Phytophthora infestans, the causal agent of late blight, is one of the most devastating and widespread pathogens affecting potato and tomato crops across the globe. US-23 (A1 mating type) has been the dominant P. infestans clonal lineage in the Midwest during 2017 and 2018 and is currently controlled with the fungicide mefenoxam. With the re-emergence of clonal lineage US-8 (A2 mating type), shown to be highly tolerant to mefenoxam and of an opposing mating type to the common US-23, there is a need for continual monitoring and isolate characterization in order to optimize control. Twenty eight P. infestans isolates collected from production fields of the Midwestern U.S. during 2017 and 2018 were tested for sensitivity to mefenoxam (100ppm) using amended plate assays. Relative growth was calculated from pathogen growth on control and amended plates. Overall, there was greater variation in relative growth on mefenoxam-amended media among isolates of the US-8 clonal lineage than isolates of US-23. In 2017, relative growth responses of US-8 isolates ranged from approximately 25 to 100%, while the US-23 isolates predominantly ranged from 10-25% (Tukey’s HSD Test). Among the ten 2017, US-23 isolates, a single isolate demonstrated complete insensitivity and was significantly different in its growth response when compared to the other nine. In 2018, there were no significant differences in growth response among all of the US-23 isolates. The finding of a mefenoxam-insensitive US-23 isolate underscores the need for continual pathogen monitoring in support of grower decision-making.