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<i>Penicillium expansum</i> is the primary cause of blue mold of apple. Pyrimethanil is a recently registered postharvest fungicide for decay control. Resistance to pyrimethanil has emerged in <i>P. expansum</i>. To monitor and characterize pyrimethanil resistance, blue mold-decayed fruit were collected in 2010 and 2011 from 5 packinghouses and the causal agents were identified to species. Isolates of <i>P. expansum</i> were tested for resistance and further classified as low resistance if conidia germinated at 0.5 µg/ml but not 10 µg/ml, moderate resistance if germinated at 10 µg/ml but not 40 µg/ml, and high resistance if germinated at 40 µg/ml. In 2010, 85% and 7% of the isolates were resistant in packinghouse A and B, respectively, where pyrimethanil had been used for 4-5 years. In 2011, either pyrimethanil or fludioxonil was used in packinghouse A, and 96% of the isolates from the fruit treated with pyrimethanil were resistant but only 4% of the isolates from the fruit treated with fludioxonil were resistant to pyrimethanil, suggesting fungicide rotation helped reduce resistance frequency. In packinghouse B, resistance frequency was reduced to 1% when fludioxonil was used instead of pyrimethnail. No resistant isolates were detected in 2010 in other 3 packinghouses where the fungicide was just recently used at a small scale, but 1% of the isolates from one of the three packinghouses in 2011 were resistant to pyrimethanil. Of the resistant isolates tested, 37-52%, 4-5% and 44-58% were phenotyped as low, moderate, and high resistance, respectively.<p><p>Keywords: Fungus, Fruits-Nuts, Apple