Question
QUESTION: I have a tree in my backyard, and recently I have noticed a sort of yellow looking substance coming out of one of the roots on the ground.. It smells sweet and has gnats and other bugs swarming it.. What I want to know is what is it? and How do I get rid of it? Please help. thank you

ANSWER: Have you had a rainy Spring? What does the rest of the tree look like? Is it leafing out normally? Are the leaves discolored, wilting, or curling? Can you remove this substance and see an open wound in the root?

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QUESTION: Yes it has been raining a lot... The rest of the tree looks healthy.  Yes there is an open wound on the root, where the substance is coming out.. It smells kind of like yeast.  And it's liquidy, like it bubbles out of a hole in the root. I tried a copper fungicide and it didn't seem to do anything.  Thank you for your information I really appreciate it.

Answer
It sounds to me that you have a bacterial problem rather than a fungus due to the smell and the liquid-like substance coming out of the root. Although most bacteria do not attack roots because roots have less openings for entry than leaves do, your root may have been damaged some how to allow a port of entry for the pathogen. Allowing the area to dry down would  definitely suppress the pathogen. If you know where your county's extension service is, they could be able to identify the causal organism if you take them a sample. Or they may be able to send the sample to a lab to be tested.
I hope that helps!