Disease triangle of large number of groups of living organism, the few members of fungi, bacteria, molicutes, nematodes, protozoa, viruses, Viroids and parasitic higher plants invade the host plant, feed and proliferate in it and induce diseases. A plant becomes diseased in most cases when it is attacked by a pathogen (biotic) or affected by an abiotic agent. Therefore, in first case, for a plant disease to occur, at least two components (i.e Plant and Pathogen) must come in contact and must interact. Even through the host and Pathogen come in contact and interact, but if the environmental conditions are not congenial or within a favourable range, then disease can not develop. Therefore, the third component, environmental conditions must be favourable for a disease to develop. Each of these three components (host, pathogen and environment) can display considerable variability and as one component changes it affects the degree of disease severity within an individual plant and within a plant population. Varieties or species may be too young or too old, or plant population grown over a large area may show genetic uniformity, all of these factors can either reduce or increase the rate of disease development by a particular pathogen. Its density may be very small or extremely more. It may be in dormant stage or may require specific environmental factors. The environment may affect both the growth and resistance of host plants and also multiplication, virulence and dispersal of pathogen.
The interactions of the three component of disease have been visualized as a triangle as a triangle generally referred to as the “disease triangle”.
Each side of the triangle represents one of the three components. The length of each side is proportional to the sum total of the characteristics of each components that favour disease i.e if the host is resistant, matured and widely spaced, the host side and amount of disease would be small or zero, whereas if the host or plants are susceptible, at susceptible stage of growth or densely planted, the host side would be long and the amount of disease could be great. Similarly, the virulent, abundant and active the pathogen, the longer the pathogen side and greater the amount of disease. Also more favourable the environmental conditions (E .g. Temperature, moisture and wind) help the pathogen or that reduces the host resistance, longer will be the environmental side and greater will be the amount of disease.
When these three components of the disease triangle are quantified, the area of the area of the triangle represents amount of disease in a plant or in a plant population. If any of the three components is zero, there can be no disease.