Paddywhack

I wouldn't jump to the conclusion that it's honey fungus unless you've seen golden fungal fruiting bodies (mushrooms) around the base. Rowan's are very sensitive to any root damage, and even compaction of the soil around their roots. The last two rowans I saw that had died had previously had roots severed due to a fence and a patio being put in. Also, how old is it? Rowans don't have a long life span, so it might have just reached the end of its life.