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Penicillium expansum is the causal agent of blue mold rot, a postharvest decay of stored fruits. This fungus also produces the mycotoxins patulin and citrinin. Control of P. expansum still relies mainly on the use of fungicides such as thiabendazole. Since its introduction, resistant strains have been reported. The aim of this work was to investigate the thiabendazole resistance and mutations in the beta-tubulin gene of P. expansum strains isolated from apples and pears with blue mold decay from Spain. A total of 71 strains of P. expansum were scored for resistance to thiabendazole and the beta-tubulin gene was sequenced. Out of 71 strains, 37 were sensitive and 34 were resistant to thiabendazole. Regarding the beta-tubulin gene sequence, 10 different genetic types were determined, with a 99.7-100% similarity. When the amino acid sequence was deduced, five different amino acid sequences were found. All except one of the sensitive strains lacked mutations in the region sequenced. Of the 34 resistant strains, only eight had mutations that involved the residues 198 and 240. All the strains with mutations at position 198 always corresponded to resistant isolates. However, a high percentage of resistant strains had no mutations in the region of the beta-tubulin gene sequenced, and so other mechanisms may be involved in thiabendazole resistance.

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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 69AE4% of the strains isolated from apples and pears were resistant to TBZ. Sensitive isolates were inhibited at 0AE25-0AE5 lg ml )1 whilst resistant isolates still grew at 512 lg ml )1 . PAT was produced by all P. expansum isolates. CIT was detected in 98AE8% of TBZ-resistant isolates and in 89AE1% 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.