Symptoms are the visible response of a plant to a causal agent over time.
Signs are structures produced by the living causal agent of a disease that are more specific than symptoms and are more useful in accurate disease diagnosis. Symptoms may be grouped according to how the plant responds or reacts to the disease-causing agent. Most living agents of diseases are fungi. Many fungi that produce plant diseases are not visible and can be seen only with a microscope. Fungi lack chlorophyl and therefore depend upon more advanced plants for survival.