Technical Abstract: The potato gene Rpi-blb1, from the wild potato species S. bulbocastanum, confers partial resistance to late blight, caused by the oomycete pathogen Phytophthora infestans. In order to determine whether a single strain of P. infestans can adapt to overcome this partial resistance source, we subjected Rpi-blb1 containing plants to multiple rounds of infection with P. infestans, with sporangia from a late blight lesion used to infect the next leaflet. A parallel line of inoculations was done using susceptible leaflets. At the end of the experiment, sporangia passaged through resistant or susceptible leaflets were compared for their ability to cause disease. Variants of the corresponding P. infestans effector IPI-O, which is recognized by Rpi-blb1 to elicit resistance, were also cloned and sequenced to determine whether variation occurred after selection on the partially resistant host. After 20 rounds of selection, no breakdown in Rpi-blb1 resistance was observed. In fact, the strain passaged through the partially resistant host produced smaller lesions on susceptible leaflets and had a lower infection frequency than the strain passaged through susceptible cultivar Katahdin. Our results indicate that continual selection by the Rpi-blb1 gene can reduce P. infestans virulence.