I got an email from the Yates Gardening Club today that reminded me about the sooty mould I used to have on my lemon tree.
It looked like this:
If you ever see this kind of black ash on your citrus leaves or lots of ants, make sure you tackle the problem. It's caused by scale insects which are sap suckers and will likely kill your tree. You need to use a pyrethrim-based spray to smother and suffocate the insects.