Signs of disease can be observed as what?

The formation of pathogen structures is a clear and direct sign of disease in plants. These structures can take various forms, such as fungal fruiting bodies, bacterial ooze, or viral inclusions, which are indicative of the presence of pathogens. These visible signs can help in diagnosing plant diseases, as they often provide evidence of the type of pathogen affecting the plant.

In contrast, changes in soil composition may influence plant health but do not directly indicate the presence of a plant disease. Variations in crop pricing are influenced by market factors rather than biological symptoms of disease, and reduction of sunlight exposure, while it can lead to plant stress or changes, does not serve as a specific sign of disease caused by pathogens. Therefore, the observation of pathogen structures offers definitive evidence of disease, making it the most accurate choice.