Sick arborvitae trees need plenty of water to help them recover from drought conditions, high nitrogen fertilizers to repair freeze injury, and polynet fencing for protection from animal damage while they heal. It is important to prune sick arborvitae trees in order to remove broken, dead or diseased branches.
Bagworms are a common problem with sick arborvitae trees. If the sick trees are caught early in the season, they can be treated with insecticides to kill bagworm larvae. If you discover the sick trees later in the summer, handpick the noticeable bags of worms hanging on the trees, and discard.
Pale patches, yellowing, dead or rust-colored spots, or greasy-looking arborvitae leaves are evidence of fungal diseases. Fungal diseases cannot be cured; they can only be stopped from spreading and prevented the following year through fungicides and pruning.
Sick arborvitae trees with wilting or curling leaves that are yellow or brown may be affected by aphid pests. Control aphids by handpicking them off the trees, spraying with water, or using sticky tape traps or various insecticides.
Treating sunscorched arborvitae trees involves adding a thick layer of mulch at the base of the trees, thoroughly soaking the ground around them with water before the first freeze of the season and spraying exposed foliage with anti-transpirant spray before winter begins.