Disease develops when:

The correct answer highlights the three critical components necessary for disease development: a susceptible host plant, a favorable environment, and the presence of a pathogen. These elements are part of the disease triangle concept, which illustrates that for a disease to occur, all three components must be aligned.

The susceptible host plant is key because it lacks the natural resistance mechanisms that could otherwise inhibit pathogen invasion or damage. The favorable environment provides the conditions—such as temperature, humidity, and moisture—that enable the pathogen to thrive and proliferate. Finally, the pathogen itself is required to initiate the infection process.

Understanding these components is crucial for effective disease management in agriculture and horticulture, allowing practitioners to adopt appropriate strategies to prevent or minimize disease emergence. In this scenario, the presence and interaction of all three components create a perfect storm for disease expression, leading to the onset of symptoms in the affected plant.