Go into the top right hand corner and search sooty mould - it will tell you all about it.

Check the plant for scale insect infestation, or possibly aphids (though its a bit early for those yet). Both of these produce sticky secretions that can lead either to sooty mould, or to pollution and dirt sticking to the leaves. If you find scale, spray with Provado or similar scale insect treatment. If you think it's just sooty mould,you'll need a fungicidal spray.