The blue mould decay of apples is caused by Penicillium expansum and is associated with contamination by patulin, a worldwide regulated mycotoxin. Recently, a cluster of 15 genes (patA-patO) involved in patulin biosynthesis was identified in P. expansum. BLAST analysis revealed that patL encodes a Cys 6 zinc finger regulatory factor. The deletion of patL caused a drastic decrease in the expression of all pat genes, leading to an absence of patulin production. Pathogenicity studies performed on 13 apple varieties indicated that the PeDpatL strain could still infect apples, but the intensity of symptoms was weaker compared with the wild-type strain. A lower growth rate was observed in the PeDpatL strain when this strain was grown on nine of the 13 apple varieties tested. In the complemented PeDpatL:patL strain, the ability to grow normally in apple and the production of patulin were restored. Our results clearly demonstrate that patulin is not indispensable in the initiation of the disease, but acts as a cultivar-dependent aggressiveness factor for P. expansum. This conclusion was strengthened by the fact that the addition of patulin to apple infected by the PeDpatL mutant restored the normal fungal colonization in apple.