Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/21/2014
Publication Date: 6/4/2014
Citation: Caiazzo, R., Kim, Y.K., Xiao, C. 2014. Occurrence and phenotypes of pyrimethanil resistance in penicillium expansum from apple in Washington state. Plant Disease. 98:924-928. Interpretive Summary: Apple fruit may rot during storage or in the market if fruit rot diseases are left uncontrolled. The fungus Penicillium expansum is the cause of blue mold, a major postharvest fruit rot disease of apples. Pyrimethanil is a fungicide that was registered in 2004 in the United States for postharvest use to control blue mold and other rots on apples. In this study, we reported that P. expansum has developed resistance to pyrimethanil with resistance levels ranging from low to high. Pyrimethanil can no longer provide satisfactory control of blue mold incited by the resistant strains of the pathogen, but another postharvest fungicide fludioxonil is effective in controlling blue mold incited by pyrimethanil-resistant P. expansum.
Technical Abstract: Penicillium expansum is the primary cause of blue mold of apple. Pyrimethanil is a recently registered postharvest fungicide for control of postharvest diseases in apple. To monitor pyrimethanil resistance, 779 isolates of P. expansum were collected from decayed apple fruit in 2010 and 2011 from five packinghouses and tested for resistance to pyrimethanil, fludioxonil and thiabendazole. Pyrimethanil resistance was 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. Postharvest fungicides were evaluated for effectiveness in controlling blue mold on apple fruit inoculated with representative isolates of low, moderate and high pyrimethanil-resistance phenotypes. In 2010, 85% and 7% of the isolates were resistant to pyrimethanil in packinghouse A and B, respectively, where pyrimethanil had been used for 4-5 consecutive 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 substantially reduced the frequency of pyrimethanil resistance. In packinghouse B, the frequency of pyrimethanil resistance was reduced to 1% when fludioxonil was used. No pyrimethanil-resistant isolates were detected in 2010 in the 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. No fludioxonil resistance was detected in either year. A significantly higher percentage of thiabendazole-resistant isolates than that of thiabendazole-sensitive isolates was resistant to pyrimethanil. Of the pyrimethanil-resistant isolates tested in the two years, 37-52%, 4-5% and 44-58% were phenotyped as low, moderate, and high pyrimethanil resistance, respectively. Fludioxonil was effective in controlling pyrimethanil resistant phenotypes on apple fruit, but pyrimethanil failed to control blue mold incited by moderate or high pyrimethanil-resistance phenotypes and only partially controlled the low resistance phenotype.