14.9 Plant disease basics: the disease triangle

Plant diseases can be analysed conveniently using the concept called the ‘Disease Triangle’ (Fig. 4); this places the three factors which must interact to cause plant disease at the three corners of a triangle. Those three factors are:

  • susceptible host,
  • disease causing organism (the pathogen)
  • favourable environment for disease.

The host is the plant itself; some can fall victim to many diseases, others only suffer particular ones. So all plants have a range of susceptibilities to a range of diseases. The pathogen is the disease. Diseases of plants are most often caused by fungi but there are some plant pathogenic bacteria and viruses.

Without the right host in the right conditions, pathogens cannot cause any harm. Some pathogens are specific to only one or a few host plants, others have broad abilities to attack almost everything. The favourable environment essentially means the weather conditions needed for a pathogen to thrive (this is an important point; it’s ‘a favourable environment for disease’ and if the pathogen is present and disease results, it’s obviously an unfavourable environment for the plant).

Disease results only if all of these three things occur simultaneously; if one or more of the factors is not present, then disease does not occur. The disease triangle was probably first recognised at the beginning of the 20th century and it has become one of the paradigms of plant pathology. It holds a position in plant pathology rather similar to that held by Ohm’s Law (which relates current, resistance, and voltage) in electrical and electronic engineering.

It is a paradigm because occurrence of a disease caused by a biological agent absolutely requires the interaction of a susceptible host with a virulent pathogen under environmental conditions favourable for disease development. The mechanisms that contribute to pathogenesis can all be thought of as modifying the disease triangle by reducing or eliminating one of the corners of the triangle. Examples (from among many) include:

  • the lack of defences in the host,
  • efficient spore dispersal by the pathogen,
  • weather conditions favouring spore production, etc.

Methods of disease control (again from among many) include:

  • breeding for resistance in the host,
  • applying pesticide to hinder the pathogen,
  • irrigating to relieve water stress.
The disease triangle
Fig. 4. The disease triangle illustrating the phenomenon of plant disease as the interior space of a triangle with the three essential factors (susceptible host, favourable environment for disease, and pathogen) at the vertices. Variation in the ‘strength’ of the contributions of these factors to the relationship (‘strength’ is indicated by the size of the circles) will quantitatively alter the severity of the disease, which will be shown by a change in the area of the central disease envelope. The diagram is intended to be used dynamically; the static disease triangle allows illustration of the continuum of host reaction from complete susceptibility to immunity, and the degree of pathogen virulence, and the environmental suitability for disease. If any one element is reduced to zero the triangle transforms into a line and the area occupied by disease collapses to zero. Less dramatic alterations in any factor change the area of the central disease envelope which is an indicator of disease intensity (incidence or severity). For example, a host with some degree of resistance will have a smaller susceptibility circle, and consequently lesser area of overlap and less severe disease. Another example could be a pathogen with greatly increased virulence, which would be shown as a larger ‘pathogen circle’ and consequently larger area of overlap and more severe disease. Based on a diagram published by the Department of Plant Pathology of the University of Wisconsin-Madison, USA at this URL:http://www.plantpath.wisc.edu/PDDCEducation/MasterGardener/General/Slide2.htm.

It is usually stated that this triangular relationship is unique to plant pathology because the immobility of plants prevents them escaping from inhospitable environments, plants have little thermal storage capacity and are therefore subject to temperature stress much more than animals (even poikilothermic animals can ‘bask in the sun’ or retire to the shade as appropriate), and the immune system of vertebrates arms them with sophisticated mechanisms to recognise and neutralise pathogens. Also, the predominance of fungi in causing plant diseases is held to reinforce the uniqueness of the plant disease triangle because fungi are also highly dependent on environmental conditions.

However, this triangular relationship is only unique to plants if you ignore the fact that members of kingdom Fungi also suffer disease, and the severity of that disease also depends on the three essential factors: a susceptible host in an environment favourable for disease challenged by a virulent pathogen.

Some plant pathologists have suggested elaborating the disease triangle by adding additional parameters, such as human activities, disease vectors, and time. Humans contribute to the disease triangle because human activity in agriculture is pervasive and, if you think about it, impacts on all three factors so far discussed, so can profoundly affect the occurrence and severity of plant diseases. This means that humans are already represented implicitly in the basic triangle configuration and this is the main counterargument against including human activity as a new vertex in a ‘disease rectangle’.

Animal and other vectors are not essential to all plant diseases even though they play a critical role in many. Vectors are therefore only worth including in those special cases, where the triangular relationship can be modified by placing the vector on the disease triangle side that connects the host and pathogen vertices; this arrangement emphasises the dependence of the pathogen on its vector.

Time is an essential dimension and has been added to the disease triangle by several authors, primarily to convey the idea that disease onset and intensity are affected by the duration that the three prime factors are aligned. Some duration of favourable alignment is necessary for disease to occur; but the length of time depends on your level of analysis. Physiological events in the host that define infection can take place in minutes or hours; disease symptoms in the field can take days or weeks to appear. Showing time as a dimension on the triangle (perhaps converting it into a pyramid) could be a more realistic adaptation of the diagram.

Updated July, 2019