We have a large (for a family/garden) orchard of about 60 mixed apple,
plum, pear and cherry trees. This year, having mostly caught up with
the more serious things that need[ed] doing it looks as if we may
actually have some time to do some pruning rather than just cutting
off the dead bits.

I've searched on the internet and have found some quite useful places
describing how to prune but there seems to be a lot of disagreement
about one thing - when should one prune plum (and cherry) trees?

The issue is silver leaf disease but there's no agreement on when to
prune to avoid it. Some sites say don't prune in winter, prune in
May, however other sites say almost the opposite. I found one site
that said "To avoid silver leaf disease, prune young trees in winter,
mid-December is a good time. For older trees, avoid pruning in May and
September to October." (What's 'young' and 'old'?)

Vary confusing! I hope I don't start an interminable thread but I
would really appreciate some guidance.

--
Chris Green