Abstract Background and AimMusts and wines produced from rotten grapes often have an earthy/musty odour, with geosmin the responsible compound. Penicillium expansum is considered a potential source of geosmin in rotten grapes from vineyards treated with copper-based fungicides. Methods and ResultsThe laboratory study assessed the influence of temperature (10–30°C), copper concentration (0–76.50 mg/L) and CO2 in the headspace (0.03–3%) on the spore count and the production of geosmin by P. expansum according to a Doehlert design. The spore count and the production of geosmin (ng/mg biomass) were significantly correlated (r = 0.78). Copper had no significant effect on the spore count but was the most important factor for explaining geosmin production. The production of geosmin was enhanced at low temperature (15°C), 0.03% CO2 (i.e. atmospheric level) and high copper concentration (76.50 mg/L). ConclusionPenicillium expansum, grown on Czapek agar, produced a significant amount of geosmin at low temperature and in the presence of copper. Significance of the StudyThis study suggests a possible explanation for the occurrence of earthy/musty odours in musts and wines made from rotten grapes.