The genome of the fungal phytopathogen Ustilago maydis contains several large gene clusters encoding putative secreted effector proteins. Brefort et al. present a functional analysis of the largest cluster, cluster 19A, which encodes 24 effectors. Although the virulence of a U. maydis strain in which cluster 19A has been deleted is severely attenuated, it can still complete the biotrophic life cycle. Specific functions for several effectors were identified and although each effector analysed makes a modest contribution to virulence, the analysis indicated that they have distinct plant cell targets.