The real cause of a problem may be the stress factors, with the disease simply being a secondary factor. The relationship of these factors is called the disease triangle. If only a part of the triangle exists, disease will not occur. Understanding the disease triangle helps us understand why most plants are not affected by the many thousands of diseases that exist. Each has a different life cycle, which includes an infectious stage.
Most pathogens are host-specific to a particular plant species, genus or family.
For instance, blackspot of rose will not attack marigolds or lettuce. However, the fungi involved are usually host-specific. The rose powdery mildew fungus will not infect zinnias or turfgrass or vice-versa. A susceptible host has a genetic makeup that permits the development of a particular disease. The genetic defense against a disease is called disease resistance.