Previous phytopathological studies of the fungal pathogen Nectria cinnabarina have been focused on its distribution and host diversity but little is known about the spread of this pathogen and the defence responses of forest trees to an infection inside host tissues. Histopathological alterations of bark, periderm, phloem and woody tissues were investigated in sycamore maple (Acer pseudoplatanus) branches following their natural attack by the advanced anamorph and teleomorph developmental stages of the fungus. Light, fluorescence, confocal laser scanning and scanning electron microscopy techniques supplemented by X-ray micro-computed tomography imaging were used to distinguish between healthy and disintegrated plant tissues. The intercellular spread of fungal hyphae was found primarily in the phelloderm. Expanding hyphae aggregations produced ruptures in the phellem and the disintegration of both phellogen and phellodermal parenchyma cells in close proximity to the expanding fruiting bodies of the fungus. Thicker hyphae of the teleomorph fungal stage heavily disintegrated the phelloderm tissues and also induced enhanced sclerification of the nearby phloem tissues that limited the spread of the infection into the sieve tubes. Both the intercellular and intracellular spread of hyphae inside the peripheral parts of sclereid clusters led to the disintegration of the compound middle lamellae but the hyphae were only rarely able to pass through these structural phloem barriers. The massive fungal colonization of both lumens and disintegrated tangential cell walls of ray parenchyma cells resulted in severe cambial necroses. Although the hyphae penetrated into the outermost annual growth rings of the xylem, no cell wall disintegration of the parenchyma cells, vessels and fibres was revealed. Despite the local cambial necroses and severe phloem ray disintegration, the bark remained attached to the examined branches and no bark cankers were formed.