I have a holly bush tucked away in a corner of my garden. The holly doesn't get much light or nutrients and as a result it seems to be prone to pests as I discovered when I reached it on my pruning rounds today. A few of the leaves had "mines" being made by the leaf miner fly Phytomyza ilicis. However, many more leaves were infested with scale insects. These live on the under-surface of the leaf. They secret honeydew which promotes the growth of a black sooty mould.
Holly Leaf Miner Phytomyza ilicis
Scale insects on Holly with black sooty mould (Both species not named!)