Many plants - especially arborvitae grown too close together
Usually, Pestalotiopsis sp. killed foliage is near the base of the plant and where foliage is relatively dense. The disease usually starts at the tip of the foliage and progresses towards the leaf base. The color of the foliage goes from green to yellowish, then to a dark brown that can look almost black. The disease may kill the smaller twigs where the infected needles died.
is a fungal disease that is considered usually a minor disease. It attacks foliage that has been injured or weakened by unfavorable weather or growing conditions.
Control is to prune out killed foliage and if necessary the twigs as soon as possible in the spring. Reduce winter injury by minimizing dehydration. Do not allow snow to accumulate or remain up and around the base of the plant for any length of time - especially during melting. Avoid over crowding the plants to allow sufficient sunlight and air circulation.