It Takes Three to Make a Plant Sick
The first “side” of the triangle is so obvious it may be overlooked. In order to have a plant disease, you must have a plant. More specifically, you need a susceptible plant, one that is able to get a particular disease. Each plant species is prone to a unique set of maladies. Crabapples and oaks get different diseases. Within a species, plant varieties differ in their susceptibility to various diseases. For example, some crabapple cultivars are decimated by apple scab while others are unaffected. The overall health and vigor of an individual plant also affects its susceptibility to disease.