Cedar Apple Rust is caused by Gymnosporangium juniperi-virginianae, a plant pathogen/fungus that primarily effects apple and juniper related species. This is a pretty interesting fungal disease that I have have been battling since getting into bonsai. Early in my bonsai experience my love for junipers is what first exposed me to cedar apple rust. I commonly purchased San Jose, Hollywood and other garden variety junipers from random local nurseries. Many of these I found were very susceptible to cedar apple rust.
Cedar apple rust has a pretty interesting 2 year life cycle. One year apples and crab apples carry it. The apple trees then release spores that blow in the wind which may end up on nearby junipers. The junipers develop gulls in late fall that remain on the junipers until the spring. When spring comes around and water is more readily available the gulls begin to grow and spread. The gulls inflate to a slimy ball of gunk each time they are exposed to water. The gunk then attaches to other foliage and branches. As the gulls and gunk grow on the foliage it also kills it. These gulls then release spores which also blow in the wind and they will reinfect the apples. This continues repeatedly year after year.
I normally only see cedar apple rust on my junipers every other year and the same goes for the apples (from what I hear). The disease must pass from one to the other and cannot be passed to the same species directly on a yearly basis.
Cedar Apple Rust Identification On Junipers
On Junipers they are little orange gulls or gross slimy balls of orange gunk (when wet). These gulls are usually found on the inside foliage and not on fresh spring growth. You will not find these commonly on shimpaku variety, procumbens, Rocky Mountain or Sierra Junipers. It is almost always found on garden varieties, San Jose being the most common in the USA as far as being used for bonsai.
The picture above is a dry gull on a juniper. You can see the discoloration on the nearby foliage as the gull begins to kill it.
Here are a variety of different gulls of different sizes and development. These are from different trees.
The picture above is a dry gull prior to being placed into the cap of water.
The picture above is the gull after about a minute in the water.
The picture above shows a wet gull (ball of slimy gunk). This was inside the cap of water for less than 2 minutes. As you can see, the gull absorbed the water and increased in size.
The above picture shows how the gull will kill the foliage/branch it attaches to. The gull started out on the yellow dying foliage/branches and is working its way to neighboring foliage/branches. This happens when the gull gets wet.
The image above shows how a gull can spread to neighboring foliage and branches. Notice inside the red circles how it attached at each of these points. All branches and foliage that the gull touches must be removed from the juniper.
Unfortunately preventative measures are the best way to treat Cedar Apple Rust on bonsai. Preventative spraying of alternating fungicides should help. If you do end up with this on your junipers - cut the infected foliage or branches off ASAP. The cedar apple rust will swell and spread on your juniper which will kill off more foliage and branches. Be sure to trash or burn infected materials and sterilize your scissors between cuts to prevent further infection.
You can also eliminate the opposite tree. If you love junipers and have control over the trees in the area, cut down the crab apple trees and apple trees. This obviously is only useful if they are on your property. If you love the apples, perhaps not owning juniper bonsai and juniper landscape plants will be your best bet. You can also stick with varieties that do not commonly get cedar apple rust. My favorite are Kishu and Itoigawa which are very resistant.
I hope this helps people understand a little about cedar apple rust. I do not have a horticulture degree, and my information is mostly from reading and my own experience with cedar apple rust on my own juniper bonsai. I do not own any apple or crab apple trees and have never dealt with this on the apples. Apparently everywhere I move my trees there are apple and crab-apple trees nearby because I seem to always end up with this every other year on at least one of my juniper bonsai.