Junipers seem to have no end of problems in western Washington. The low growing varieties are especially prone to dieback and browning. In general, junipers are adapted to dry, barren, rocky or sandy soils (which provide excellent drainage and aeration) and full sun. It is little wonder that plants in heavy clay, waterlogged soils, heavily mulched with bark and/or black plastic begin to suffer. Still, before the problem can be corrected and the plant put on the road to recovery, the cause must be identified.
Arborvitae leaf blight is caused by the fungus Didymascella thujina. Infected plants begin to look fire scorched. On close examination the infected leaves are ashy gray, with brown to black fruiting bodies. The fruiting structures are imbedded in the leaf tissue and the entire structure drops out, leaving deep pits. In the fall, infected leaves may drop from the plant.
This disease is prevalent as the plants become overgrown or thick. Plants are unable to dry out and the moisture and increased humidity are ideal for disease development. Pruning and thinning will increase the air circulation allowing foliage to dry out. Reducing overhead irrigation will also help. Remove infected branches and fallen debris where possible.
Juniper twig blight is caused by the fungus Phomopsis juniperovora. The scale-like leaves turn yellow and die, then the whole branch or twig dies. Usually one branch or one side of the plant is affected at first. It is often severe toward the heart of the tree where branches join the trunk. Prune out infected twigs and branches, avoid watering in the evenings or overhead, increase air circulation, avoid wounding branches and protect with a fungicide until the disease is under control.
Don't be too quick to blame diseases.
Other common problems to check:
Cultural decline can be caused by a number of practices or soil conditions. Wet, heavy, clay soils or waterlogged soils can cause plants to do poorly. Junipers are frequently mulched with bark. On hillsides the bark can wash downhilll and gradually bury the crown of the plant. Remove the beauty bark and expose the crown.
Sometimes plants are buried too deeply. This can happen even when the plant is planted at the proper depth, because soil in the planting hole settles. Plants which had been in the container too long might have had numerous circling roots which continue to grow around and around. After many years they eventually choke and girdle each other, and the plant begins to suffer, struggle, then die. If all else has failed, dig around the plant and examine the soil and roots for rot or other problems.
Dwarf Mistletoe is a leafless parasitic plant that can grow into and use the sugars of the host plant. Prune it out.
Ice coatings can severely injure or kill hybrid junipers if the ice lasts for several days. "Individual branches or whole plants may die from the after effects" (Pirone, Diseases and Pests of Ornamental Plants).
Dogs find junipers to be a favorite "rest stop" as they wander through the neighborhood, marking their territory. This fertilizer burn usually has a detectable odor, and often it is on the street side of the plant or the side most likely to be passed by a dog.
Juniper scale can also cause twigs, branches and entire plants to become sickly-looking, yellow and die. The infested twigs are always covered with tiny white scales. With a hand lens, one can see the volcano shaped female scales, oblong males and yellow specks which are the scale crawlers. Pesticides are best applied to the vulnerable crawlers.
Aphids on juniper seem more of a problem during the growing season. Sometimes ants, pirating the honeydew, or ladybeetles are more noticeable than the black aphids themselves. Aphids can devitalize the plant or branches if they are abundant.
Juniper webworm is a caterpillar that makes messy nests of webbing, dead needles and grass. The striped, brownish caterpillar is well hidden among the mess it makes. This pest is rare in our area. Parasites or natural predators seem to control it well. The adult is a small moth.
Cypress tip moth or leaf miners are the most destructive of the insects. The ever-so-tiny caterpillars mine the needles causing them to turn brown. Look for holes in the needles, or tiny silk cocoons in the crotch of needles or twigs. These insects can build up until the entire plant appears to be brown. Fortunately the branches remain alive and once the leafminers are controlled the tree will green up again. Pesticide timing is critical. Contact pesticides must be applied when adults are flying (usually about May); systemics should be aimed early in the life cycle to kill young larvae before they damage the leaves severely.
Spider mites can build up to damaging numbers. Close examination of off-color foliage with a hand lens will reveal irregular brownish rasped areas on the leaf, revealing mites, mite eggs, or webbing. Mites are usually more of a problem in hot dry climates or seasons. Pesticides don't work against eggs so a second or third application is sometimes needed. Use enough pressure to penetrate dense foliage.
It is a wonder junipers survive as well as they do considering this rather formidable list of problems. (Again, juniper critics will think this list too small.) Fortunately, all of the problems don't hit at once and nearly all of the problems are correctable. Check with your local WSU Extension Office for registered pesticides or other treatments currently used for these juniper problems - if such control is needed.
Online Guide to Plant Disease Control
Hortsense: Managing plant problems with Integrated Pest Management