• Elena Levin - Department of Postharvest Science, ARO, The Volcani CenterRishon LeZionn, Israel
• Samir Droby - Department of Postharvest Science, ARO, The Volcani Center, Rishon LeZion, Israel
• Michael Wisniewski - Appalachian Fruit Research StationU.S. Department of Agriculture – Agricultural Research Service, Kearneysville USA.
• Christopher Dardick - Appalachian Fruit Research StationU.S. Department of Agriculture – Agricultural Research Service, earneysville USA.

Penicillium expansum is a ubiquitous postharvest pathogen that infects a wide range of fruits, including pome and a variety of stone fruit. Virulence factors that mediate pathogenicity in postharvest-decay fungi have not been fully defined, especially in regard to if and how the pathogen is able to modulate fruit defense mechanisms, particularly at the initial stages of infection. In this chapter, we present an extensive analysis of the potential effectors of P.expansum that may be involved in its pathogenicity on apple fruit and examine their role in suppressing fruit resistance mechanisms in the initial stages of infection. The ability of P. expansum to secrete factors that are able to down-regulate ROS production was initially assessed. Results demonstrated that there are proteins secreted by P. expansum that are able to down-regulate host-response to presence of the pathogen. A combination of different bioinformatic and genetic approaches were used to identify and characterize potential effectors in P. expansum. In particular, the small cysteine-rich proteins, Pescr1 and Pescr2, were investigated. Additionally, the role of LysM proteins, NLPs, and one proteolytic enzyme, peptidase S8, in the virulence of P. expansum was also examined.