Rust spores need a moist environment in which to prosper. The fungus develops mostly on leaves but also on stems. In appearance they can develop either as patches or as pustules (like septic spots). Normally they range in colour from being orange to dark brown. The colour can change from lighter to darker as the seasons change from spring to late summer and autumn.
None other than the Rust itself although the presence of this fungal infection may indicate an already weakened plant so check for other diseases or infestations.
Treatment and control
General tips: Immediately remove and destroy all infected leaves and plant parts. Collect and destroy any fallen leaves.
Spray with a suitable fungicide: For roses and other ornamental plants, products with one of the following active ingredients will enable control although directions should be checked to ensure suitability for specific plants: RoseClear Ultra and RoseClear Ultra Gun! (contain triticonazole and acetamiprid), FungusClear Ultra (contains triticonazole) or FungusClear 2 Gun! (contains myclobutanil). Continue spraying at fortnightly intervals throughout the season.