Unfortunately, roses are not care-free plants, at least not in the Midwest. They are susceptible hosts to several diseases, namely powdery mildew, rust, black spot and rose rosette.
Powdery mildew: The name is apt for this disease, as the foliage of affected stems turns to a whitish cast as if dusted with flour. Powdery mildew is usually more likely to infect roses when they are located in low sunlight, low air circulation locations. For control, prune out heavily affected stems and apply funginex, myclobutanil, chlorothalonil, triamedifon, fenarimol or horticultural oil according to label directions. Cleaning up and disposing of the fallen leaves is also effective as a reduction of innoculum measure.
Black spot: The name is apt for this disease too, as the main symptom is a yellowing of the leaves and the development of quarter inch black spots. After a couple of weeks of infection, the leaves eventually fall off. Adequate air flow is very important for the prevention of black spot, as is avoidance of overhead watering. As with powdery mildew, disposal of fallen leaves is very helpful in reducing the incidence of black spot. Fungicides registered for control of powdery mildew are also effective for control of black spot. Growing a rose cultivar with a history of resistance such as Buck roses, Easy Elegance, Canadian Explorer Series, Knock Out Series, Parkland Series and Rugosa roses goes a long way towards control as well.
Cankers: Cankers commonly occur on plants that have been weakened by winter injury, black spot or poor nutrition. They first appear as small reddish spots on the stems. They enlarge and eventually encircle the entire stem, causing individual canes to die. Pruning these stems out of the plant is the best control measure. Disinfect pruning tools with a 1:10 solution of bleach solution, (1 part bleach, 10 parts water) between cuts.
Rose Rosette: This mycoplasma induced disease produces striking symptoms, as if the plant has been sprayed with a broadleaf herbicide. The stems begin developing as flattened, thickened and twisted versions of the normal looking ones, usually with a reddish cast. Unfortunately, there is nothing that can be done to control this disease, other than to dig out and destroy the plants. This is recommended as soon as the disease is confirmed to avoid possible spread to other plants in a rose garden.
Rose rust: Rust diseases are common on many plants including turfgrass, hawthorn, coneflower and wheat. Rust tends to occur in the fall, and is common where low air circulation prevails. Spraying with the aforementioned fungicides provides some relief, however, increasing the spacing between plants, perhaps even removing and replacing some plants is most important.