True or False: A visible response of the host to a casual agent over time is a symptom. A sign is a physical structure produced by the causal agent of the disease.

The statement is indeed true. In the context of plant pathology, a symptom is understood as the visible response manifested by the host organism as a result of infection by a causal agent, such as a pathogen (fungus, bacteria, or virus). Symptoms may include changes in the plant’s appearance, such as wilting, chlorosis (yellowing of leaves), or stunted growth, and these changes signify the host's reaction to the disease over time.

On the other hand, a sign is defined as a physical structure that is produced by the causal agent itself, such as fungal fruiting bodies, bacterial ooze, or insect frass, which can be observed without interpreting any response from the host. The distinction is critical for diagnosing plant issues accurately, as it allows an understanding of both the effects of the pathogen on the host and the actual characteristics of the pathogen itself.

This clear differentiation helps in effectively managing and treating plant diseases by directing focus on both the symptoms presented by the hosts and the signs indicating the presence of the disease-causing organisms.