Which of the following would be considered a sign of a plant disease?

Visible fungal growth is a clear indicator of a plant disease because it directly points to the presence of a fungal pathogen. This growth often manifests as mold, mildew, or other fungal structures that can be observed on the surface of the plant or in the surrounding soil. Fungi are known to be significant pathogens that can cause various forms of plant diseases, impacting the health and vitality of the plant.

In contrast, while wilting leaves, abnormalities on leaf margins, and discolored fruit may all suggest that a plant is experiencing some form of distress, they do not exclusively indicate a specific disease. For example, wilting can be caused by insufficient water supply or root damage; abnormal leaf margins might result from environmental stress or pest damage; and fruit discoloration can stem from timing of harvest or nutrient deficiencies. These symptoms are broader and can be linked to multiple causal factors rather than being definitive signs of a disease like visible fungal growth.