Early blight and brown spot, caused by Alternaria solani and Alternaria alternata, respectively, are important foliar diseases of potato, affecting both tuber yield and quality. Most of the commercial cultivars lack resistance; therefore, application of foliar fungicides remains a primary management strategy. Correlation coefficients comparing EC50 values for conidial germination and mycelial growth of A. solani and A. alternata in response to boscalid and fluopyram, respectively, were low, indicating that the association between these two in vitro assays was very weak. Baseline sensitivities of Alternaria spp. to difenoconazole, metconazole, and pyrimethanil using mycelial growth assays demonstrated high intrinsic activity against the two pathogens. Six out of 245 A. solani isolates exhibited reduced-sensitivity to pyrimethanil in in vitro assays and reduced-sensitive isolates were not controlled except at 100 μg/ml in greenhouse efficacy tests. The DMI chemistries and pyrimethanil remain valuable options for fungicide rotation programs in areas of high disease pressure.