I can't really tell from the photos, but a quick internet search suggests that some of the diseases that can affect rowan trees are:
Fire blight, a bacterial disease that attacks the rowan through its open wounds and causes dead branches and defoliation.
Crown gall, a bacterial disease that causes tumor-like growths on the roots and stems of the rowan.
Powdery mildew, a fungal disease that infects the rowan's foliage, stems and fruit with a white, powdery coating.
European mountain ash ringspot-associated virus, a viral disease that causes yellow spots and rings on the rowan's leaves and reduces its growth and fruiting.
Silver leaf disease, a fungal disease that causes the rowan's leaves to turn silver and die.
Might any of those be relevant?
Is the trunk showing any signs of unusual cracking, or weepage from it?