I'm new to this group and apologise if this question has been asked and
answered before.

We have modestly trimmed Leylandi and Castelwellan hedges for about 25
years. Recent years have seen dieback patches develop which are unsightly
until new growth comes. I see it affects many other gardens, but not all.

I've had numerous answers to the question why, and what to do - spider,
drought etc.

Any definitive ideas welcome please!

Thanks

Len