 a damaged cane is an open door for pests and diseases. Avoid long intervals between watering. Have a neighbor water your roses while you are on vacation.
Keep the area around the bush clean and free of dead and diseased leaves and branches. (Do not put the debris in the compost pile.) Inspect your plants often and act as soon as you see a problem. Prune out any canes with cankers, discoloration, or sunken areas, cutting 1 or 2 inches below the infected area. Dip shears in a solution of rubbing alcohol or household bleach and water at a ration of 1 to 10, before moving to the next bush. If diseases or insects become a problem, use only those pesticides recommended by your Extension Office for specific problems and follow the directions exactly when mixing and applying chemicals.
The most common diseases are powdery mildew and black spot. Caused by fungi, they can be controlled with fungicides, but rarely eliminated entirely.
Powdery mildew occurs most commonly in spring and autumn when the days are warm and the nights cool. The leaves, buds and stems are covered with white or gray powdery coating. New growth may be stunted and curled and may drop off. The buds are malformed and fail to open.
Black spot is most prevalent when moisture remains on the leaves from overhead watering, long rainy periods, or heavy dew. The fungus over-winters in the canes and on infected leaves and reappears on otherwise vigorously growing plants in the spring and summer. Complete defoliation may even occur on susceptible varieties. Black spot looks just like the name implies--black spots (sometimes surrounded with yellow) on the leaves grow in size until the leaves drop off. When on the canes, the spots are reddish-purple.
For black spot and mildew, spray both the top and undersides of the leaves as soon as they appear in spring with the recommended fungicides. Continue spraying every 7 to 10 days throughout the growing season.
Another fungus disease, Botrytis blight, occurs during long periods of frequent showers. The flower bud fails to open and the interior of the bud is usually filled with a cob-webby mold. Control is just to cut all infected buds along with several inches of stem and remove from the garden. Other diseases that can affect roses are mosaic, stem canker, and crown gall. They are not usually a problem in clean beds where sanitation is practiced and no fallen debris is allowed to remain. If you suspect a problem with your roses that is not powdery mildew or black spot, call your Extension agent.
Roses are attacked by many insects. The most common in this area are described below. Remember when using pesticides that they are nondiscriminatory, killing even the "good bugs", so learn to identify specific insects and use only recommended pest management for them. Experiment with companion plantings and natural predators. While enjoying your evening walk in the rose garden, be on the lookout for problems and act early.
- Japanese beetles. The Japanese beetle attacks rose flowers and foliage during July and August. This beetle is about 3/8" long with metallic green and coppery-brown wing covers. Japanese beetles can be successfully controlled by commercial traps placed away from the garden to attract beetles away from the roses. Treating your yard and garden in early spring for grubs will help to keep the mature beetle population down during the summer months. If only a few are seen, just hand-pick and destroy.
- Aphids. Several species of aphids occur on stems, leaves, and buds and often occur in large numbers. By sucking the plant juices, they stunt the plants. Consult your local Extension Office for both chemical and nonchemical control.
- Rose Stem Borers. A hole in the end of a rose cane or stem probably indicates the presence of a borer which has burrowed down deep into the stem tissue. Infested stems usually die back and the entire plant can be lost if borers are not controlled. Cut and destroy infested stems, then seal the fresh cut with white glue. Make a habit of sealing the ends of stems whenever you gut flowers to bring 