Host Density as a Factor in Plant Disease Ecology

At the structural, physiological, and genetic levels of host-pathogen interac­ tion, plant pathological studies have been relatively even-handed in their treatment of both host and pathogen. However, in considerations of the numerical interactions between populations of these protagonists there has been an overwhelming bias towards studies of the pathogen. Whether treated directly as the number of spores (4 1), parasite density ( 10), or inoculum intensity (46), dealt with indirectly in terms of the number of lesions produced on the host ( 107, 1 18), or combined with other factors within one of the various concepts of inoculum potential (50), pathogen numbers have been subjected to intensive investigation. By contrast, few attempts have been made to isolate and quantify host density as a factor in disease even though many pathologists accept the axiom that growing plants in dense stands contributes to the development of severe epidemics ( 17, 5 1, 1 19). The relative neglect of plant density as an experimental variable probably reflects the preoccupation of pathologists with agricultural crops, where planting density is generally regarded as a nonnegotiable quantity for such purposes as disease control. Moreover this attitude is probably reinforced by the widespread tendency to express disease incidence in terms of the proportion of plant tissue affected by disease, a procedure which obscures quantitative differences in density between host stands by reducing these all