My lemon tree which has given lemons in the past is now looking very sad with sticky glue on the leaves and black dust sticking to it. What can I do to clean it up?
The problem you describe is very common with lemon trees in a porch or greenhouse. Almost invariably they suffer from attacks by scale insects which release the sticky glue. Soon after, fungi called ‘sooty moulds’ begin to grow on the sugary glue. The best way to control the problem is to watch for scale insects, or greenflies which also cause the problem, and treat them with Derris or Malathion. The glue and black mould is difficult to remove but sponging with lukewarm water helps … if you have the patience!