Leafminers

• Leafminers are the larvae of moths that tunnel into the needles and twigs of arborvitae trees to feed on them from the inside. Usually, only the tips of the twigs are affected, but trees can lose as much as 80 percent of their needles and still recover. Symptoms include brown leaves and the appearance of leafminer webs and pellets of excrement. Prune infested tips, if the infestation isn't serious. Avoid using insecticides because they will kill beneficial predators, like parasitic wasps, that prey on leafminers and usually keep their populations from exploding.
  
  

Scales

• Scales are unusual insects because they have no identifiable body parts, such as a head or legs. The juniper scale is one of several species that attack arborvitae trees. Juniper scales are round white or gray insects with a yellow dot in the middle. Scales can cause leaves to turn yellow or brown. In addition, they kill branches. Scales also secrete a sweet, sticky substance called honeydew that attracts black sooty mold. When scale populations become large enough, natural predators like ladybird beetles and parasitic wasps appear. These beneficial insects help keep scale populations in check. Minor infestations can also be controlled by wiping scales off by hand. Heavily infested branches can be pruned.
  
  

Pestalotiopsis

• Pestalotiopsis is a fungal infection that causes needles to turn yellow, brown and black, starting from the tips and working its way down to the bases of the needles. Symptoms usually appear near the bottom of the plant first and include twig dieback, the appearance of cankers, and damping-off. Pestalotiopsis usually infects trees that are already weakened by other diseases or insect pests. Prune dead foliage and twigs as early as you can in the spring.
  
  

Cercospora

• Cercospora leaf blight usually appears on the lower, inner branches first and works its way outward and upward until only the top is green. Diseased needles turn brown and then grayish, and small, fuzzy structures form on the dead needles. As the disease progresses, the inner needles fall off and the only needles that are left are those at the tips of branches. Eventually, even the lower branch tips die. Cankers can also develop and slimy, cream-colored spores are discharged from them. Cercospora spreads quickly when the weather is wet and warm. Although fungicides can be applied as protection, specific control measures haven't yet been developed for this disease.
  
  

Phomopsis

• The phomopsis fungus kills only new growth on arborvitae trees, causing the needles to change from yellowish-green to red-brown to gray. Small black spots can appear in the gray areas. After attacking the young needles, the fungus invades small stems. This fungus is more severe when the weather is warm and wet, and even older trees can die if seriously attacked over a period of several years. Control phomopsis by pruning and burning or burying all diseased plant parts, but prune only in dry weather and sterilize pruning tools with a bleach solution or rubbing alcohol between cuts. Avoid overhead watering, if possible. Avoid injuring the tree.