All rust diseases are caused by fungi that have alternate hosts, meaning that they complete parts of their life cycle on two different plant species. For example, cedar-apple rust infests both junipers and apples, but at different times of the year.
Junipers become infected in late summer and fall, and galls develop in the spring.
These fungal spores infect the young leaves and sometimes young fruit of apples and crabapples. Prune out and discard galls on junipers. Plant resistant cultivars. Avoid wetting the leaves when irrigating your plants. Fungicides are available.