This study identified sources of fungal spores causing postharvest infection of pome fruit and established relationships between inoculum concentrations of Penicillium expansum and wet core rot of apple. Twenty-four of 35 picking bins contained pathogenic fungi on the wood surfaces, including Alternaria sp., Mucorpiriformis, and five species of Penicillium. Pathogenic fungi were isolated from 9 of 12 packing shed water systems used to dip or dump fruit. P. expansum was recovered most frequently in mean concentrations of 133 and 44 spores/ml in dip and dump water, respectively. Relationships between the natural logarithm of the inoculum concentration of P. expansum and wet core rot were linear. Spores in diphenylamine emulsion caused more core rot than spores in water. This was probably due to increased penetration of the emulsion as well as injury caused to the core tissues by a component of the formulation. The sanitation of bins and packing shed water is important in the control of postharvest decay.