If you have sooty mould it means your plants have scale insect which is sucking the sap out of your trees, then the scale insect exudes a sugary honeydew which the sooty mould then grows on.
So. you need to suffocate the scale insect by using Pest Oil which you dilute in water and spray on. Best done in the evening or on an overcast day or some scorching of the leaves can occur.
Scale insect usually indicates the plant is under stress for some reason, it can be too dry, or the plant could be diseased or simply hungry.
Citrus are gross feeders, and do not like anything around their root zone, as they have fairly surface feeder roots. Best thing for cirtus is to remove any grass out to the drip zone (edge of where the leaves cover) and mulch heavily. They love nitrogen, so try to get males of the house to pee around the citrus
Otherwise you can use compost as a pick me up, sheep pellets/cow poo/ wood ash/ seaweed whatever you can lay hands on really to help create a nice tasty mix for them. They appreciate enough water, but prefer well draining soil.