"Bob Hobden" wrote in message

All Citrus trees are susceptible to Scale Insect which excrete "Honeydew"
loved by ants and on which Sooty Moulds flourish. The white woolly bits on
the leaves and stems are the Scale Insects in breeding phase. If it's not
too big a tree and you don't like chemicals you can spray them off with a
sprayer filled with water with a few drops of washing up liquid. Pump it
right up so it's a powerful jet and start at the top and do every leaf and
stem.
If it's a big tree and/or you don't mind chemicals then go see your local
Garden Centre for something that works on Scale.
They will come back, they always do on Citrus, so it's not a once only
job.

To add to Bob's excellent post, I would advise that you control the ants
(and there will be ants as they 'farm' the scale insects and spread them).
White Oil will kill the sooty mould and this can be made up at home - dozens
of recipes for it on the Net.