Marcia, There a several reasons why boxwoods turn yellow. Phytophthora parasitica is one. It's caused by waterlogged soil. A soil soil drench with a fungicide may save a lightly infected plant. Root feeding nematodes also cause leaf yellowing. A soil drench of a neem product should help. There are several other reasons for yellowing leaves. I suggest you contact the Virginia Cooperative Extension for more information. Best And Happy Yardening, Nancy.