For a disease outbreak to occur, several factors are necessary. The occurrence of a disease epidemic is a result of a complex interaction between, the pathogen, the host plant and the environment.
A plant becomes diseased when it is attacked by a pathogen. This is the result of an interaction between two organisms; the host and the pathogen. The environment has a major influence on this process and determines the incidence and severity of a disease. For example, if conditions are too dry or too cold the pathogen may not be able to attack the plant.
Traditionally, the major factors affecting disease spread have been linked together in the Disease Triangle, (Figure 1.1). Each side of the triangle represents one of the three components. Each of the three components can display considerable variation and as one component changes it can affect the disease severity. For example, the pathogen may be of a less virulent race, some plants become more resistant with age or environmental conditions may be too dry for the pathogen to infect the plant.
While there are generally more disease problems in wet years, different seasonal conditions will suit different diseases. In most seasons, conditions will suite a particular disease. For example, wet springs will favour the disease take-all, while dry springs favour crown rot.
Figure 1.1 The disease triangle: The amount of disease is dependent on the interactions between host, pathogen and environment (Illustration by Kylie Fowler)
What is a Disease?
Disease Cycle: Stages of Development