What are the conditions necessary for disease occurrence?

The correct answer identifies three essential factors that contribute to the occurrence of plant disease. Firstly, a susceptible plant is necessary because not all plants are vulnerable to all pathogens; a disease can only manifest if the host plant lacks resistance to the causative organism. Secondly, the presence of a disease-producing organism—such as a fungus, bacterium, or virus—is critical, as it is the source of the disease. Lastly, a favorable environment must be present, as certain conditions such as temperature, moisture, and light can influence the development and spread of the disease. The interplay of these three components creates an environment in which disease is likely to occur.

In contrast to the other options, the first choice mistakenly includes an infected plant but lacks the necessary pathogen, which is fundamental for disease development. The third option mentions only a disease-producing organism but overlooks the need for a susceptible host and environmental conditions. Lastly, the fourth option suggests a resistant plant, which does not align with the notion that susceptibility is key for disease occurrence, even in conditions of high humidity. Thus, the correct option captures the complete picture of disease occurrence by incorporating the necessary susceptibility, the pathogen, and the appropriate environmental circumstances.