Chemicals are indispensible tools in the ongoing effort to minimize crop losses due to plant diseases. Various active ingredients within fungicides are especially useful for managing diseases caused by many fungal plant pathogens. Although often not recognized, various substances formed by plants and present before infection provide significant self-defense against potential plant pathogens, ranging from outright immunity through various levels of resistance. A variety of chemical substances are present on 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 shown that some of these compounds have an inhibitory action on certain plant 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 was shown to impart resistance in certain types of tomato plants to powdery mildew by inhibiting spore germination. Additionally, proteins and enzymes on plant surfaces can inactivate pathogen enzymes that are essential for disease development. These preformed compounds, together with various types of structural plant disease defenses, often drive what we recognize as resistance to diseases in plants. Even if these plant derived chemical and structural disease defense systems are not sufficient to totally prevent disease, they along with disease management tools applied by growers contribute to the overall level of disease suppression obtained on a particular crop.

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.