Spatial and Temporal Distribution of Mutations Conferring QoI and SDHI Resistance in Alternaria solani Across the United States.

The application of succinate dehydrogenase inhibiting (SDHI) and quinone outside inhibiting (QoI) fungicide chemistries is a primary tactic in the management of early blight of potato, caused by Alternaria solani. Resistance to QoIs in A. solani has been attributed to the F129L mutation, while resistance to SDHIs is conferred by five different known point mutations on three AsSdh genes. In total, 1,323 isolates were collected from 2013 through 2015 across 11 states to determine spatial and temporal frequency distribution of these mutations. A real-time polymerase chain reaction (PCR) was used to detect the presence of the F129L mutation. Molecular detection of SDHI-resistant isolates was performed using SDH multiplex PCR specific for point mutations in AsSdhB, AsSdhC, or AsSdhD genes and mismatch amplification analysis PCR detecting the point mutations in AsSdhB. Previous work in our research group determined that substitutions of histidine for tyrosine (H278Y) or arginine (H278R) at codon 278 on the AsSdhB gene were the most prevalent mutations, detected in 46 and 21% of A. solani isolates, respectively, collected in 2011 to 2012, and uniformly distributed among six sampled states. In contrast, the substitution of histidine for arginine (H134R) at codon 134 in the AsSdhC gene was the most prevalent mutation in 2013 through 2015, identified in 36% of isolates, compared with 7.5% of isolates recovered in 2011 to 2012. Substitutions of histidine for arginine (H133R) at codon 133 and aspartic acid for glutamic acid (D123E) at codon 123 in the AsSdhD gene were detected in 16 and 12%, respectively, in the A. solani population by 2015 and were recovered across a wide range of states, compared with 15 and 1.5% of isolates collected in 2011 to 2012, respectively. Overall, SDHI- and QoI-resistant isolates were detected at high frequencies across all years, with evidence of significant spatial variability. Future research will investigate whether these results are due to differences in parasitic fitness.