An extremely valuable weapon in the battle to manage plant diseases may reside within the genetic composition of the plant. This plant genetic disease management tool is commonly referred to simply as disease resistance or tolerance. These names are often used interchangeably; however, the definitions of each term denote a significant difference. Resistance can be thought of as the ability of a plant to exclude or overcome the effect of a plant pathogen, whereas tolerance is the ability of a plant infected by a pathogen to grow without dying or sustaining serious injury or yield loss. Therefore, resistance focuses on infection prevention, whereas tolerance permits the plant to grow without serious injury or yield loss after infection. Disease resistance and tolerance are not all or nothing conditions. For example, resistance can range from its highest level, which we call immunity, through degrees of useful resistance, and finally to its lowest level, when a plant is highly susceptible to a particular pathogen. Also, resistance and tolerance usually are specific to one or at most a few diseases, and not a broad range of plant ailments. The mechanisms within plants that create disease resistance and tolerance are many and varied. Successful suppression of pathogen activity by a plant is tied to how a particular pathogen gains entrance into a plant to initiate disease and also how a plant defends itself from that infection by one or more physiological (biochemical) or morphological (structural) changes. One key advantage of strong genetic resistance or tolerance is that this disease management tool will be active for the life of the plant, without any input by the grower. On the other hand, when products such as fungicides are used to manage diseases, the products have to be applied so as to be in place over the entire growth period of the plant when disease is expected. Also, disease management provided by plant genetics often targets diseases for which no other known effective disease management tools are known. Building disease resistance or tolerance into plants is an ongoing activity of plant breeders, using classical as well as modern genetic manipulation techniques to achieve this goal.

To contact Mike Matheron go to: matheron@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.