Symptoms
In early spring, large, elongated patches of rust may develop on the stems and leaf stalks. These are up to 2.5cm (1in) in length and are bright orange due to the presence of masses of spring spores. The fungus can persist in the stems for many years, producing a fresh crop of spores each spring. Other diseases such as rose canker can invade the cracks in the stem caused by this stage of the rust. The spring spores infect the leaves to produce numerous orange pustules, this time containing summer spores, which continue to spread the disease through the growing season. Towards the end of the summer, black pustules develop, containing overwintering spores. Severe infection by rust can lead to premature defoliation, which weakens the plant sufficiently to cause dieback or even death.
Cause
There are a number of different rose rust fungi, the most common being Phragmidium tuberculatum. The overwintering spores can survive on fallen leaves or on the soil surface. They can also become attached to objects such as stakes, fences and rose stems. The fungus can also survive the winter in the stem lesions, as described above.
Non-chemical control
Pick up and burn or otherwise dispose of fallen and infected leaves. Do not plant roses too close together. Prune to keep the bushes open, and cut out and destroy any rust lesions on the stems, as well as old and weakened wood. However, to control rust these cultural methods should be combined with chemical treatment.
Chemical control
Spray plants with a fungicide in spring before the overwintering spores germinate, taking care to cover both sides of the leaves and the stems. In summer, only the lower leaf surfaces need to be protected. Suitable chemicals include mancozeb (Dithane), penconazole (Scotts Fungus Clear) and myclobutanil (e.g. Systhane Fungus Fighter or Roseclear 3, which also contains an insecticide), all of which will also help to control powdery mildew and blackspot on roses. Avoid using products that contain an insecticide, if no pests are present.