We planted a small river birch this spring, and it has already grown and spread out nicely. About a month or so ago, its leaves started turning yellow with spotting on them. We researched and then started spraying it with something called Bioadvanced insect, Disease, and Mite Control. The tree would look better for days and then develop increasingly more yellow leaves. Clearly, what we're doing isn't working. Please help us with our little tree.