There are two main issues commonly causing holly leaves to turn yellow. It could be that your hollies are iron deficient and lacking that essential nutrient. Secondly, your root systems could be struggling to stay healthy. The plants could be experiencing a disease known as root rot. Some things that contribute to root rot are poorly drained soils or compacted soils. Over mulching can contribute to root rot as well. If you suspect this, give us a call and we will help you fix your problem.