Certain chemicals serve as indispensable tools in the continuing effort to minimize
crop losses due to plant diseases. For example, active ingredients within fungicides
help growers to manage 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 can range from various levels of resistance up to outright immunity.
Numerous 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 are responsible for what we recognize
as resistance to diseases in plants. Even if these plant derived chemical and structural
disease defense systems can not 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.
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