Roses make a beautiful, showy addition to a garden, but as attractive as they are, they can be susceptible to a number of diseases. Climbing roses are no exception. While the versatility of the climbing rose makes it attractive to gardeners, climbers can actually be more vulnerable to disease because they are often planted against walls and in cramped areas where poor airflow and close proximity to other plants can cause fungal infections to spread rapidly.
Rose rust is caused by several different fungi that infect the rose's leaves. It usually appears at the beginning of the summer as small yellow spots above orange bumps full of spores on the underside of the leaves. These spots turn black as the growing season progresses. While this disease does not appear to be serious, it can be deadly. A severe infection can weaken the whole plant.
To treat rust, prune out the infected foliage as soon as you see it, and remove any dead, dying or pruned foliage from the garden immediately. To prevent rust from developing and spreading, you can spray the rose all over with chemicals such as mancozab, myclobutanil, penconzale or triticonzale. If you want to use an organic deterrent, consider a neem oil and water mix sprayed directly on the foliage.
The black spot fungus appears as round black spots on the leaves of the roses. It can progress to a large lesion surrounded by a yellow ring before the leaf shrivels and drops from the plant. Black spot can also cause the rose blooms themselves to become deformed, and the stems can also develop black areas. While black spot alone will not kill the rose, it does weaken it so that it is more susceptible to other more deadly diseases, as well as extreme weather conditions. While the best preventative comes from planting disease-resistant cultivars, there are several things you can do to protect the roses you already have.
Begin treatment, spraying with chlorothalonil or triforine, as soon as you notice the signs of black spot, and should continue to spray it once a every week and again after each rain. You can also dust the plants with "Massey dust," which is comprised of mostly sulphur with a little lead arsenate. Because the disease tends to quickly infect wet leaves, be sure to leave plenty of space between your rose and other plants for good air flow, and always water from the bottom of the plant using a soaker hose. Take preventative measures against black spot immediately after the growing season by removing all old leaves and prunings from the garden area and burning or bagging them, and again during the spring pruning.
Climbing roses are particularly susceptible to powdery mildew. This fungus initially looks like small spots of a whitish-gray dust on the surface of the leaves and stems. With time, the spots spread over the entire leaf surface and create wide patches on stems. This fungus is highly contagious, and can spread to nearby plants as the spores are spread by wind and water. The fungus will overwinter in the garden on diseased leaves and garden debris, so you must practice good garden hygiene, removing any prunings or fallen leaves immediately and burning or bagging them.
Because powdery mildew is far more common in gardens where the plants are crowded close together or that grow close to buildings or on walls, allow adequate space between your climbing roses. Once the plant is infected, begin treatments as soon as you see the first spots of mildew, continuing use throughout the growing season, particularly in humid or rainy weather. Some of the treatments for powdery mildew are neem oil, potassium bicarbonate, sulfur or lime sulfur, or Bacillus subtilis. You can also use a baking-soda solution to hold the fungus at bay, although it will not treat the fungus once it is present.