Rust diseases attack a wide range of plants – but each type is specific to one plant or one group of plants and won't spread to unrelated plants. Generally, they are among the most difficult plant diseases to control once established.
As their name suggests, many rust diseases are rust coloured, and normally they range in colour from being orange to dark brown. The colour can change from lighter to darker as the disease matures and the seasons change from spring to late summer and autumn.
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).
None other than the rust pustules itself – often associated with yellow or pale patches. The presence of this fungal infection may indicate an already weakened plant, so check for other diseases or infestations.
Treatment and control
Immediately remove and destroy all infected leaves and plant parts. Collect and destroy any fallen leaves. Severely affected plants may have to be destroyed.
Keep plants growing as strongly as possible, without any stress, and keep high nitrogen feeds to a minimum. High potash feeds may help to strengthen the leaves and prevent the disease taking hold.
Spray with a suitable fungicide
For roses and other ornamental plants, treat plants with a systemic insecticide. Continue spraying at regular intervals throughout the season, as recommended on the packaging.