Plants are not susceptible to many potential pathogens for several reasons. Structural characteristics of plants can provide various levels of resistance against potential plant pathogens. Also, various substances formed by plants and present before infection provide disease defense mechanisms of a chemical rather than a structural nature. Plants exude a variety of chemical substances from the surfaces of plant parts such as leaves, stems, fruit, seeds and roots. These antimicrobial materials include phenolic compounds, tannins and fatty-acid like chemicals. Experiments have demonstrated that some of these compounds have an inhibitory action on certain pathogens. For example, toxic exudates on leaves of a specific variety of sugar beet are present in sufficient concentrations to inhibit spore germination of certain fungal pathogens. Another compound imparted resistance in certain types of tomato plants to powdery mildew by inhibiting spore germination. Additionally, proteins and enzymes on plant surfaces have been shown to inactivate pathogen enzymes that are essential for disease development. These preformed compounds, together with structural defenses, often drive what we recognize as resistance to diseases in certain varieties or types of plants.

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.