Question
We have an eighteen year old privacy stand of Arbovitae trees that are turning brown or dying in patches all over the 20 ft. high trees. What could some of the reasons that cause this condition..

Answer
Here are a few of the foliage diseases of arbovitae.

Phomopsis Blight of Juniper

Many gymnosperms especially those in the cypress family are affected by the fungal pathogen - Phomopsis juniperovora.

For most people, the first symptom noticed in the spring (early summer) is the die-back of the new shoot growth (tip blight). The new growth changes from light yellow green to red brown to ash gray as it dies from this fungal disease. Phomopsis juniperovora only kills the new growth - if more than just the new growth is killed, other fungal diseases or environmental injury is involved. Infection starts on immature scale leaves or needles, whereas mature needles are resistant. This infection progresses into the underlying shoots, causing a small lesion at the junction of healthy and diseased tissues. Small stems (less than one third inch in diameter) are usually girdled by these lesions, causing tip death. Older branches (more than one third inch in diameter) are more resistant to infection, and lesions that form on this size stem usually heal. In advance stages of infection, small black spots (the fungus fruiting bodies called pycnidia) can be seen with the unaided eye or a magnifying glass. The pycnidia are embedded in the dry, ashen gray lesions on the stems and needles but often break through as they mature, releasing spores (conidia) that are spread by rain and wind. The spores can tolerate some drying and may remain viable within diseased tissue for as long as two years.

Infection occurs in the spring from conidia that overwintered on shoots or stems killed the previous year. Late season infections may occur if cultural practices (improper watering and maintaining high fertility) encourage prolonged plant growth. Prolong wet and warm conditions increase the severity of infection and symptoms on the susceptible hosts. The central part of the plant is often more affected than the outer portion, with the new growth showing almost continuous infection. Under certain conditions favorable for fungal development, entire young shrubs and trees may be affected, and all the evergreens needles and stems will die and turn brown. Severe infection for several years in a row, may result in the death of larger or older plants.

Phomopsis Tip Blight

The Phomopsis fungi attack a wide range of plants. On juniper, the fungus is Phomopsis juniperovora and it causes dieback of the new shoot growth. The affected tips turn brown as they die. Over several years this fungus can kill the plant. Some junipers are much more susceptible than others.

Cercospora Blight

There are more than 1,200 species in the genera Cercospora. And these fungi attack a wide range of plants. Cercospora sequoiae juniperi attacks junipers. Most selections of Eastern Red Cedar and Chinese junipers have good resistance to this disease. However, Rocky Mountain juniper is known to be one of the most susceptible juniper species. This disease can also attack the Oriental Arborvitae.

The disease spreads best during warm, wet weather. The spores penetrate new to one-year-old foliage during this time. At about 75 degrees, complete infection takes about sixteen hours. Symptoms may be visible between two to three weeks after infection. The pathogen is able to over winter in the infected foliage on the trees. The disease usually starts near the base of the plant and works upward. It also starts usually on the inner foliage and works outward. The affected foliage turns bronze to light brown color then grayish. Small, fuzzy or hairy spore-bearing structures, easily seen with a hand lens, form on the dead foliage. Eventually the dead foliage and twigs falls off the branch. With time all the inner foliage and twigs are gone, leaving only foliage at the very tips of the branches. Occasionally adventitious buds may develop new growth on the "naked " lower branches. Left uncontrolled, the tips of the lower branches die too. Thus leaving only green growth at the very top of the plant. On highly susceptible trees, the fungus can also kill the entire plant, sometimes in as little as three years. Initially the green tips of the affected branches help to separate this disease from Phomopsis and Kabatina Blights. Phomopsis and Kabatina kill from infection (canker or wound) out to the tip in the same growing season.

Pestalotiopsis Blight

Pestalotiopsis sp. 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. Usually, the 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.

Cercospora Blight would be my guess with the limited discription.
Therefore, make two applications of mancozeb or Bordeaux in early June and again in early July. A third application in mid- to late-July may be necessary during wet summers. Fungicide applications at other times are ineffective. Take care to cover the foliage thoroughly, especially the lower two-thirds of the tree crown. Yearly fungicide applications may not be required once the disease is controlled; however, carefully monitor trees for recurrence yearly