Early blight of potato (Solanum tuberosum L.) is caused by Alternaria solani Sorauer. The single-site mode of action chemistries of the succinate dehydrogenase inhibitors (SDHIs) and quinone outside inhibitors (QoIs) have been widely used for early blight control but resistance has developed rapidly to a number of fungicide chemistries. QoI resistance in A. solani has been attributed to the F129L mutation, or the substitution of phenylalanine with leucine at position 129. Resistance to SDHI fungicides in A. solani is conferred by five known point mutations on three AsSdh genes. Over 1,300 A. solani isolates collected from 11 states in 2013 through 2015 were characterized for the presence of mutations associated with QoI and SDHI resistance through real-time, SDH multiplex, and mismatch amplification analysis (MAMA) polymerase chain reaction (PCR). Approximately 95% of isolates collected from 2013 to 2015 possessed the F129L mutation. Additionally, 95% of the A. solani population was determined to be SDHI-resistant, with the most prevalent mutation being on the AsSdhC gene. All A. solani isolates collected that were characterized as possessing the D123E mutation, or the substitution of aspartic acid for glutamic acid at position 123 in the AsSdhD gene, were evaluated for boscalid and fluopyram sensitivity in vitro. Furthermore, 15 isolates characterized as being SDHI-sensitive or -resistant, including five D123E-mutant isolates, were evaluated in vivo for percentage disease control to boscalid and fluopyram. Sensitivity of D123E-mutant isolates to fluopyram ranged from 0.8 to 3 ug/ml, and were found to be sensitive or only slightly higher than those of baseline isolates, ranging from 0.1 to 0.6 ug/ml. Disease control of all five D123E isolates evaluated in vivo was reduced significantly with the application of fluopyram compared to SDHI-sensitive isolates. Fitness was compared among 120 SDHI-sensitive and -resistant A. solani isolates using the parameters of spore germination and mycelial growth in vitro and aggressiveness in vivo. Spore germination and mycelial growth between SDHI-sensitive and -resistant isolates was not significantly different. However, D123E-mutant isolates were significantly more aggressive in in vivo assays compared to other SDHI-resistant and SDHI-sensitive isolates. These results illustrate the importance of implementing fungicide resistance management strategies.