ABSTRACT

There has been a great deal of increase in the use of herbicides in the world during the past decades (Altman and Campbell, 1977; Altman and Rovira, 1989; Heydari and Misaghi, 2003; Moorman and Dowler, 1991; Utkhede, 1982). New and more effective herbicides are continuously being developed to replace the old ones. In the course of their development, herbicides are routinely screened for the absence of phytotoxicity to crops. However, little attention has been given to the possibility that they may also be toxic to plant-associated microorganisms such as those that cause disease and those that protect crops against diseases (Heydari and Misaghi, 2003; Weller, 1988). Consequently, most herbicides are biologically active against nontarget microorganisms (Heydari and Misaghi, 2003; Rodriguez-Kabana and Curl, 1980). Some herbicides have been shown to cause changes in populations of some microorganisms, including bacteria in the soil (Atlas et al., 1978; Heydari and Misaghi, 1998a; Moorman and Dowler, 1991) and in the rhizosphere (Heydari and Misaghi, 2003; Mudd et al., 1985). Herbicides are also known to cause changes in the incidence and severity of some plant diseases, possibly by affecting plant pathogens (Ahmad et al., 1995; Altman and

30.1 Introduction ......................................................................................................................... 773 30.2Plant Pathogens and the Diseases Caused by Them ........................................................... 774 30.3Stress in Plants Induced by Plant Pathogens ....................................................................... 774 30.4Biological Control of Plant Pathogens ................................................................................ 775 30.5Impact of Herbicides on Pathogen-Induced Stress in Plants .............................................. 776 30.6 Effect of Herbicides on Population of Biocontrol Bacteria in the Rhizosphere ................. 778 30.7Effect of Herbicides on Biocontrol Activity of Bacteria in the Rhizosphere ..................... 779 30.8Mechanism of Herbicide-Mediated Alterations in Pathogen-Induced Stress ..................... 781 30.9Mechanism of Herbicide-Mediated Changes in Biocontrol Activity of Bacteria ............... 782 30.10Conclusions ......................................................................................................................... 782 References ...................................................................................................................................... 783

Campbell, 1977; Altman and Rovira, 1989; Mekwatanakarn and Sivasithamparam, 1987; Rovira and McDonald, 1986).