Résumé
This study investigated the role of genetic variation among host plants and fungal pathogen on disease prevalence and pathogen-induced phenotypic changes. Four different maternal sibships of Silene latifolia showed no difference in susceptibility to disease by the anther smut fungus Microbotryum violaceum after inoculation of an infectious mixture containing haploid sporidia of both mating types into the growing rosette. Three different combinations of sporidia were used. Two selfing combinations, one using fungus collected from the same population as the host plants, one from a second population, and one cross between strains from the two populations were generated. The three sporidia combinations did not differ in their success in infecting the host plant and sporulating in the flowers. However, this fungal disease altered host plant phenotype in several important ways. Disease retarded flowering in those male plants that initiated flowering in their first season, and it increased the number of flowers produced by both male and female plants. Different fungal strains differentially augmented flower production, indicating that there is genetic variability for important fitness-related life-history traits in natural populations of the fungus M. violaceum in Europe. Furthermore, in this short-lived perennial host plant, diseased plants were more likely than were healthy ones to initiate flowering in their first growing season. Since this shift from vegetative growth to reproduction should enhance fungal reproductive success, we interpret this as a manipulation of the host plant phenotype by the fungus. In addition we found a significantly female-biased sex ratio across the four maternal sibships in this study and higher infection rates for female plants than males.