With the wide variety of roses come a wide variety of diseases. Some are more serious than others. Some are just cosmetic. A few have no known treatment, leaving the infected rose bush no hope of recovery. While prevention is the best option, some of the diseases can be treated if caught in time.
Fungal Disease Treatments
Roses are susceptible to several fungal diseases including black spot, rust, blight, brown canker and powdery mildew. Spraying with a fungicide at seven- to 10-day intervals is sufficient to remedy these diseases. In periods of vigorous growth or during heavy rains, the fungicides may have to be applied twice a week to provide adequate coverage.
Rose rosette disease is the most serious of the viruses affecting roses, and symptoms can vary from one rose type to another. Stunted and discolored leaves, deformed flowers and excessive thorniness are among the symptoms displayed. There is no control for this virus. Once the rose plant is infected, the best that can be done to stop the spread of the disease to other plants is to remove and destroy the affected plant.
Crown gall is caused by a bacteria living in the soil. The bacteria enter the plant either directly through the roots or through wounds to the rose bush in the root area. The effect is abnormal cell growth, which causes a tumor-like growth. There is no chemical treatment for this disease. If galls are mild, they can be pruned away. If the disease has become serious, the plant and the soil surrounding it should be destroyed to prevent the bacteria from spreading.
Most fungal diseases are prolific when conditions are warm and moist, such as in the spring. The spores from fungi can overwinter in plant debris such as fallen leaves and plant cuttings. Aphids play a significant role in spreading powdery mildew as the honeydew they create on leaves is a breeding ground for the spores. Eriophyid mites spread rose rosette disease.
Fungicide programs begun in spring and continued throughout the growing season are effective in preventing these diseases. Sound maintenance practices that include removing plant debris also reduce the risks. Regular pruning provides good air flow so plants can dry quickly. A practice of watering early in the day so that water evaporates can limit the chance for fungus spores to invade healthy plant tissue. Use of an insecticide that treats for aphids can reduce the occurrence of powdery mildew. A miticide used on plants near the location of a previously infected rose bush can control the mites that cause rose rosette disease.