privit hedge desease

Sounds like a fungus, the most common for privet is honey fungus in which case you may see brackets on the wood lower down. Cut it down to a few inches on each stem and burn them, it may take a few years to grow back but it will. Clean pruning tools carefully afterwards so not to spread spores. Clear under the plants of weeds, ivy, dead leaves and any other debris and give the soil a top-dressing of fish blood and bone meal lightly forked in to help build up the strength. I'm afraid though that the outlook is not very positive, particularly as you say it is spreading.

John

did you prune it in the autumn? could it be wind/frost damage?

if its honey fungus peel the bark back on a sick bit. if it
smells strongly of mushrooms and/or has a film of white growth it is definitely hf. according to the rhs there is no cure and will not grow back. i have lost several plants to it. the rhs recommended removing the privet for at least 1m either side of the sick ones, burning the affected wood and planting with less susceptiple species. i used blackthorn& hawthorn. i lost 2 silver birch, lilac, laurel, flowering current and forsythia as well as 10ft of privet hedge. so far nothing else has succumbed and i am planting resistant species as and when i can.

however if you build up the soil above the basic root stock/soil boundary that can apparently cause privet die back. dont know why but that is what our local radio plant expert said. so lets hope its this instead.