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.
The cooler fall temperatures bring welcome relief from the summer heat. As we trade bare feet for shoes, the grass underfoot in our lawns is relieved to see some cooler, wetter weather. But the autumn weather also makes many fungi happy, and several fungal diseases of lawns may show up this time of year.