Fig rust is a fungal disease that occurs in high humidity or damp regions, it rarely occurs in dry conditions.
copper sulfate used around the base of the tree as an amendment allows the tree to take up the copper which helps the tree build rust defenses.
Removal of all infected leaves as soon as detected also helps with control of the disease (it tends to be found more in areas with high numbers of conifer trees
such as junipers and pines, they might be the carrier).
We use copper sulfate as a surface amendment once a year around all our orchard trees, until testing shows there is enough
copper in the tree leaves, (usually around 3 years), then we apply every other year.
Rust fungus does not limit itself to just figs, pears, plums, apples, peaches, in fact just about any fruit
tree can be effected by rust disease.
If you have a lot of junipers (more of the culprit than pines) removing those within a 100 yard
circle of your infected trees can help greatly to reduce the recurrence of rust disease if the other measures are also completed.