New privet hedge plants

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It looks like sooty mould, the black coating.
I'm thinking you have probably had a lot of sap sucking insects on the plants all the summer..

Possibly scale insect, although aphids are also possible.

 

Thanks for reply Pete so do you think I should do something to prevent

cheers for reply Pete I'll investigate this further

 

Possibly a bit late this year, but I would spray with a systemic insecticide.
It might stop re-infestation next year.

Have a close look at the shoots, the kind of scale insect I'm thinking of is like sticky yellow blobs that you can wipe off the stems.
Tried to take a picture of one of my citrus plants with scale insect.