Abstract

The succinate dehydrogenase inhibitor (SDHI) fungicide boscalid is an excellent broad-spectrum fungicide but has not been registered in China to control Penicillium digitatum , the causal agent of green mold of citrus. The present study evaluated the risk and molecular mechanisms for boscalid resistance in P. digitatum . Resistance induction with four arbitrarily selected sensitive isolates of P. digitatum by ultraviolet (UV) irradiation on conidia plated on boscalid-amended potato dextrose agar (PDA) and consecutive growing on boscalid-amended PDA produced five highly resistant isolates with EC 50 values greater than 1000 μg/mL and two resistant isolates with EC 50 lower than 200 μg/mL. Boscalid resistance of the five mutants with EC 50 values above 1000 μg/mL was stable after successive transfers on PDA for 16 generations. However, for the other two mutants with EC 50 lower than 200 μg/mL, the EC 50 values decreased significantly after successive transfers. There was significant cross-resistance between boscalid and carboxin ( r  = 0.925, P  < 0.001), but no significant cross-resistance was detected between boscalid and fludioxonil ( r  = 0.533， P  = 0.095) or between boscalid and prochloraz ( r  = −0.543， P  = 0.088). The seven resistant mutants varied greatly in the mycelia growth, sporulation , pathogenicity , and sensitivities to exogenous stresses including NaCl, salicylhydroxamic acid (SHAM), and H 2 O 2 . Alignment of the deduced amino acid sequence showed that there was no point mutation in the target enzyme succinate dehydrogenase (Sdh) subunits SdhA, SdhC, or SdhD in each of the seven resistant mutants, and the mutation of a conserved histidine residue to tyrosine (H243Y) in the subunit SdhB (i.e., iron‑sulfur protein) occurred in only three highly resistant isolates. Molecular docking indicated that mutation H243Y could n