What condition can be both a sign and symptom of brown rot?

Gum exudation is indeed both a sign and symptom of brown rot, making it the correct choice among the options. In the context of plant pathology, a "sign" refers to any physical evidence of a disease that can be observed, such as the presence of the pathogen itself or damage caused by it. In this case, gum exudation indicates the plant's response to infection.

A "symptom," on the other hand, is a physical expression of the disease from the plant's perspective, often showing as stress or damage. When a plant exhibits gum exudation, it is reacting to the infection caused by brown rot pathogens, which leads to the formation and secretion of gum. This duality makes gum exudation significant, demonstrating both the direct effect of the pathogen and the plant's response to it, thus serving as both a sign indicating the disease's presence and a symptom reflecting the plant's condition.

The other options, while they may indicate plant stress or other issues, are not directly associated with brown rot. Leaf curling, stunted growth, and root galls are symptoms that could relate to various conditions, but they do not specifically reflect the dual nature of both sign and symptom as gum exudation does in