Our crepe myrtle trees have a lot of sap dripping. The leaves and bark look wet and there is a white substance on the bark and some leaves turning yellow. What is wrong with the trees? I have never seen this before. How do I treat this?
The sap is most likely Honeydew, which is the secretions of insects.
Aphids are likely, though it could be other pests.
Here are some links with more information.
You should treat with Neem Oil or a horticulture oil.