| Summary: | Blue mold caused by <i>Penicillium italicum</i> is one of the two major postharvest diseases of citrus fruits. The interactions of pathogens with their hosts are complicated, and virulence factors that mediate pathogenicity have not yet been identified. In present study, a prediction pipeline approach based on bioinformatics and transcriptomic data is designed to determine the effector proteins of <i>P. italicum</i>. Three hundred and seventy-five secreted proteins of <i>P. italicum</i> were identified, many of which (29.07%) were enzymes for carbohydrate utilization. Twenty-nine candidates were further analyzed and the expression patterns of 12 randomly selected candidate effector genes were monitored during the early stages of growth on PDA and infection of Navel oranges for validation. Functional analysis of a cell wall integrity-related gene <i>Piwsc1,</i> a core candidate, was performed by gene knockout. The deletion of <i>Piwsc1</i> resulted in reduced virulence on citrus fruits, as presented by an approximate 57% reduction in the diameter of lesions. In addition, the mycelial growth rate, spore germination rate, and sporulation of Δ<i>Piwsc1</i> decreased. The findings provide us with new insights to understand the pathogenesis of <i>P. italicum</i> and develop an effective and sustainable control method for blue mold.
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