Brown rot caused by Monilinia fructicola and M. laxa can result in considerable losses in peach production worldwide. Respiratory Inhibitors (RIs) have been used to control the disease in Turkey and worldwide. However, resistance against RIs may have been developing in Monilinia populations in Turkey. The sensitivity level of the total of 128 isolates of Monilinia fructicola and M. laxa from peach fruits in Turkey were tested against certain RIs: azoxystrobin, boscalid, and commercially used signum (26,7% boscalid, 6,7% pyraclostrobin). Fungal growth from both mycelia and conidia was evaluated as fungicide response phenotypes. A new approach was developed to define the sensitivity levels of the isolates. The mean IC50 values of each fungicide on representative collection from each species were used as discriminatory doses in view of both phenotypes. The whole collection was scanned at discriminatory doses and the relative growth (RG) of each isolate was obtained. The distribution of RG data sets in the quantile ranges was considered to define sensitivity levels of the isolates against each fungicide. Resistance levels for mycelial and conidial growth assays varied within and between the species against the three RIs. According to the pairwise correlation analyses between responses of the species to the fungicides, the significant correlation indicating a cross-resistance was only found between boscalid and signum on conidial growth assays for M. fructicola. This study represents in vitro fungicide responses of a large population of Monilinia pathogens from peach in Turkey for the first time which will help to reconsider disease control strategies and suggests a fungicide sensitivity definition be used in all in vitro fungicide assays as a common and comparable strategy at the global stage.