World Applied Sciences Journal 31 (7): 1237-1247, 2014 ISSN 1818-4952 © IDOSI Publications, 2014 DOI: 10.5829/idosi.wasj.2014.31.07.14361 Corresponding Author: Safaa M. Ezzat, Central Laboratory for Environmental Quality Monitoring (CLEQM), National Water Research Center (NWRC), Cairo, Egypt. 1237 Biocontrol of Phytopathogenic Bacteria Isolated from Drainage Water and Causing Bacterial Blight Disease Safaa M. Ezzat Central Laboratory for Environmental Quality Monitoring (CLEQM), National Water Research Center (NWRC), Cairo, Egypt Abstract: The present investigation was conducted to explore the incidence of Xanthomonas oryzae pv. oryzae (Xoo) causing bacterial blight disease of rice crop in drainage water reused for irrigation and to evaluate the potentiality of using aqueous extracts from Acacia nilotica plant as natural bio-control for the pathogen and alternative to synthetic pesticides. Forty eight water samples were collected from El-Gharbia drain in two different seasons (summer and winter,2013) and assayed for traditional indicators of microbial water pollution (total and fecal coliforms). Results obtained and compared to WHO guidelines reported high incidence of fecal coliforms and categorized water reused from El-Gharbia drain as possibly suitable for irrigation of cereal, industrial and fodder crops under restrictions. Further bacteriological investigations revealed positive detection of Xoo in drainage water. Maximum peaks for the pathogen's occurrence were recorded in summer (71.40%) compared to winter season (28.60%) favored by the high temperature. The antagonistic effect ( in vitro) of aqueous extracts (leaves and fruits) of Acacia nilotica plant was screened against Xoo isolates at different concentrations. The Pearson's correlation coefficient between the applied concentrations and inhibition zones showed significant correlation (p<0.05) for leaves extract and highly significant correlation (p<0.01) for fruits extract which was significantly comparable in its antagonistic effect to the synthetic bactericide "Streptocycline". The phytochemical analysis of the fruits extract revealed the presence of chemical compounds reported for their antimicrobial activity. The study concluded that removal of weeds from canals and ridges could effectively reduce the natural habitats for the pathogen and control its spreading through drainage and irrigation water. Aqueous extracts from Acacia nilotica were suggested as natural biocontrol for the pathogen either as protective agents for seeds and/or disinfectants in irrigation water. Future field studies are recommended for the development of these interesting extracts into an exploitable commercial product. Key words: Acacia nilotica Botanical extracts Irrigation Pesticides Reused wastewater Xanthomonas oryzae pv. oryzae. INTRODUCTION expenses [3]. In Egypt, there are two major crop seasons, Microbial pollution of irrigation water has been summer season (May-October). The most important crops recently recognized as a major threat not only for human are wheat and berseem in winter, cotton, rice and maize in health but also for crops yield and quality. Polluted water summer [4]. Rice (Oryza sativa L.) is an economically harbor a vast array of microbes that could infect important cereal crop in Egypt. Rice production reached susceptible plants causing diseases which result in losses about 5.9 million ton in the year 2011-2012, with an up to 40% of global food production [1]. Nowadays, increase of 4.2% than year 2010. Yield losses have been non-official reuse of drainage water in irrigation by diminished through a series of preventive measures taken individual farmers constitutes a real problem [2]. Plant by the Egyptian authorities including, cultivation only nutrients and organic matter as well as drainage water once a year, planting resistant varieties, good fertilizers availability are tempting factors for farmers to save their and pesticides management. Apart from these preventive namely, the winter season (November-May) and the