What is the definition of a 'sign' in relation to plant pathogens?

In the context of plant pathology, a 'sign' specifically refers to visible evidence of the plant pathogen itself. This can include structures such as fungal hyphae, spores, or bacterial lesions that can be directly observed on the plant. Signs are tangible manifestations of the pathogen's presence and can be essential for accurate identification and diagnosis of the disease affecting the plant.

In contrast, the plant's response to infection, which is characterized by symptoms such as wilting, yellowing, or stunted growth, is termed a 'symptom.' This distinction is important as symptoms reflect the plant's reaction to the pathogen rather than evidence of the pathogen itself. The mention of invisible damage relates to the underlying effects that a pathogen may have on plant health, but it does not fit the definition of a sign. Environmental conditions, while critical to plant health and disease occurrence, are separate from the definitions of signs and symptoms as they do not directly indicate the presence of a pathogen. Understanding these distinctions is crucial for effectively diagnosing and managing plant diseases.