We prune it about every three months. The issue became obvious after it was last pruned. The leaves on one of the trees went black and started dying ...
Welcome to the Bunnings Workshop community @WhimsicalManiac. It's terrific to have you join us, and many thanks for your question about headge health. ...
It appears that there might be some insect infestation and sooty mould @WhimsicalManiac. I'd recommend you start treating the whole hedge with Eco-Org ...