Sensitivity of Monilinia fructicola from stone fruit to thiophanate-methyl, iprodione, and tebuconazole

Sensitivity in Monilinia fructicola to three fungicides was determined by measuring mycelial growth in fungicide-amended media. Resistance to thiophanate-methyl was found in 39 of 52 isolates (75%) collected from 1992 to 1998 (historic population) and 22 of 100 isolates (22%) collected in 2002 (current population). Three groups having distinct ranges of values for 50% effective concentration (EC50) to thiophanate-methyl were identified. Benzimidazole-sensitive (benS) isolates had EC50 values less than 2.0, low-resistant (benL) isolates between 2.0 and 30.0, and high-resistant (benH) isolates greater than 30.0 mug/ml. One (2%) isolate from the historic and three (3%) isolates from the current population were benH. Inoculation of untreated nectarine blossoms with four isolates from each of the three groups, individually or combined using equal numbers of conidia from each isolate, showed that the benS, benL, and benH isolates were equally pathogenic and competitive. Use of thiophanate-methyl at 300 mug a.i./ml (half dosage) and 600 mug a.i./ml (full dosage) effectively reduced the percentage of blighted blossoms caused by the benS group but not that caused by the benL and benH groups. The benH isolates caused significantly greater percentage of blighted blossoms than the benL isolates at both dosage levels. None of the tested isolates of M. fructicola were resistant to either iprodione or tebuconazole.