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A convenient way of thinking about plant diseases is by using a concept called the "Disease Triangle".

Disease represents the interaction between three factors (the three corners of the triangle): a susceptible hot, a pathogen (disease causing organism) and a favorable environment. If all of these factors are present, disease results; if one or more of the factors is not present, then disease does not occur.

Methods of disease control (which we'll discuss in greater detail later) can be thought of as modifying the disease triangle by reducing or eliminating one of the corners of the triangle. For example, if you use resistant ornamental varieties in your garden (e.g., a powdery mildew resistant phlox variety), you are eliminating the "susceptible host" and can thus reduce or prevent disease. Similarly, for some diseases, by removing diseased plant material, you can reduce or eliminate disease because you are eliminating the pathogen. Finally, you can reduce or eliminate a "favorable environment" for disease by doing something as simple as not over-watering in your garden.