Garden Pests and Diseases: What is wrong with nt Lilac?, 3 by Gitagal

Gitagal wrote:
This is an old Lilac bush--but, overall, it is growing well and its blooming
has improved from the last couple of years since I have been pruning out some of the
really old, yukky, stems and the new ones seem to be growing OK.

This bush has been growing in this site for ages. My neighbors did put up a 6' fence
all along their property about 5 years ago.
Much of the Lilac bush that faces the fence gets no sun at all. Air circulation is also poor
back there. I will show you a picture of the whole bush and how it is located in the next post.

Right now--a good part of this bush has these leaf issues that emanate from
one particular stem. Not an old stem either. It is in the center of the shrub.

Many of these leaves are turning black on many of these branches. Even high up.

Anyone have any ideas? Suggestions?

Thanks, Gita

Pictures 1-2 and 3 shows various shots of the sick leaves

Pics. 4 &5 showa the healthy parts of the same bush. These branches face the garden.