From Tom’s How Can a Leaf Rust? Roya in Guatemala which I humbly suggest should be required reading for folks in the specialty coffee trade.
See also: JimSeven’s Leaf Rust: End of Days?
The knowledge of how to fight Roya, and the money for fungicide, the labor to treat it, and to cut back the trees deals a huge blow to the income and profits of the coffee farm. The real impact is on organic farms, whether certified farms or organic-by-default farms, on casual coffee farmers who have little technical knowledge, and on smallholder farms in general. In a couple crop cycles, Roya unmanaged is the death of the coffee farm.