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This study aimed to investigate the effect of copper sulfate (from 0 to 8 mmol kg−1) on radial growth rate and lag time of two moulds responsible for vine grapes spoilage: Penicillium expansum strain 25·03 and Botrytis cinerea, strains BC1 and BC2.A new model was developed to describe tailing and shoulders in the inhibition curves. Because of tailing, the minimum inhibitory concentration (MIC), was not defined as the concentration at which no growth was observed, but as the concentration at which the lag time was infinite. The concentrations at which μ = μopt/2, (Cu50), were in the range of 2·2–2·6 mmol kg−1. Radial growth rate of P. expansum and the reciprocal of the lag time were linearly correlated (r = 0·84). In contrast, in the range 0–4 mmol kg−1, an inhibition of growth of B. cinerea was observed whereas germination remained unaffected (i.e. the lag time was constant). In the range 4–8 mmol kg−1, the radial growth rate of B. cinerea was almost constant (c. 1 mm day−1), but germination was inhibited (i.e. the lag time was increased).The MIC values were 4·7 mmol kg−1 for P. expansum, 8·2 and 7·3 mmol kg−1 for B. cinerea strain BC1 and BC2, respectively, demonstrating that some isolates of these moulds are resistant to copper.Copper concentrations at 4 mmol kg−1 would be sufficient to control the development of these isolates, but the toxicity of copper should be extended to other isolates and evaluated in vineyards.