The Limiting Effect of Disease Resistance on Yield

The great majority of vascular plants possess natural means of defense that enable them to avoid or resist infection by the majority of plant pathogens present in their environments. In general, the means used by plants to defend themselves against attack by pathogens can be divided into two categories: those that are preformed and those that are induced by infection. The former includes morphological or histological structures that may primarily serve other purposes in the plant and that exist irrespective of the presence of the pathogen. Such constitutive means of resistance are most often associated with race-nonspecific resistance. Induced or active resistance is due to defense mechanisms activated in the host plant as a response to 'infection or attempted infection. It includes the formation of physical or chemical barriers and the synthesis and accumulation of antifungal compounds that inhibit or prevent pathogen growth. Active defense is most often implicated in gene-specific resistance, but it may also be involved in nonspecific and nonhost resistance. It has been used widely by plant breeders in their effort to produce disease-free cultivars. Although disease resistance has proven an invaluable tool to be used in plant breeding and hence in plant production, this article will be devoted to the fact that active defense processes in plants occur at the expense of the energy resources of the host and therefore may be a limiting factor in plant growth and yield.