Comparison of methods to detect resistance of Penicillium expansum to thiabendazole

Aims: Because of the lack of a standard method, the aim of this work is to evaluate the suitability of the broth microdilution method CLSI M38-A in determining the resistance level of some Penicillium expansum isolates to thiabendazole (TBZ). The ability of the isolates to produce patulin (PAT) and citrinin (CIT) has been also assessed. Methods and Results: Penicillium expansum isolates (128) were assayed (apples, pears, grapes and five reference strains). It was observed that 69.4% of the strains isolated from apples and pears were resistant to TBZ. Sensitive isolates were inhibited at 0.25-0.5 μg ml ^-1 whilst resistant isolates still grew at 512 μg ml^-1. PAT was produced by all P. expansum isolates. CIT was detected in 98.8% of TBZ-resistant isolates and in 89.1% of the TBZ-sensitive isolates. Conclusions: The preliminary screening method combined with the adaptation of the method CLSI M38-A, can be a good strategy to be used in assessing the in vitro activity of TBZ against a large number of isolates. Significance and Impact of the Study: The proposed methodology can be a contribution to the standardization of susceptibility tests to fungicides against P. expansum.