If your Leyland cypress trees are turning brown, you may want to check for a few common pests and pathogens. Fungal diseases are the most common cause of a browning tree and are usually caused by lack of airflow or excessive moisture. Pruning away the dying foliage may help — sterilize your pruning shears before each cut and dispose of the infected branches away from your trees. Poor planting site conditions can also cause the tree to turn brown, including dry, hard soil or standing water.