes to the Xoo strains were similar and only a few genes show contradictory reaction in different donors,which suggests that the genetic background would produce an effect on the resistant expression of some R genes. Four resistant donors,IRBB4,IRBB5,IRBB7 and CBB23,were crossed with four susceptible cultivars,WufengA,JitianA,Jingang30 and IR24,respectively,using the resistant donors as male parent. The analysis of F1 populations revealed that the recessive resistance gene xa5 and dominant resistance gene Xa23 conformed to recessive and dominant expression model,respectively,while the other two dominant resistance genes,Xa4 and Xa7,conformed to dominant expression model only in some of F1 populations but not for others. In conclusion,expression of resistance for xa5 and Xa23 was independent of genetic background,while Xa4 and Xa7 show specific behaviors of resistance related to the receptor background. This study also suggests that Xa7 is not suitable for application in hybrid rice,for the F1 generation of its cross with two male sterile lines were highly susceptible,and Xa23( CBB23) had important value in improving BB resistance of hybridrice,as the F1 generations from its cross with 4 susceptible parents,including sterile lines and inbred rice,were all show highly resistant.

Utilization Situation and Prospect of Gene xa5 Against Pathotype Ⅴ of Rice Bacterial Blight

In recent years,the strong virulence pathotype Ⅴ of rice bacterial blight grew up quickly in Southern China,which has become a major population and spread to the rice regions of Jiangsu and Zhejiang provinces in Southern China.Since pathotype Ⅴ caused serious bacterial blight disease in rice production regions,it is urgent to breed and promote resistant varieties against pathotype Ⅴ.The most economic and effective measure to control rice bacterial blight is to breed resistant cultivars for widely planting using resistance genes.The Institute of Plant Protection,Guangdong Academy of Agricultural Sciences,and Agricultural Science Research Institute of Panyu District of Guangzhou used IRBB5 carrying the recessive xa5 gene from IRRI that is resistant to pathotype Ⅴ to breed resistant varieties with rice blast resistance source,through hybridization,multiple cross,pedigree selection and synchronous resistance evaluation.We successfully bred series of new resistant rice varieties such as Baijiangzhan,Baijingzhan and Baisizhan,which showed resistance to strong virulence pathotype Ⅴ(grade 1-3)of bacterial blight and rice blast(mediate to high resistance),good grain quality(level 3 of rice quality of Guangdong),and yield equivalent to major cultivars(compared with region trial control cultivars of Guangdong).These new resistant varieties were promoted and planted in the strong virulence pathotype Ⅴ region along the west coast of Guangdong,which showed favorable superiority and wide application prospect in controlling rice bacterial blight with resistance varieties.