United Kingdom 
Hi,
I have an Oak bonsai [or rather a tree that will one day possibly become a bonsai] but every summer it get's this sticky substance on it's leaves, followed by a white powdery mildew that then infects everything that's near it.
I've tried treating it with mildew spray with limited success, but I also don't want to kill the insects that the tree [even though it's small and on my balcony it attracts a lot of small wildlife to a council estate balcony and that makes me happy :-)]
What can I do, and what is it. I really love this tree and one day it could make a wonderful bonsai...
All the best
Barry Robinson
6 Aug, 2016
Sounds like you possibly have an aphid infestation. Aphids excrete a sticky substance known as 'honeydew'. Normally this would attract a lot of ants and other insects and is also prime conditions for mold, mildew.
If possible, move the tree to where it can get more light & better air circulation. Inspect leaf undersides for aphids. A magnifying glass would be very useful. They are little green bugs & very very tiny. Spray the tree down with a 50/50 mixture of dish soap & water. Clean off dead bugs & repeat application.
6 Aug, 2016