Sensitivity of Alternaria spp. from potato to pyrimethanil, cyprodinil, and fludioxonil

Early blight, caused by Alternaria solani, and brown leaf spot, caused by a number of small-spored Alternaria spp. including Alternaria alternata sensu stricto, Alternaria arborescens, and Alternaria tenuissima, are observed annually in all midwestern potato production areas. However, Alternaria spp. have developed reduced sensitivity and/or resistance to many single-site mode of action fungicides such as quinone outside inhibitor (QoI; FRAC group 11), succinate dehydrogenase inhibitor (SDHI; FRAC group 7), and anilinopyrimidine (AP; FRAC group 9). Mean in vitro sensitivity EC50 values (effective concentration where fungal growth is inhibited by 50%) of A. alternata (n = 16), A. arborescens (n = 3), A. tenuissima (n = 5), and A. solani (n = 58) in response to the AP fungicides pyrimethanil and cyprodinil and the phenylpyrrole (PP) fungicide fludioxonil were determined via mycelial growth assays. Significant fungicide by isolate interactions were observed for all Alternaria spp. evaluated in vitro, indicating reduced-sensitivity of some isolates to individual fungicides. EC50 values for three non-baseline A. solani isolates collected in 2010, 2011 and 2013 were within the baseline for all three fungicides. A significant correlation was observed between pyrimethanil and cyprodinil EC50 values among A. alternata isolates, but no relationship was observed with the other fungicides or in A. solani. In greenhouse evaluations, a significant loss of disease control was observed for some non-baseline A. solani isolates, and this was more pronounced in the AP fungicides, pyrimethanil and cyprodinil. No significant correlation was observed between in vitro EC50 value and area under the dose response curve based on greenhouse assays, likely due to the limited number of isolates evaluated. Further research is needed to determine if these reductions affect control of early blight and brown leaf spot in potato under field conditions. Results from this study indicate that fludioxonil and cyprodinil are potentially good additions into fungicide rotation programs or as co-pack chemistries for control of leaf spot diseases and fungicide resistance management.