Late foliar diseases (especially leaf rust) reduce assimilate supply during post-anthesis, determining fewer assimilates per grain and thereby inducing grain weight reductions. Although the assimilate reduction hypothesis is the most accepted to explain decreases in grain weight due to late foliar diseases, it has not been clearly established whether those reductions could be completely ascribed to source limitations or whether diminished grain weight could be the consequence of reductions in grain weight potential. The objective of this work was to determine whether grain weight reductions due to leaf rust during grain filling could be associated with source–sink limitations. Two experiments (during 2007 and 2008 growing seasons) including healthy and diseased wheat crops were conducted under field conditions. Source–sink manipulation treatments and grain water content measurements were made to test the source- and sink- limitation hypotheses due to the appearance of late foliar diseases during grain filling. Leaf rust was induced to appear exclusively during grain filling, and in both years, it reduced grain yield and grain weight in both experiments. However, except for distal grains, there were no significant differences between healthy and diseased plots in maximum grain water content, indicating that late foliar diseases did not affect the potential size of the grains. The reserves stored in stems were remobilised to the growing grains in both healthy and diseased crops. However, the reserves remaining at physiological maturity were significantly reduced in diseased crops. Reduction in grain number by trimming the spikes increased the grain weight in diseased but not in healthy crops. Grain weight of trimmed spikes in diseased crops reached similar values to those of healthy crops. These results support the hypothesis that foliar diseases could cause source limitation for grain filling beyond differences in grain weight potential when the crops are severely affected by late foliar diseases such as leaf rust.