ASSESSING FITNESS COST IN PENICILLIUM EXPANSUM ISOLATES WITH RESISTANCE TO MULTIPLE FUNGICIDES

Jonathan Thomas Puglisi

doi:10.7273/000007908

Blue mold, primarily caused by Penicillium expansum, is the most economically significant postharvest decay of pome fruit in the Pacific Northwest (PNW) and globally. The disease leads to losses directly through fruit decay and indirectly through contamination of processed products with the mycotoxin patulin (PAT). Management of blue mold typically combines rigorous sanitation practices with postharvest fungicide application, i.e., thiabendazole (TBZ), pyrimethanil (PYR), fludioxonil (FDL), and difenoconazole. However, frequent fungicide use has contributed to the emergence of P. expansum populations resistant to TBZ, PYR, and FDL in the PNW. Compounding this issue, high PAT levels were reported in several processed apple lots in Washington State during 2021 and 2022. The relationship between fungicide resistance, fungal fitness, and PAT production in P. expansum remains unclear. This study addresses this gap by assessing the fitness and PAT production in vitro and on apples of 15 P. expansum isolates, ranging from fungicide-sensitive isolates to single-, dual-, or triple-resistant to TBZ, PYR, and FDL, collected from packinghouses in the PNW. In vitro, resistant isolates exhibited reduced germination on nutrient-poor media, while the isolates resistant to PYR and FDL (PYRRFDLR) and those resistant to the three fungicides (TBZRPYRRFDLR) showed increased susceptibility to osmotic and oxidative stress. However, on nutrient-rich media, no obvious fitness costs were observed for germination, mycelial growth, sporulation, and PAT production between the isolates with different fungicide sensitivity phenotypes. Likewise, virulence, sporulation, or PAT production on detached apples were not significantly influenced by fungicide resistance profiles. Cumulative fitness assessments revealed that most resistant phenotypes exhibited reduced fitness under low-temperature in vitro and increased fitness under fungicide selection pressure conditions in vivo. Additionally, the expression of several fitness-related genes varied in response to both fungicide elicitation and fungicide sensitivity. The findings from this study indicate that while the evolution of fungicide resistance may be associated with certain fitness costs, it does not impair the pathogen’s ability to cause decay loss or pose mycotoxigenic risks in processed products under commercial conditions.

ASSESSING FITNESS COST IN PENICILLIUM EXPANSUM ISOLATES WITH RESISTANCE TO MULTIPLE FUNGICIDES

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