Field isolates of Monilinia fructicola Change resistance pattern to greater sensitivity to thiophanate-methyl in recent populations

The objectives of this study were to characterize Monilinia fructicola isolates that cause brown rot in stone fruit in Brazil according to resistance patterns to thiophanate-methyl (TM) and aggressiveness in detached fruit, comparing two sets of population. A total of 135 M. fructicola isolates were collected from three states in Brazil and divided in two sets: historic population (2003-2010) and 2017 population. Based on nucleotide substitutions at codon 6 and 198 in beta-tubulin gene and culture medium growth at 1 mu g mL(-1) and 500 mu g mL(-1) of thiophanate-methyl, the isolates were classified as: Sensitive (S), Low Resistant (LR) and Highly Resistant (HR). HR and LR isolates, confirmed by the presence of E198A and H6Y mutations, respectively, were found more frequently in the historical population (8.6% HR and 39.6% LR) than in 2017 (0% HR and 4% LR). Interestingly, the frequency of S isolates in 2017 increased three-fold (29 to 96%) compared to the historic population. Thirteen isolates were classified as S-LR, with no mutation detected in the partial beta-tubulin gene. The ex vivo assays with two peach cultivars confirmed that the LR and HR isolates were equally pathogenic and aggressive in the absence of TM. TM completely prevented M. fructicola sporulation and effectively reduced the incidence and lesion size of brown rot either for S and S-LR isolates in both cultivars. Therefore, the possibility of re-registration of thiophanate-methyl for the management of brown rot in Brazil was discussed.