Roses, like almost every member of the plant kingdom are susceptible to a number of diseases. But you can avoid many of these rose problems if you take a few precautions. First pick a good site for your roses or rose garden away from heavy shades trees and in an open area if possible. Then get the soil ready by making sure it is well drained and fertilized. The healthier the rose is the less like it will succumb to diseases. The most common diseases affecting roses are Blackspot, Powdery Mildew, Stem Cankers, Botrytis Blight, Mosaic and Crown Gall.
Blackspot is a fungal disease that can cause the defoliation of rose bushes by early fall and weaken the rose bush by creating severe cankers on the rose canes. You can see it by noticing round black spots on the upper surface of the leaves which start at the base of the plant and begin to move upwards. Of course you will also notice that all of the leaves are turning yellow and falling off your rose bush. Check your rose canes and if you see reddish purple spots then you can be pretty sure you have Blackspot. Blackspot is spread by splashing water and it is often more serious during rainy periods. In the fall you will need to remove all of the dead leaves and if possible relocate your roses to a dryer area. You should apply a fungicide in the spring to kill off any remaining fungus.
Powdery Mildew is also a fungus disease, but it attacks young leaves which causes them to curl, twist and start to turn a purplish color. Later on the leaves become covered with a white powdery fuzz. This disease is spread by the wind and is particularly acute during any period when it is hot and humid during the day and cool at night. Powdery mildew affects the top portion of the plant and it is actually discouraged by water or moisture that collects on the leaves. The remedy for this disease is to prune out all affected canes and to apply a fungicide to all new growth in the spring.
Stem Cankers can be caused by several factors but they all produce the same effects. Brown, oval-shaped or shriveled areas on the canes and wilting leaves that point outward. Cankers need to be pruned out ever year and provide adequate protection to the pruned plant over the winter months. There are no known and effective chemical treatments for cankers.
Botrytis Blight is a fungus disease that tends to be found on older rose plants. It normally attacks dying plant tissue and it likes damp and even wet conditions. This disease is sometimes called gray mold because of the color of the fuzzy mold that often appears on entire flowers. If you clean up the plant and the garden well you can normally eliminate a repeat performance of this disease.
Mosaic is caused by a virus that appears as bright yellow patterns on the leaves of the rose. Once a rose gets the virus it cannot be removed. If you have a plant that is severely infected you need to remove it before it infects other plants in your garden.
Crown gall is a remarkably resistant bacterial disease that can survive in the soil for up to twenty years. It causes dark colored masses or galls to appear on rose stems near the ground. Your best bet is to check the base of all plants before you buy them because they can contaminate the soil in that area for up to five years or longer.