Abstract: Polyploids are naturally ubiquitous throughout the plant kingdom. Their abundance indicates that polyploidy confers an adaptive advantage. In the case of artificially mitotically-doubled polyploid plants, some studies have shown an increased resistance to biotic and abiotic stresses compared to their diploid progenitors. In a previous study we observed that under water deficit conditions tetraploid (4x) plants show a higher leaf water status and a higher photosynthetic rate compared to their diploid (2x) progenitors. These results suggest that 4x plants could be more tolerant of water deficit. Based on these observations and previous studies we expected that polyploidisation could also be advantageous in the response of apple to biotic stress caused by pathogens. Hence the main objective of the present study was to determine if polyploidisation has an effect on the tolerance of apple plants to apple scab (Venturia inaequalis). Three susceptible (Gala, G40 and G61) and one Rvi6 resistant genotype (G58) were artificially doubled from nodal segments using colchicine. Visual evaluation of infection severity, according to the five ‘Chevalier classes’, showed a higher susceptibility in 4x plants for the three susceptible genotypes. An additional assessment of the sporulation showed also a clear increase in sporulation area on 4x plants compared to the 2x plants. The 4x plants of the Rvi6 resistant genotype showed a clear decrease in infection severity compared to the 2x plants. No sporulation was observed in this genotype. Based on these observations, polyploidisation has a clear impact on the susceptibility or tolerance to Venturia inaequalis compared to the diploid state of the host. Polyploidisation of susceptible genotypes resulted in a higher scab susceptibility, while polyploidisation of an Rvi6 resistant genotype resulted in a higher scab tolerance. Further physiomorphological and molecular research is however necessary to elucidate the mechanisms behind this changed tolerance.