Abstract

One of the major constraints in rice cultivation in most of the states of our country is the infection of bacterial leaf blight (BLB) causing a reduction of up to 60% in grain yield. In its greatest severity, it can cause a yield loss of about 80% in susceptible cultivars. The bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo) is responsible for this disease. BLB was not a major constrain in our state up to 20th century. However, with higher adaptation of high yielding and hybrid varieties along with higher application of nitrogenous fertilizers, BLB is also causing a serious concern of our state. Earlier studies made on standard differentials revealed the presence of Pathovar Ia. From the study it was found that in Assam the prevalent pathotypes of X. oryzae pv. oryzae are III, V, VIII and IX. The most popular rice variety of our region-Ranjit has also become highly infested by the disease. Therefore, to combat this disease and increase the productivity of the rice cultivation, the introgression of resistance genes of BLB in the popular rice varieties of Assam is of utmost importance. A number of rice genes have been identified along with their closely linked molecular markers that confer resistance against X. oryzae pv. oryzae. Out of the resistant genes, Xa4, xa5, xa13 and Xa21 have been introgressed in different combinations in the background of popular rice varieties of our country and exhibiting resistance by the introgressed varieties. For the introgression of the resistant gene, identification of parental polymorphism between the recipient variety and the donor is the first and foremost importance. For introgression of xa5, xa13 and Xa21, polymorphic molecular markers have been identified between the donor Improved Sambha Mahsuri and Ranjit, Shraboni; RM122 for xa5, xa13 for xa13 and pTa248 for Xa21. However, it was observed that in the popular varieties-Ranjit and Shraboni, the resistance gene xa5 is already present. However, xa5 alone could not confer the resistance to the available pathovar of BLB in Assam. Therefore, introgression of other resistance gene is very important.