A good defense against disease is to begin with a healthy rose bush and keep it well watered and well fed. The garden should be free of weeds and debris and any cutting on the rose bush done using a garden shear that has been sanitized. Too much moisture, rose bushes planted too close together, too little sun, dirty garden tools, exposure to diseased plants and careless handling will make the rose bush more susceptible to disease.
Dead and dark areas along the cane are an indication of cankers, which is caused by fungi. The fungi enter the plant via any pruning cuts, which is one reason to always sanitize your gardening shears before and after use. Highly susceptible areas are cuts made too high above a bud eye, areas where thorns have been removed or at a graft union. Cankers will inhibit the flow of nutrients, causing the flowers and leaves to wither. Once a plant is infected, the best option is to remove and destroy the plant so the disease does not affect the other roses.
Crown gall is a disease that shows up near the soil level or roots of the rose bush. Caused by bacteria, crown gall shows up as tumor-like growths. One way a plant catches this disease is through wounds inflicted by careless garden tools or grafts. A rose bush with crown gall will have stunted growth and eventually dies. All types of roses are susceptible to crown gall. Like a rose bush suffering from canker, the bush may eventually need to be destroyed, but first try cutting out the affected area and spraying it with a disinfectant.
Powdery mildew is one of the most common rose diseases. A whitish, powdery substance appears on the plant and the leaves brown and curl. Instead of opening, the buds will shrivel. Common in humid climates, roses that receive too little sun or are crowded together are most vulnerable to this disease. Rose bushes can be sprayed weekly with Ortho Funginex for prevention or twice weekly if the plant has the disease.
Signs of fungus can show up on the leaves of the rose bush. Rust, spot anthracnose and rose blackspot all show their signs on the leaves of an infected plant. With rust, reddish, wart-like growths appear on the underside of the leaves. Eventually the leaves will wither and drop. During the fall, the growths or spores will blacken. A leaf showing signs of spot anthracnose will have red-rimmed white spots that turn yellow. The leaves become riddled with holes and drop. With rose blackspot, black spots appear on the leaf. Areas around the spots yellow and eventually the leaf drops off. They can all be treated with a fungicide such as Funginex. However, rose blackspot is difficult to cure during the season. Removing the lower leaves is advised, as it can affect other rose bushes.