To assess the impact growth substrate has on the resulting sporangia, field and laboratory based inoculation experiments were carried out using three isolates of Phytophthora infestans clonal lineage US-23. Isolates were grown in pure culture on agar plates as well as on detached Solanum lycopersicum leaflets in moist chambers. Sporangia harvested from each of these culture types and for each isolate was used as inoculum in two independent field trials and one laboratory based trial in the summer of 2015. Disease assessment at 144 hours after inoculation was very consistent across all trials. In every case, culture derived sporangia produced significantly less disease than sporangia of the same isolate washed from sporulating leaflets. Genes associated with necrotrophy and host cell death were up regulated in culture grown sporangia, while genes associated with biotrophic growth and disease were more up regulated in sporangia from leaflets.