Citrus tree blackflies

I have a Citrus tree growing in my yard(I don't know which fruit tree it is, probably sweet lemon). The tree leaves completely are being taken over by these black molds kind of structures(only on backside of the leaves). Almost whole tree is in this state. I read on plantix that this is citrus blackfly... But I did not get what to use in order to eradicate this. Also should I cut the leaves off or else use some pesticide? Please suggest me what to do..