The black powdery stuff on the citrus is called sooty mould, sooty mould often comes about as there are sap-sucking insects on the tree. It also sounds like an insect problem on the roses, this is called scale.
To fix the problem I would recommend spraying a spraying oil such as Enspray Oil 99, I would also recommend spraying a liquid copper spray as well to ensure fungal problems do not arise.
All the best.
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