This is a problem I have in my coastal garden in the East Neuk of Fife. about four years ago noticed it on one plant at the end of a long established Escallonia hedge, then next season a slight exhibition of symptoms in another some distance along the same hedge. This autumn the entire 124ft of 8ft high hedge became almost completely defoliated, those few leaves clinging on showing the dark spots with tiny white dots in the middle.
This winter has been wet and dull certainly, but paradoxically other specimen Escallonia varieties elsewhere in the garden - Iveyii and Macrcatha - less hardy here, normally do thin out and look sickly over winter have remained noticeably verdant and healthy this year! Perhaps this is because it has been mild. The hedge is now sprouting new growth.
The prospect of grubbing up such a long run of a beloved hedge is daunting. I wonder why this has suddenly manifested itself and whether there is any possibility of natural recovery long term. The practicalities of spraying such a massive hedge seems impractical.