The blue mold decay of apples is caused by Penicillium expansum and associated with the contamination of patulin, a worldwide regulated mycotoxin. Recently, a cluster of the 15 genes (patA-patO) involved in patulin biosynthesis was identified in P. expansum. BLAST analysis revealed that patL encodes a Cys6 zinc finger regulatory factor. The deletion of patL caused a drastic decrease of the expression of all pat genes which led to an abscence of patulin production. Pathogenicity studies performed on thirteen apple varieties indicated that the PeΔpatL strain can still infect apples, but the intensity of symptoms compared to the wild type strain is weaker. A lower growth rate is observed in PeΔpatL strain when this strain grows on nine of the thirteen apple varieties tested. In the complemented PeΔpatL:patL strain, the ability to normally grow in apple and the patulin production are restored. Our results clearly demonstrate that patulin is not indispensable in the installation of the disease but acts as a cultivar dependent aggressiveness factor for P. expansum. This conclusion is strengthened by the fact that the addition of patulin on apple infected by the PeΔpatL mutant restores the normal fungal colonization in apple.