Responses to pathogenic irritants and insects are termed symptoms. Some symptoms are common to numerous insects and diseases. Other symptoms are almost unique to certain irritant-host relationships. The sum of all the symptoms expressed by a host from the time it is initially infected until it either recovers or dies is known as the symptom complex.
Infection of a plant by pests is not a static condition. The symptoms expressed early in the infection may be quite different than those expressed later. For any one irritant, however, the symptom complex is usually specific. To diagnose a disease or insect problem correctly requires recognizing the specific changes each major pathogen or insect can create in a host.
Symptoms may be influenced by an assortment of factors including the species of the host, the environment, the quantity of inoculum or insects, and the stage of development of the pathogen or insect. Furthermore, symptoms can result from other abiotic causes such as a damaging environment, improperly applied chemicals, animal injury, and mechanical damage. For this reason, it is often necessary to isolate and identify the specific agent of plant injury or to consider other possible sources of irritation before the cause of plant symptoms can be established.
While specific symptoms cover a wide range often separated only by subtleties, collectively they can be grouped into major categories that permit description and comparison (Figure 6-13).
Plants may wilt from lack of water. If such a symptom is environmental in origin, the plant will recover when watered. If insects attack a plant’s root system or pathogens destroy the xylem tissue, the wilting may be permanent. When fungi invade the tender stem tissue of a young seedling, damping-off develops, the plant wilts, and drops over to die.