Differential effect of triazoles on mycelial growth and disease measurements of Monilinia fructicola isolates with reduced sensitivity to DMI fungicides

Demethylation-inhibitor fungicides (DMIs) are key components for managing Monilinia fructicola populations, even those with demonstrated reduced DMI sensitivity because few effective alternatives are available. Rotational fungicide programs that include DMIs would therefore benefit from selection of the most effective DMI. The goal of this study was to investigate the in vitro efficacy and in vivo potential of three triazole fungicides to reduce brown rot disease caused by M. fructicola isolates sensitive or with reduced sensitivity to DMIs. Baseline isolate SCDL71.01 was sensitive to propiconazole, tebuconazole, and fenbuconazole with mean EC50 values ranging from 0.018 to 0.023 μl ml−1. Single spore isolates collected from an orchard in middle Georgia (GADL) were significantly less sensitive to all three triazoles compared to SCDL71.01; however, tebuconazole and fenbuconazole were significantly more effective compared to propiconazole in vitro. Protective and curative applications of each triazole on SCDL71.01 inoculated fruit prevented disease development. However, there were significant differences in efficacy of these triazoles when fruit were inoculated with GADL isolates. In protective applications, fenbuconazole and tebuconazole were significantly (P<0.05) more effective compared to propiconazole in reducing important disease measurements of GADL isolates. In curative applications, tebuconazole alone was most effective (P<0.05) in reducing disease measurements of GADL isolates. Both, in vitro and in vivo results indicate differential performances among triazoles against isolates with reduced sensitivity to DMIs.