Yellow leaves can be due to a due to a number of things. Drought, root problems, and soil pH that is too high for the particular type of red oak are all possible causes. If the older leaves are turning color it maybe due to the fact that it is near the end of their season and the effects of summer droughts are just shutting things down a little earlier than normal. If even young leaves are yellow it may be soil pH being too high or because the soil is staying to soggy wet for some reason. Also if the tree is at least 10 years old it may be suffering from a root encirling and strangling the trunk just below the soil surface.
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