What are signs of disease in plants?

The correct choice identifies signs of disease in plants as the structures of the pathogen. This is because signs of plant disease are typically physical manifestations that indicate the presence of a pathogen. These can include observable structures such as fungal fruiting bodies, bacterial ooze, or mycelium, which are directly related to the plant's infection.

When identifying disease in plants, it is critical to differentiate between symptoms and signs; symptoms are the plant's responses to disease, such as wilting, yellowing, or leaf drop, while signs refer specifically to the pathogen itself. Recognizing pathogenic structures plays an essential role in diagnosing plant diseases and implementing effective management strategies.

In contrast, the other options refer to broader contexts that do not directly indicate the presence of a pathogen. A plant's response to stress refers to the physiological reactions, which can be caused by various factors, including diseases but are not the signs themselves. Environmental conditions affecting plant health can influence disease prevalence or severity but are not indicative of the disease per se. Changes in farmer practices may affect how diseases manifest or are managed but do not provide direct signs of disease in plants.