There are a group of fungi that are referred to as rust and they require alternate hosts in order to survive. Pictures are of Apple cedar rust Gymnosporangium juniperivirginianae that is a fungus who’s life cycle is between Juniperus virginiana (upright junipers) and apples and crab apples. The fungus spores come into contact with juniper twigs and awls (needles) and forms a gall on the juniper.(picture 1)
It usually will develop in the fall and in the spring around 18 months later, when mature, these galls swell considerably and repeatedly produce orange, gelatinous telial horns during rainy spring weather and releases its’ spores (picture 2)
The spores are carried by the wind onto apple leaves which develop into yellow blotches on the leaves, fruit or twigs during the summer and later in the season it spores are released where it re-infects the juniper and the cycle continues.
It does not kill either of its’ host, but can do harm especially on stressed plants.
I know on one hand it is bad, but it does exist, and apples and crab apples are going to grow near junipers in their range and on some levels it is pretty cool, certainly unusual.