By Petea72
United Kingdom 
I am trying bonsai some sycamore tres and up to this year they were doing pretty well. but over the last month they have developed some black spots and other discolouration and was wondering i have not fed them properly or could there be another reason .I know its a bit late in the year but am worried about next years growth ! A couple of them are four years old so i would hate to lose them.
3 Oct, 2013
Sounds like tar spot - a picture would help. Remove the affected leaves and burn them (and any fallen leaves) to prevent spores spreading.
Doesn't cause much trouble to full size trees but bonsai might be a bit weaker. Wash with baking soda (1 teaspoon in a pint of water with some soap) to remove spores but it doesn't have a "cure".
It's worse in mild damp summers than hot dry ones.
3 Oct, 2013