Roses, common problems, brown canker
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Brown Canker is caused by the fungus Cryptosporella umbrina. The disease is commonly found on outdoor roses and occasionally on greenhouse-grown roses. The fungus is capable of attacking any portion of the plant above ground and can result in death of the entire stem.
Symptoms: 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. A whitish patch can be seen as the small spots are massed together. Oftentimes little damage occurs the first year; however, in time the white lesions continue to enlarge and brown cankers (several inches long) form, girdling the stem resulting in death. The cankers may extend down into the crown of the plant and may destroy the entire plant.
Disease Cycle: The fungus overwinters in infected canes and spores can be spread to healthy canes by splashing water, wind, and pruning tools. However, the pathogen can only enter plant tissue through wounds.
Control: If a new rose planting is to be established, care should be taken to select disease-free planting stock to prevent the introduction of brown canker. In established rose plantings, all dead and dying canes should be pruned out and destroyed. In removing diseased canes, make cuts well below the diseased areas. Before each cut is made it is advisable to dip the pruning shears in a 1:10 chlorine bleach:water dilution. Since this pathogen enters the stem through wounds, care should be taken to avoid stem injury.
Treatment: The ONLY treatment is to cut off the cane below the infection and remove it from the garden. If any is left, it can infect the entire bush, so you must get it all. Make sure that you dip your pruning shears after every cut, in a bucket of 1:4 mixture of household bleach and water. You do not want to spread the disease with your pruners. Chemical controls are mostly ineffective for these canker diseases. All of the 'Canker' afflictions can be treated in this manner.
You might also want to read about the Brown Canker disease and its method of control at the University of Minnesota Extension website.
The following website addresses provide additional information.
Rose Cultural Practices and Weed Control
Rose Diseases and Abiotic (Environmental) Disorders
Rose Insect and Mite Pests and Beneficials