Which of the following factors is NOT part of the plant disease triangle?

The plant disease triangle is a fundamental concept in plant pathology that illustrates the interaction between three essential components for disease to occur: a susceptible host, an infective pathogen, and a favorable environment. These three elements must coexist for a plant disease outbreak to happen effectively.

Through this framework, a susceptible host refers to a plant that is vulnerable to disease due to genetic factors or conditions that promote susceptibility. The infective pathogen is the organism, such as a fungus, bacterium, or virus, that causes the disease. Finally, the favorable environment is the set of conditions (such as temperature, moisture, and other environmental factors) that support disease advancement and pathogen survival.

While plant age can influence disease susceptibility and resistance—older or younger plants may have different levels of resilience—it does not form a core component of the triangle itself. The triangle focuses specifically on the three critical elements that definitively lead to the occurrence of disease. Thus, plant age, while relevant in certain contexts, is not one of the foundational elements of the plant disease triangle.