I have a Japanese Maple which has been healthy throughout last year. But
this spring it became noticeable that a number of new top stems have died
back. I've now cut these back to main branches. I've no idea what killed
them and we haven't had that cold a winter to cause that amount of dieback.
New growth has already started lower down the main stem, and at the moment
that looks healthy and vigorous. But I'm wondering whether the symptoms
could be early signs of verticillium wilt although there's no sign of the
new growth wilting so far?

I don't know much about Japanese Maples but recently learned that vw is the
main threat to growing them successfully.