I have a beautiful 4-Clump River Birch that I had planted in my full sun back yard, 23 years ago, now full grown. Last year and this year, in the center of the tree, from bottom to the top the leaves came out "yellow" like when they turn in the Fall. The rest of the tree. on both sides of the yellow center section, are the normal Green color. Every Spring, for the last 5 years, I applied Bayer Tree & Shrub Protect & Feed and the tree did beautifully, until last and this year, when the Yellow leaves opened up. Our area is on Clay. WHAT IS THE PROBLEM. HELP!! PLEASE. Thank You!