In the context of plant pathology, which term refers to a symptom that can be seen with the naked eye?

In plant pathology, the term that refers to a symptom that can be seen with the naked eye is "symptom." Symptoms are observable changes in the plant's appearance that indicate the presence of a disease, such as wilting, yellowing, or necrosis. They serve as visual cues to identify health issues in plants and can range from changes in leaf coloration to the presence of lesions or growth abnormalities.

The other terms, while related to plant pathology, have different meanings. A sign typically refers to physical evidence of a pathogen, such as fungal structures like spores or mycelium, rather than symptoms displayed by the plant itself. An indicator could relate to various cues that signal the presence of disease, but it does not specifically denote visible symptoms. A response might describe how a plant reacts to external factors, including diseases, but again does not specifically refer to the visible symptoms themselves. Therefore, the correct term to describe a visible symptom is indeed "symptom."