Two of the most commonly cultivated ornamental plants in North America are junipers and apples/crabapples. You see them side by side in gardens everywhere, you see them lined up one next to the other in nurseries… and yet these are two plants you should never be grown close to one another. The recommended distance between the two is 500 feet (150 m): in other words, you should not grow crabapples if your neighbor 5 houses down has junipers, or vice versa!
The disease involved is cedar-apple rust (Gymnosporangium juniperi-virginianae), so named because it was first discovered growing on “red cedar”, one of the common names of Juniperus virginiana. (Note that it does not infest any of the true cedars (Cedrus) nor any “white cedars” (arborvitae or Thuja), only junipers).
It’s one of those strange diseases with two alternate hosts. During one phase, it attacks junipers (Juniperus spp.), causing the growth of brown galls that repeatedly produce orange, gelatinous pustules in the spring. During the second phase, the disease must necessarily move, via wind-blown spores, to an apple or crabapple (Malus spp.) where symptoms include yellow spots on top of the leaves and brownish growths underneath. Fruits are also infected. In the very worst cases, it can even cause crabapple or apples to drop all their leaves.
Of course, you could treat this disease every two weeks from spring to late summer with a fungicide (a sulfur-based product would be appropriate) and pruning out the galls on junipers might also help (at least if your neighbors also collaborate), but why even bother? There are rust-resistant apples and crabapples you could try (ask a reputable nursery to point them out), plus not all junipers are subject to the disease either (North American juniper species in general are the most highly susceptible). Or simply avoid the issue by planting junipers and crabapple/apple trees far from each other.
Other Juniper Rusts
There are very similar diseases (other rusts of the genus Gymnosporangium), all of which spend part of their life cycle on junipers, but whose alternate hosts are other trees and shrubs of the Rosaceae family: hawthorns, serviceberries, mountain ashes, pears, quinces. So to make your life easier, never plant them near junipers either!