Biotrophic fungi divert assimilates for their growth and thus compete with their host for carbon allocation. Bancal et al. modulate competition by shading the flag leaf of the wheat host, Triticum aestivum, and/or by varying the inoculation level of the fungi, Puccinia triticina, and then use a mixed experimental–modelling approach to combine sporulation data with an existing carbon partitioning (source–sink) model for wheat during the grain filling period. They find that fungal sporulation has a competitive priority for assimilates over grain filling, making it quite insensitive to variations in incoming radiation. Plant carbohydrate reserves are also tapped into by the combined fungi–grain sinks more frequently than by the grain sink alone, resulting in a decrease in the duration of grain filling.