Spatial and temporal variation in disease levels of a floral smut (Anthracoidea heterospora) on Carex nigra

1 Data are presented from a 4-year study on the incidence of infection by the nonsystemic smut fungus Anthracoidea heterospora a of florets of the sedge Cal ex nigra. 2 Beetles of the species Phalacrus substriatus may act as vectors, transmitting A. heterospora between different C. nigra tussocks. Path analysis clearly showed that the number of vectors on individual tussocks was an important factor explaining levels of infection of A. heterospora. 3 The year-to-year correlations in incidence of A. heterospora disease for individual C. nigra plants were high, indicating that the same plants consistently experience high levels of disease. There were also high year-to-year correlations in the abundance of P. substriatus beetles on individual plants. 4 A second natural enemy of the sedge, the gall mite Phytoptus caricis, also showed high between-year correlations in levels of infestation. Since P. caricis competes for the same resource as A. heterospora, we predicted that high levels of P. caricis infestation should have a negative effect on disease levels. However, there was in general a positive effect of P. caricis infestation on A. heterospora disease levels. 5 This study shows that for understanding levels of disease of A. heterospora, both within-year effects as well as transient effects must be included. Phenotypic variation among plants in susceptibility to infection by A. heterospora could explain the observed patterns of disease but they could also be the result of host choice by the beetle.