Which three conditions must exist for a plant disease to develop?

The correct answer identifies the essential elements required for the development of a plant disease: the presence of a pathogen, host susceptibility, and a conducive environment.

In this context, the presence of a pathogen is critical because diseases are specifically caused by organisms such as fungi, bacteria, viruses, or nematodes that infect plants. If there are no pathogens present, disease cannot occur, even if the other conditions are favorable.

Host susceptibility refers to the vulnerability of a plant species or individual plant to a particular pathogen. Some plants have natural defenses against certain diseases, while others may be weak or genetically predisposed to be more easily infected. If the plant is not susceptible, it can resist the disease regardless of pathogen presence or environmental conditions.

Finally, a conducive environment is essential for the disease to manifest. This refers to environmental factors such as temperature, humidity, and moisture levels, which can either promote or inhibit the growth of pathogens. For instance, high humidity and warm temperatures can create ideal conditions for fungal diseases to thrive.

The other options do not encompass all three necessary conditions for disease development. While some may reference environmental factors or the presence of pathogens, they fail to account for host susceptibility or include conditions that are not essential to the disease cycle.