I lost a big piece of a huge privet hedge and a large fir together over a couple of years. Also a floribunda, himalayan honeysuckle and choisya, all very mature large plants in the same area, turned brown and died.
We still don't know what caused it but Honey Fungus was suspected.
The fir came down, the hedge was replaced with Laurel and the H.H. and Choisya came back from the root after a year or two.
It was a depressing time though.
Interesting that it might have been an aphid....