Reduced Sensitivity in Monilinia fructicola to Propiconazole Following Prolonged Exposure in Peach Orchards

The baseline sensitivity of Monilinia fructicola in a peach orchard not previously exposed to demethylation-inhibiting (DMI) fungicides was determined for propiconazole, using the concentration in an agar medium required to suppress radial growth of mycelium by 50% (EC50) The baseline sensitivity was found to be approximately 0.03 μg/ml. Prolonged, regular exposure of the natural population of M. fructicola to propiconazole in the test orchard over a 3-year period (29 total applications) resulted in a wider range of sensitivity (EC50 of 0.02 to 2.16μg/ml) among isolates than was observed in the initial population (EC50 of 0.02 to 0.15 μg/ml). Comparisons with isolates from commercial orchards where DMI fungicides were used regularly showed that sensitivities were comparable to, or less than, those of isolates from the population in the test orchard that had been exposed to propiconazole for the 3-year period. M. fructicola in South Carolina peach orchards might now be less sensitive to DMI fungicides than when those fungicides were first introduced for brown rot control, although effective disease control in the field has been maintained.