|I have two Sweet Gum trees (over 10 yrs old) that are turning yellow in color on leaves and also have dark black spots on the leaves (almost like they have been burned along the edges). I have had to treat them before for lack of iron but never have seen the black spots on leaves.|
That sounds like a root problem. Sweet gum is not fond of your area (thus the iron chlorosis). In a clay soil when it rains a lot the roots can become waterlogged and start to die from lack of nitrogen. Something has damaged the roots whether it is to much water, weed and feed fertilizer, trenching, etc. If the initial damage is not too severe and the plant has time it may recover. Otherwise it is likely to die back some or all the way.