Roses are susceptible to a variety of diseases, according to the University of Minnesota Extension. Since a rose bush should last many years, it is important to buy and plant disease-free specimens from disease-resistant varieties. Give your rose adequate air circulation and soil with good drainage, and check the plant frequently for signs of disease or poor health.
While many fungal diseases require damp conditions, powdery mildew, caused by the Sphaerotheca pannosa fungus, can occur in both dry and wet weather. According to the University of Colorado Extension, the fungus "produces a white, talcum powder-like growth on the top and bottom of the leaves and stems." Serious infections can cause buds not to open and leaves to turn a purpleish color, curl up and fall off the plant. Treat this disease with weekly applications of a fungicide, such as triforine or chlorothalonil, and by removing diseased portions of the plant.
Botrytis blight, caused by the Botrytis cinerea fungus, is a disease that primarily affects flower buds, particularly those of hybrid roses. This disease can cause buds to shrivel and die. Look for black lesions on flower buds or gray-brown growth on buds and stems. Roses are particularly susceptible to Botrytis blight in cool, damp conditions. Promptly remove infected buds and stems to prevent the release of spores. Spray the plant with a fungicide such as chlorothalonil or mancozeb as well.
Unlike most other fungi, brown canker, caused by the Cryptosporella umbrina, can attack the plant anywhere above the ground. According to the University of Minnesota Extension, during a brown canker attack "small red to purple spots appear on the current year's canes and with time, these spots usually develop into gray-white lesions on the stem surface." If the infection is untreated, it can cause large lesions which can girdle and kill the rose plant. Dip a pair of shears in a solution of one part bleach and 10 parts water and cut all infected portions of the plant off 2 to 3 inches below where the infection is visible. Dust the plant with sulfur to treat the infection.
Rose mosaic is a common rose disease caused by the Rose Mosaic Virus. According to Rose-roses.com, this virus causes "a fern like zig zaggy pattern" of "yellow to whitish lines or spots in the leaves," a characteristic which gives the disease its name. It can also cause yellowing of rose veins and swirly blotches called watermarks. Rose mosaic reduces the health of the plant, making it more susceptible to winter weather and other stresses, but rarely kills it. Unfortunately, it cannot be cured, but the disease is not contagious.