Continued sub-culturing of Phytophthora infestans on artificial media results in virulence reduction. Typically, inoculation of the pathogen back on to host plant tissue restores virulence. This response suggests that the change is not due to genetic mutation but may be the result of a more sophisticated and fluid method of regulation. We investigated the virulence of P. infestans after continued sub-culturing on artificial media and analyzed changes in effector expression within the pathogen at key stages. The isolate was sub-cultured (6X) and maintained at 18°C on Rye-A medium over 4 months. Sporangial suspensions of the 0, 3^rd, and 6^th sub-cultured isolates were inoculated onto susceptible (‘Katahdin’ potato) and resistant (‘Katahdin’ potato transformed with RB). Disease severity was determined and lesion size was measured. The mixed model generated for lesion development showed that sub-culturing 3 times resulted in a non-significant 1% decrease in the likelihood of lesion development while sub-culturing 6 times resulted in a significant 22% decrease in the probability of lesion development (p-value 8.36e-11). RNA was extracted from inoculated plants and RT-qPCR analyses were conducted to assess changes in effector expression. At 72 hours post inoculation, the relative expression of the Avr3 effector was significantly reduced by an average of roughly 5-fold from the 0 sub-culture event to the 3^rd, with no significant change in expression to the 6^th. Further work is underway to continue elucidating this host-pathogen interaction.