Author : Nguyen Dac Khoa

Summary : Knowledge of plant-pathogen interaction and pathogen population structure can contribute to sustainable manipulation and deployment of host resistance in disease control. A molecular technique that is reliable but simpler than other methods such as RFLP was used to study population structure of Xanthomomas oryzae pv. oryzae (Xoo), which causes bacterial blight, a worldwide destructive disease of rice, particularly serious in Asia. IS-PCR using primer J3 designed based on the endogenous high-copy insertion sequence IS1112 isolated from the pathogen was able to identify Xoo from other pathovars, detect polymorphisms between Xoo strains, and differentiate the two genetic lineages of the pathogen population. This simple technique when combined with virulence analysis constitutes a good tool to study population structure of Xoo especially in differentiating the four groups of race 9 in the contemporary pathogen population in the Philippines. Analyses of 249 bacterial isolates collected during two consecutive years from near-isogenic rice linens IRBB5 (xa5), IRBB7 (Xa7), gene pyramid IRBB61 (Xa4+Xa5+Xa7), and the recurrent parent IR24 (susceptible) deployed in rice fields at Calauan and Santa Cruz (Philippines) showed that lineage C, composed of races 3 and 9, dominated the indigenous Xoo population (98.8%), with race 9 constituting 94.8%. In contrast, lineage B, represented by race 2 diminished (1.2%). It appears that race 3C is the source of variability observed in the pathogen population. Being under host selection pressure for a longer time (10 years), the Xoo population in Calauan was shown to be more diverse compared to that in Santa Cruz (2 years). However, the pathogen populations at these two locations had similar tendency for genetic diversification over two years.