Assessing boscalid efficacy and resistance of large-spored Alternaria pathogens of potato and tomato crops

Abstract Large-spored Alternaria species of the section Porri were evaluated for resistance to boscalid through laboratory assays. Most isolates had EC50 values above 100 µg/mL, indicating widespread fungicide resistance. Only isolates of A. solani demonstrated sensitivity with EC50 values below 7 µg/mL. Isolates of A. linariae, A. protenta and A. alternariacida were resistant. A moderate level of resistance to the fungicide was found in A. grandis (mean EC50 = 76,8 µg/mL). Even though the resistance levels varied among strains, sampling location did not appear to be a significant factor. One of the primary targets of boscalid within fungal cells is the succinate dehydrogenase (SDH) enzyme complex, which consists of several subunits. Mutations in these genes have been identified to be a key mechanism of boscalid resistance in various fungal pathogens. Our study revealed the absence of H134R and H278Y mutations within the SdhB gene among the Alternaria isolates. We further analyzed the sequences from Sdh gene subunits B, C, and D in the isolates originating from potato, including A. alternariacida, A. grandis, A. linariae, A. protenta, and A. solani. This analysis identified several single nucleotide polymorphisms (SNPs) that effectively distinguished these isolates from the tomato-derived A. linariae ones.