A sign of plant disease is generally:

A sign of plant disease is typically identified as a visual mark or a change in a plant's structure. Signs are physical manifestations that can be directly observed and are indicative of a disease, such as lesions, discoloration, or abnormal growths. These visible changes can help diagnose the specific problem affecting the plant.

In contrast, other options relate to different conditions. Symptoms are often the plant's response to stresses or diseases, such as wilting or leaf drop, rather than direct indicators. Physical injuries caused by pests may not necessarily indicate a disease but rather damage caused by feeding or other types of infestations. Lastly, changes in soil quality pertain more to the health of the soil as a growing medium rather than direct signs of plant disease. Understanding these distinctions helps in accurately diagnosing and managing plant health issues.