Identification of succinate dehydrogenase inhibitor-resistance mutations, T78I, N85K/S, and H151R, in the SdhC gene in the tomato leaf mold pathogen, Passalora fulva

Abstract

Tomato leaf mold caused by Passalora fulva is a significant disease in tomato production. We isolated several types of boscalid-resistant isolates in the Gifu and Mie Prefectures of Japan. Sequencing analysis of succinate dehydrogenase (Sdh) subunits B, C, and D genes strongly indicated that four amino acid substitutions—T78I, N85K, N85S, and H151R in SdhC—conferred boscalid resistance. We conducted SNP assays to detect each mutation using qPCR techniques and revealed that all 35 resistant isolates had one of these mutations in the SdhC. Among the four resistance types, N85K isolates exhibited the highest, N85S isolates showed the lowest, and T78I and H151R isolates displayed moderate resistance to boscalid. These mutations also conferred cross-resistance to other succinate dehydrogenase inhibitor (SDHI) fungicides, including penthiopyrad, pyraziflumid, fluopyram, and isofetamid. A predicted SdhC protein structure, created by I-TASSER, suggests that the amino acid at position 151 is located close to those of positions 78 and 85, likely forming the SDHI-binding pocket of the protein.