Rusty-red spots on the underside of the leaves are an indication of this fungal problem. The leaves can show small yellow spots on the tops of the leaves. On touching the rust spots will leave an orange powder on your fingers. Only in severe cases will plants affected with rust lose vigour, but it should be treated when it appears, mostly in late autumn.
Worth trying a milk spray – 1 part full cream milk to 10 parts water sprayed up under the leaves to contact the rust area.
The best fungicides to use are ‘Mancozeb Plus’ or ‘Eco-rose’. These sprays will kill the spores. Best if sprayed under the leaves for direct contact with the pustules.
Rust is a problem mainly associated with a small number of rose varieties and only in certain weather conditions.