One major challenge in parasitology and epidemiology is determining whether the richness of parasitic and infectious diseases simply tracks host diversity or is largely determined by exogenous factors, such as climate-forced variables. We addressed this issue by analysing a 30-year survey of fungal diseases in French forests. We first combined generalized linear models and stepwise analyses to select the habitat descriptors that may account for variations in parasitic fungal species richness. Our results suggest that host species diversity is not a major determinant of parasite richness. Temperature seasonality, host abundance, and the composition of host species assemblages may play a greater role. Then we used structural equation modelling to investigate the links between these habitat descriptors and parasitic fungal species richness. Our results showed that climatic and host species descriptors had not only direct effects on species richness, but also indirect effects (via host species and sampling effort, respectively). Our results also showed that the direct effects of climate and host species were roughly equal. We therefore conclude that it is important to take into account exogenous factors when investigating the potential causes of spatial variation in the richness of parasitic diseases, in particular for plant diseases.