ABSTRACT
Examples of iatrogenic diseases have been provided by all major groups of crop protection chemicals. There is little evidence that the toxic principles in insecticides and fungicides produce changes in host plants which increase their proneness to disease. Pathogen growth and reproduction depends on extraction of nutrients from their hosts, and, inevitably, host composition has effects on disease severity. Seedling diseases provide the vast majority of examples where natural defense mechanisms appear to have been undermined by pesticides. In considering the effects of pesticides on crop ecosystems, therefore, it seems desirable to divide attention between effects on competitors and antagonists and the consequences of changes in crop production practices which have arisen through the widespread use of pesticides and which have influenced disease development. The most obvious consequence of removing weeds is that crop plants grow better. Space which would have been occupied by weeds can be taken up by the crop species.
