All summer my Forsythia "spring glory" would have new shoots and strong growth while other established growth would die back (turn brown, then dry rot).

I pruned out dead limbs as fast as they grew in and died which was almost biweekly.

Neighboring plants show no signs of stress etc.
I do not use a sprinkler/watering system unless we go several days without rain. This summer we had ample rain.

New home construction landscaping.
Plants approx 2yrs old, poor clay soil in a well drained location. Some organic amendments in root area. 1/2 day full sun or more.
Zone 8 / Zone 7 borderline.

Any ideas ?