Which three factors are necessary for a plant disease to develop?

The development of plant diseases is dependent on three critical components: a supportive environment, a host, and a primary causal agent.

The supportive environment provides the necessary conditions for the disease to thrive, whether it be appropriate humidity, temperature, or other climatic factors that favor disease development. The host refers to the plant species that can be infected by the disease; without a susceptible host, the disease cannot progress. The primary causal agent includes pathogens such as fungi, bacteria, or viruses that are responsible for causing the disease. Together, these three factors create a scenario conducive to the onset and spread of plant diseases.

The other options involve elements that do not directly encompass all three necessary factors required for disease development. For instance, heavy rain and a full moon do not consistently correlate with disease onset, while fertilizer and specific weather conditions are not fundamental necessities in the absence of the other two essential components.