Which factors are required before a pathogenic disease can develop?

• A. A susceptible host plant, a pathogenic agent, a environment favorable for development of the pathogen
• B. A resistant host plant, an absent environment, and a non-pathogenic agent
• C. A susceptible host plant, a non-pathogenic agent, and an environment unfavorable for the pathogen
• D. An immune host, a benign agent, and no environment

Answer: (A)

Disease can develop only when three things come together: a susceptible host, a pathogenic organism capable of causing the disease, and environmental conditions that support the pathogen's growth and infection. If the host is susceptible and the pathogen is present, the infection can proceed only if the environment provides the right moisture, temperature, and other factors that enable the pathogen to infect and thrive. When any one of these elements is missing or unfavorable, disease is unlikely to occur.

So this option is the best because it includes all three necessary components: a susceptible host plant, a pathogenic agent, and an environment favorable for the pathogen. The other options omit at least one of these essential elements—such as a resistant or immune host, a non-pathogenic or benign agent, or an environment that is not conducive—so they do not support disease development.