The interaction between Alternaria dauci and two carrot cultivars differing in their resistance to leaf blight was investigated by microscopy. The fungal development between 1 and 15 days post-inoculation was quite similar in the susceptible cv. Presto and the partially resistant cv. Texto: After conidial germination, leaf adhesion of the pathogen was achieved with mucilaginous filaments; hyphae penetrated the leaves directly with/without the formation of appressoria-like structures or via stomata; the fungus spread by epiphytic hyphae with hyphopodia and subcuticular mycelia. Intense necrotic plant cell reactions occurred under the fungal structures. At 21 days post-inoculation, typical features of fungal development were noted for each cultivar: growing hyphae emerged from stomata in cv. Presto, whereas conidiophores without conidia were observed in cv. Texto. Leaf tissues of both cultivars were strongly damaged and vesicle-like structures (assumed to be plant phenolics) were abundantly present between mesophyll cells. A real-time PCR method was developed for in planta quantification of A. dauci. Between 1 and 15 days post-inoculation, the fungal biomass was equivalent in the two cultivars and was about fourfold higher in cv. Presto than cv. Texto at 21 and 25 days post-inoculation. Taken together, our results indicated that A. dauci was able to colonize both cultivars in a similar manner during the first steps of the interaction, then fungal development in the partially resistant cultivar was restricted due to putative plant defence reactions. The results of this study enhance the overall understanding of infection processes in the A. dauci-carrot pathosystem