Juniper can be a beautiful and low-maintenance addition to any garden or other outdoor area, as long as proper soil conditions and cultural practices are maintained throughout the juniper's life cycle -- and as long as the grower protects the juniper from diseases and other ailments that could threaten its health. An important component of protecting junipers from disease is to regularly monitor the plants for any sign of disease symptoms. New growth turning yellow is one of the most common indicators that a plant is in poor health.
Cedar-apple rust is so named because it most commonly infects cedar and apple trees. However, this disease is also notorious for its ability to easily spread to other nearby host plants -- and juniper is one of its favorite targets. If you have any nearby apple or cedar trees that show signs of cedar-apple rust -- orange horn-shaped clusters of gelatinous fungal fruiting bodies being the most conspicuous of these symptoms -- the chances are good that the juniper has been infected with spores released from the apple or cedar tree.
Spruce Spider Mite
Perhaps a more likely diagnosis is that the juniper has been infested by spruce spider mites. Feeding damage from these pests causes the whole juniper plant to discolor and diminishes new growth. Juniper needles will turn yellow and brown, and branches will wither and fall off. On seriously infested parts of the juniper, you may see a white cottony mass protecting the mites. Treatments should combine natural and biological controls with a chemical pesticide application.
Another common juniper pest that causes new growth to turn yellow is the juniper scale. The symptoms of feeding damage from this pest are nearly identical to those of the spruce spider mite. Additionally, scales secrete a sweet excrement called honeydew when they travel, so you may also notice an oily film over the juniper leaves that are most infested. Just as is the case with spruce spider mites, control methods should combine natural and biological methods with a well-timed insecticidal spray program.
Other Possible Causes
A handful of other ailments exist that can cause growth on a juniper to turn yellow. Simple drought stress due to under watering causes yellow new growth. Somewhat ironically, over watering can cause yellow growth as well. Adjust watering schedule and monitor the juniper for a return to normal color to confirm this diagnosis. If a dog urinated on the parts of the plant that are yellow, keep the dog away from the juniper and see if the juniper takes on normal colors. Salts in dog urine can scorch juniper foliage.