I have a row of six conifers forming the boundary at the bottom of my
garden. They are about 20feet tall. The other side of my boundary is a field
used as a car park for a small club. I've only been in the house for a few
months so wanted to check what the boundary looked like from the other side.

When I checked, there are quite large patches stretching high up the length
of the conifers. Any ideas on what may be causing this and why on the back
only? I wondered if maybe the people who maintain the area being my garden
have been putting something on it or damaged them in some way?

Any suggestions appreciated.

Many thanks

Jeff