![]() ![]() Adding more than one parameter while retaining the pyramidal shape is possible by drawing a base with four line segments. A three-dimensional disease pyramid or tetrahedron ( Figure 3) has been the most common figure drawn after addition of a single parameter. Of these, only time is absolutely required so other elements represent special case applications. Suggested additional parameters have included humans, vectors, and time. Some plant pathologists have elaborated on the disease triangle by adding one or more parameters (1). ![]() ![]() A variant representation of the plant disease triangle showing an unequal relationship among the environment, pathogen and host determinants, which are associated with line segments. Aside from this null case, the alternative quantitative representation ( Figure 2) treats disease as a degree of intensity (i.e., incidence or severity) rather than as a phenomenon.įigure 2. If any one element is reduced to a null variable, the geometric figure transforms into a line and the area occupied by disease collapses to zero. So too, the degree of pathogen virulence and environmental conduciveness may be conveyed equally well. Used in this sense, the disease triangle illustrates the continuum of host reaction from complete susceptibility to immunity. For example, a host with some degree of resistance, but not immune, will result in an overall lower level of disease. Alternatively, the three factors may be associated with the line segments (i.e., triangle sides) then, line length and interior volume can show variation in the strength of the relationship in a quantitative sense ( Figure 2). In a qualitative sense, the disease triangle concisely illustrates the phenomenon of plant disease as occupying the interior space of a triangle with the three essential factors at the vertices ( Figure 1). factors may affect independently host and parasite, and they may affect the interrelations of these organisms." However, perhaps none of these pioneering plant pathologists prior to Stevens was so explicit in their treatment of this three-way interaction as Gäumann (5), who analyzed examples of crucial environmental, host, and pathogen determinants and their effects on disease development. the abundance of a very large number of fungous diseases is directly connected with or conditioned by climatologicalįactors. For example, Duggar (4) wrote in 1909: ". The disease triangle drawing most likely was first published by Stevens in 1960 (6), although earlier plant pathologists certainly recognized the interaction among plant, pathogen, and environment. Finally, the predominance in phytopathology of fungi, which are also highly dependent on environment, may have contributed to the development of this paradigm. The sophisticated immune system found in mammals is absent in plants, and this places an emphasis on the host's genetic constitution. This triangular relationship is unique to phytopathology in comparison to veterinary and medical sciences because terrestrial plants possess little thermal storage capacity and their immobility precludes escape from an inhospitable environment. The three necessary causal factors of disease are positioned at the vertices. The equilateral plant disease triangle after Stevens (6). Conversely, plant disease is prevented upon elimination of any one of these three causal components.įigure 1. Thus, the figure illustrates one of the paradigms in plant pathology that is, the existence of a disease caused by a biotic agent absolutely requires the interaction of a susceptible host, a virulent pathogen, and an environment favorable for disease development (1,6). The disease triangle ( Figure 1) is one of the first concepts encountered by college students in an introductory plant pathology course (1) and often may be re-encountered in higher level classes as a fundamental principle of the factors involved in disease causation. The Disease Triangle: A plant pathological paradigm revisited. Francl Department of Plant Pathologyįrancl, L.J.
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