Interactions between ecological communities of herbivores and microbes are commonly mediated by a shared plant.A tripartite interaction between a pathogenic fungus-host plant-herbivorous insect is an example of such mutual influences.In such a system a fungal pathogen commonly has a negative influence on the morphology and biochemistry of the host plant,with consequences for insect herbivore performance.Here we studied whether the biotrophic fungus Podosphaeraferruginea,attacking the great buet Sanguisorba officinalis,affects caterpillar performance of the endangered scarce large blue butterfly Phengaris teleius.Our results showed that the pathogenic fungus affected the number and size of inflorescences produced by food-plants and,more importantly,had indirect,plant-mediated effects on the abundance,body mass and immune response of caterpillars.Specifically,we found the relationship between caterpillar abundance and variability in inflorescence size on a plant to be positive among healthy food-plants,and negative among infected food-plants.Caterpillars that fed on healthy food-plants were smaller than those that fed on infected food-plants in one studied season,while there was no such difference in the other season.We observed the relationship between caterpillar immune response and the proportion of infected great buets within a habitat patch to be positive when caterpillars fed on healthy food-plants,and negative when caterpillars fed on infected food-plants.Our results suggest that this biotrophic fungal infection of the great buet may impose a significant indirect influence on P.teleius caterpillar performance with potential consequences for the population dynamics and structure of this endangered butterfly.