Chrysanthemum Rust 
(Puccinia chrysanthemi)

First found in England in 1895 but known in Japan long before this. It spread throughout Europe and now is fairly common although its importance has been eclipsed by Chrysanthemum White Rust.

Symptoms. 
Usually appearing in late summer, most common on indoor plants and less common on outdoor chrysanthemums. Dirty brown pustules on the undersides of the leaves and yellowish green spots on the upper surface of the leaf. 
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Biology 
The disease produces urediniospores, other types of spores are unknown to this disease, and it either over winters as urediniospores or as Mycelium on old leaves within plant tissue. 

Treatment 
This disease has been little seen for many years but has re-emerged again recently. Consequently recommended treatments are for old fungicides like Bordeaux mixture and copper fungicides.

It would be worth trying modern fungicides used against Chrysanthemum White Rust as these were not available when Chrysanthemum Rust was prevalent.

Hot water treatment has been found to have some affect in its control.