Orange Tree

My leaves on my orange tree are curling and underneath the leaf is something that resembles ground pepper. It's easy to rub off but not sure what it is. Has anybody any ideas please and treatment.

i dont know what it is but i spray my fruit /olive/fig trees with a solution of washing up liquid and blast them off.
so far (although time consuming ) it works and better than using insecticide
what i have is usually aphids

Sounds like Leaf Curl, caused by the Mirador bug. You need some Anti-Mirador spray.

Pete.