Rust is a fungal disease that seems to be more prevalent in West Coast gardens; it is rarely indicated east of the Rocky Mountains. Rust appears as its name implies, as red-orange spots (raised looking like warts) on undersides of leaves and yellow blotches on top surfaces. Long, narrow rust spots or streaks may also form on young canes. If left on roses, these spots will develop into large groupings of rust ‘warts’ and in autumn they will turn black. Ultimately this disease will cause the entire defoliation of the rose plant.
Rust thrives in cool, moist weather (18 to 21 degrees Celsius, 64 to 69 degrees Fahrenheit), especially in rainy, foggy or misty conditions. This disease will develop on leaf surfaces that remain wet for 4 hours (as can occur during summer fogs, heavy dews or extended rains). Reproduction of rust spores occurs every 10 to 14 days throughout summer. If rust infected leaves are left on the damp ground, insects, rain and wind can spread the disease spores. In these conditions it is imperative to keep an eye on your rose bushes. Any appearance of rust must be dealt with immediately. Remove any affected leaves and bag them or burn them. Do not add the diseased parts to the compost as they will return to haunt your garden the following year when recycled back into the soil. Spray the rose bushes with wettable sulphur (do not use sulphur on days when the temperature exceeds 80 degrees Fahrenheit or 26+ degrees Celsius or you will injure your plants) or fungicidal soap.
One way to combat rust and the other fungal diseases, mildew and blackspot, is to minimize excessively water logged soil around rose plants. Good soil drainage is essential for moisture control; adding organic matter, double digging beds or planting raised beds are effective means in providing a good healthy environment for roses. Also it is best to avoid working in a wet rose garden so as not to help spread rust spores (this is also sound advice regarding minimizing the spread of blackspot and mildew). Try to limit wetting the foliage on rose plants while watering and provide good air circulation between plants by spacing them well apart from one another (planting hybrid teas and smaller rose bushes 3 feet apart and larger rose bushes 4 feet apart). Prune to keep the centers of rose bushes open for air circulation as this will assist in keeping them drier.
Remember that rust spores overwinter on leaves and canes and are distributed by wind and water. A good autumn cleanup will help prevent spores from hiding in your garden throughout winter. If winter ends up being cold and harsh, chances are that rust will be limited, especially if the following summer is hot, conditions not favorable to the disease. Happy gardening!