Chemicals are indispensable tools in the continuing 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 can enable plants to defend themselves against potential plant pathogens. The level of defense against potential plant pathogens can range from various levels of resistance up to outright immunity. A variety of chemical substances are present on the surfaces of plant parts such as leaves, stems, fruit, seeds and roots. Chemicals with antimicrobial properties include phenolic compounds, tannins and fatty-acid like materials. Experiments have shown that some of these compounds have an inhibitory action on certain plant pathogens. As an example, toxic exudates on leaves of a specific variety of sugar beet are present at a sufficient concentration to inhibit spore germination of certain fungal pathogens. Another compound in certain types of tomato plants was shown to impart resistance 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.