This is a very aggressive fungus that attaches itself to any part of the leaf surface.
When established, it starts showing as a yellow spot below the leaf skin, gradually growing until it is a small coin size. The leaf skin then perforates, exposing the rust coloured spores that look like reddish brown powder. The wind will distribute the spores over a wide area, where they quickly establish and grow.
The best treatment is to cut heavily infected leaves and dispose in a closed plastic bag – do not leave on the compost heap. Un-perforated spots may be treated by breaking the leaf skin and applying a drop of formalin/formaldehyde. Take care to avoid run-off and do not spill any in the centre of the plant. The formalin leaves a dead spot on the leaf but is a very effective treatment. General fungicides have no effect once the fungus is established.