Diseases of Roses

Diseases

Blackspot

This fungal disease can cause almost complete defoliatiation of bushes by early fall, resulting in a weakened bush on which cane die-back and cankers become severe. Blackspot is identified as circular black spots that appear on the upper surface of the leaves, starting at the bottom of the plant and moving upward. Infected leaves turn yellow and fall off prematurely. The fringed margin and black color distinguish this leaf spot from others. Infections on canes are identified as reddish-purple spots. Splashing water spreads blackspot. Infection occurs after leaves are wet for several hours, making it more serious during rainy periods. Some roses are less susceptible than others, so cultivar selection is important. The fungus overwinters in fallen leaves and stem cankers. Raking and removing these leaves in the fall may help provide some control. Avoid wetting the leaves when watering and locate plants where there is good air circulation. Fungicide spray programs need to be started as soon as new leaves appear in the spring. Mulching also helps keep water from splashing up onto the leaves.

Powdery Mildew

Powdery mildew is a fungus disease that affects young leaves, causing them to curl and twist and develop a purple coloration. As the disease progresses, leaves become covered with white powdery fuzz. Whereas blackspot is usually most severe on the lower part of the plant, mildew affects the top part of the plant. Mature leaves are less likely to be affected. Mildew is spread by wind and develops rapidly during periods of warm, humid days followed by cool nights. Mildew is actually discouraged by the presence of water on the leaves. Infection can be reduced through sanitation and fungicide spray programs. Prune out all dead and diseased canes to reduce initial fungus infection. Because new growth is especially susceptible, thorough coverage of new growth with fungicide is important. Plant roses in areas where they receive good air circulation and where the foliage can dry off rapidly in the early morning.

Stem Cankers

There are several fungi that cause cankers on roses. The different fungi can cause different-looking cankers, but they usually produce brown, oval-shaped, sunken, or shriveled areas anywhere on the cane. The cane dies, and leaves wilt from that point outward. Sometimes small black specks can be seen on the cane surface within the borders of the canker. These are fungal spore-forming structures. Cankers should be pruned out each year. Make the cut well below the affected tissue. Protect the plant from cold or freeze injury by providing adequate cover over the winter. Keep plants vigorous with proper culture and disease control. Canker is a disease of stress. If plants are kept actively growing, they stand a better chance of avoiding cankers. There are no effective chemical controls for canker disease.

Botrytis Blight

Botrytis blight is a fungal disease that generally attacks dying tissue. It is frequently found on older flowers and other plant parts. Under certain conditions it may also attack healthy tissue. Botrytis favors moist, wet conditions, often causing the disease to attack entire flowers and produce a gray fuzzy mold. This disease is often called gray mold. Good garden sanitation and removing spent flowers often result in good control of this disease. When this is insufficient in providing adequate control, a preventative spray program may be necessary.

Mosaic

Rose mosaic is caused by a virus. Bright yellow patterns made up of wavy lines may appear on the leaves of some varieties. Other varieties may show no yellow lines, but may be stunted and weak due to virus infection. Virus-infected plants cannot be cured. Plant virus-resistant roses if possible. Try to control insects, especially aphids, since they help spread the virus. If you are pruning virus-infected plants, don't prune healthy plants unless you first disinfest your pruners. Dipping the blades in a 10 percent solution of chlorine bleach and water for 60 seconds can do this. A 25 percent concentration reduces the time needed to about 10 seconds. Severely infected plants should be removed and destroyed to reduce the spread of the virus to other plants.

Crown Gall

Crown gall is a bacterial disease that can survive 15-20 years in the soil. It causes irregularly shaped, rough, dark-colored masses (galls) to appear on stems near the soil line. These galls can appear as small swellings or be several inches in diameter. Severely infected plants become stunted and fail to grow properly. There are no effective controls for crown gall. Avoid buying plants with suspicious swellings or gall on lower stems or crowns. However, do not confuse crown gall with normal swellings that you see as a result of the budding process. Protect plants from injury on stems during cultivation. Maintain vigor with fertilization and watering. Remove and destroy badly infected plants and do not replant in that area for at least five years. Crown gall is not specific to roses and can affect apples, raspberries, honeysuckle, euonymus, and many vegetables. For this reason, roses should not be planted where plants susceptible to crown gall have been removed because of the disease.

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