About treating this tree with fungicides and or pestacides.
Its a great tree. I have seen more than one expensive disaster from incorrect dosing of fungicides in particular.
You mentioned spider mites, I have seen damage by mites, both true spider mites and false spider mites, that looks like fungal, bacterial or viral problems. Anytime I think something is off, and possibly a fungus, I tripple check to make sure it is not really mites. Treat for mites first if you think you have both. Clearing up mites and other insects may actually take care of the 'fungus'. Often bacterial and fungal pathogens are not primary, but rather secondary, in that they enter, and take advantage of damage done by the mites and or insects.
Once you are sure the insect problem is under control. Better than 50% of the time fungal issues can be cured by selecting a better location for the tree to grow in. Junipers need sun, and good air movement. Site the tree so it gets 8 or more hours of sun in summer. Don't crowd other trees or plants around it, keep enough space that the air can move freely, 6 inches between the tips of this tree's foliage to the next tree's is a workable minimum. It is not a precise science. Good air movement is what is needed.
Last, if you still think you have a fungus. Treat with a product labelled approved for junipers, and follow directions, both for the tree's sake, and for your own health, wear protective gear. Gloves, safety glasses are minimum.