Rapid Emergence of Boscalid Resistance in Swedish Populations of Alternaria Solani Revealed by a Combination of Field and Laboratory Experiments

Early blight, caused by Alternaria solani, is a common potato disease worldwide. Reduced field efficacy of the fungicide boscalid against this disease has been reported in several countries. Boscalid resistance has been mostly studied with in-vitro and/or greenhouse experiments. Field studies validating this phenomenon are largely missing. Here, for the first time in Scandinavia, we validated boscalid resistance in a Swedish population of A. solani both in the field and in the laboratory. Field trials between 2014 and 2017 in Nymö showed significant efficacy reduction by year. The target regions of the A. solani genes encoding the succinate dehydrogenase subunits (Sdh) B, C and D of samples collected from Nymö, and additional fields in south-eastern and central Sweden, were analysed for substitutions associated with loss of boscalid sensitivity. In 2014, the SdhC-H134R mutation was found at several sites at a low frequency, while, in 2017, the majority of the samples had either the SdhB-H278Y or the SdhC-H134R substitution. No mutations were detected in the gene encoding the SdhD subunit. Spore germination tests showed a high sensitivity (EC50 < 1 μg mL−1) of isolates lacking the substitutions. This was supported by a significant decrease in their radial growth rate, from 0.1 to 10 μg mL−1 boscalid. However, the mutated isolates had EC50 > 100 μg mL−1 and their growth rates hardly decreased at concentrations above 1–10 μg mL−1. These results add to the current knowledge of fungicide resistance development in field and indicate that early blight management in southeast Sweden should no longer rely on boscalid.