Infection by Pathogens and Population Age of Host Plants

Abstract

(1) The incidence of disease caused by host-specific systemic fungal pathogens was compared in populations of three host plants, Valeriana sambucifolia, Trientalis europaea and Silene dioica. (2) The study was conducted in an archipelago area in the Gulf of Bothnia, Sweden, subject to isostatic readjustment (land uplift). In this area primary succession is in strikingly different phases on different islands. The height above sea level of an island is correlated with island age and the time since it was first colonized by plants. Approximate ages of host-plant populations could thus be estimated. (3) In all three systems, disease incidence peaks during an early-intermediate phase of population development. In V. sambucifolia maximum disease (84%/,) occurs in forty to forty-five-year-old populations, in T. europaea in fifty to 100-year-old populations (-45%), and in S. dioica in fifty to 120-year-old populations (-600%0). Populations with an estimated age of greater than fifty years for V. sambucifolia, > 400 years for T. europaea and > 300 years for S. dioica show low disease incidences (< 10%). (4) A significant positive correlation between disease incidence and mean host population density was detected only for the Valeriana-Uromyces system, where the pathogen has an annual cycle, in contrast to the other two pathogens, which are perennial. (5) Three hypotheses could account for the observed population age-disease incidence pattern: (i) host population size and density effects, (ii) founder effects and selection for resistance, and (iii) changes in the environment during primary succession.