Whodunnit? What's killed our cherry tree?

Last autumn we transferred a 2 year old cherry tree (don't know what type other than it produces edible cherries!) from a pot to a newly prepared raised bed made with new wooden "sleepers". I then put strawberry plants around the base of the tree. Last year it was happy in the pot and gave us a handful of cherries. This spring it produced a large number of buds and then two tiny leaves smaller than a fingernail. The buds never opened, the leaves died. I've looked for obvious fungal infestations but my untrained eye has spotted no toadstools, no white slime under the bark etc. When I've broken off a twig the centre of it is a deep soft brown that smells sickly sweet.

Does anyone have any ideas please? I'd like to replace it but am wary of planting another tree in the same place if it's something like honey fungus.