DENSITY-DEPENDENT MORTALITY IN PINUS-SYLVESTRIS CAUSED BY THE SNOW BLIGHT PATHOGEN PHACIDIUM-INFESTANS

Relationships between disease incidence and the density of host plant populations were investigated in the Pinus sylvestris - Phacidium infestans host-fungal pathogen association, in which the season of death of plants killed up to 3 years previously could be accurately determined. Significant (P < 0.05), positive density-dependent relationships between the proportion of plants dying in the winters of 1987-1988, 1988-1989 or 1989-1990 and the original stand density were detected in 12 of 26 comparisons. Of the remaining comparisons, all but three had positive regression coefficients for the same association. Plants killed up to 2 years previously contributed to inoculum production. The use of "standing dead" as a predictor in the analyses showed that the proportion of plants dying in the winters of 1988-1989 or 1989-1990 was generally better correlated with standing dead in the previous summer than with the density of the original population. Significant (P < 0.05), positive density-dependent associations were also found between the proportion of living plants in 1990 infected with P. infestans and the number of standing dead plants in all nine comparisons. In contrast, only four of the nine associations between these proportions of infected plants and population density were significant. The strength of the density-dependent relationships varied substantially within and between sites. Much of this variation appears to be due to differences in the stage of development of the epidemics occurring at different sites.