Answer: I assume you are referring to a type of prostrate, spreading juniper. They love a well drained soil and a sunny location. If they get "wet feet" they will quickly begin to exhibit dieback of branches or the entire plant. Check your drainage and watering schedule to make sure the plant roots are not in soil that gets too soggy. Raingutter downspouts are another common cause of this condition.
Junipers also are subject to diseases that can cause sections of the plant to turn brown. These fungus infections are increased by wet conditions, such as could be created by a sprinkler system that wets the foliage every day or two.
Set watering schedules to once a week. Give them a 1/2 to 1 inch sprinkling (a coffee can makes a good sprinkler gauge) and then allow them to dry out for several days. Prune out infected areas and remove clippings to avoid reintroducing the disease spores to healthy foliage.
A final possibility is spider mites. These mites are especially attracted to hot, dry areas, typically near a sidewalk, curb or rock border. A close examination will reveal the tiny webs they leave as their feeding progresses to the point of causing brown sections in a juniper.
Products with neem are good, organic mite control options. Avoid using horticultural oil (often recommended for mite control) on your "blue" junipers or they won't be blue anymore, until new growth replaces the oil-darkened foliage.
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