STUDIES ON CHARACTERIZATION AND STRAIN DIVERSITY OF LOCAL ISOLATES OF XANTHOMONAS ORYZAE PV. ORYZAE IN RICE

Rice is an important staple food of Pakistan grown on an area of 2.1 million hectares with an annual production of 4.0 million tons. Such an important crop is threatened by a highly destructive disease known as bacterial leaf blight. Bacterial leaf blight disease is a worldwide problem especially in the Asian continent and has been studied widely in different countries. However in Pakistan for the first time the present extensive study was carried out on the disease. During this study, the surveys conducted in the years 2002 and 2003 revealed that the disease is prevalent in all the four agroecological rice zones of Pakistan. However its prevalence was higher in zone-2 (Punjab province), as compared to other zones. Moreover it was found that prevalence of disease was higher in the year 2003 than 2002 despite similar climatic conditions and same varieties sampled during both the years. This shows that the disease has a growing trend. The causal agent of bacterial blight disease, Xanthomonas oryzae pv. oryzae strains were isolated from foliar samples of rice and were characterized by different classical and molecular methods. Aggressiveness of the isolates was analyzed and it was found that all categories, slightly aggressive to highly aggressive isolates are prevalent in Pakistan. However the percentage of highly aggressive isolates was higher in zone-3 (Sindh province). Races of the pathogen were also identified based on their virulence on 12 isogenic rice lines. Five races of X oryzae pv. oryzae were discovered in Pakistan. Wide variability in virulence among the bacterial isolates and great complexity in the susceptibility of rice cultivars were observed. None of the single genes used were found to be resistant against all the five races. Genomic finger printing of all the isolates obtained was performed using repetitive-sequences and PCR. Seven different major genotypes of X oryzae pv. oryzae were found. Hierarchical analysis of spatial variation of the rice bacterial blight pathogen across diverse agro ecosystems in Pakistan was also carried out by comparing the origin of isolates, their pathotypes/race as well as their aggressiveness. It was found that there existed regional as well as varietal linkage correlated to their genotype, their virulence as well as their reactions towards different resistance genes. This illustrates that diversity within the pathogen population is not randomly distributed among different regions of Pakistan. Instead unity exists within the diverse strains of X oryzae pv. oryzae