Question
I live in Eastern Ontario, and have a property with many mature oak trees. Last year the trees started "dripping" a sticky substance from early spring to late fall (i.e. when the tree has leaves). This is continuing this year. My research so far indicates that it is likely caused by either aphids or a bacterial infection. All the trees are affected. I'd like to know how I can definitively identify which is the source of my problem, and what can be done about it. These are large, mature trees so spraying is not an option.

Answer
I would guess it is either aphids or scale insects. Both are sucking type insects and secrete a substance called honeydew which is high in sugar content and sticky. The aphids will cause the leaves to curl or not look normal. The scale insects look like a turtle shell on the twigs. And the foliage may not show any signs of a problem. Both can be controlled with the use of a systemic either sprayed on the foliage fro aphids or put into the soil for aphids and scale insects. For a large tree the soil application will be the easiest.
The problem usually is the tree is not as healthy as it needs to be usually due to some type of stress-drought, etc. I would recommend that you fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree under the foliage and watered in good. Do this now and again in the late summer. I would check with your local nursery and ask for a systemic insecticide applies to the soil for sucking insect control.