Bitter rot of apple has become a major problem in Pennsylvania in recent years. In response to grower requests for management recommendations, research is being conducted to deliver timely science-based management tools, with an initial focus on the characteristics and disease cycles of the causal fungal species. While a state-wide survey of bitter rot apples uncovered six Colletotrichum species, over 75% of the infections were caused by C. fioriniae. The timing of C. fioriniae infections is being investigated by detecting and quantifying C. fioriniae inoculum sources and dispersal patterns throughout the season, and by detecting quiescent C. fioriniae infections in immature apples. Spore dispersal was detected in orchards at full-bloom and continued throughout the season. Spores were detected in nearby forests at quantities as high or higher as in heavily infected orchards, and abundant endophytic C. fioriniae infections were detected in various asymptomatic forest understory plant leaves. Immature apples from untreated and inoculated trees had some quiescent infections 3 weeks post-bloom, increasing to nearly 100% incidence by 7 weeks post-bloom. We have therefore established that C. fioriniae infections can occur early in the growing season in Pennsylvania, which may necessitate season long control efforts. Ongoing research is looking at whether forests are a source of orchard infections, whether early quiescent infections always persist until harvest to cause necrotrophic infections, and whether apples are equally susceptible throughout the growing season.