Stem canker, is a rose fungus that may cause stems to die back, and leaves to yellow and drop off. The most common species of fungi that cause this is Ciniothyrium.
The fungi will enter the plant at a wound caused by thorns, or at a cut stem.
A sunken canker will develop and expand in all directions through the tissues.
The leaves on the rose plant will wilt or yellow as the rose fungus cuts off the flow of water and nutrients to the plant.
Stem canker usually occurs during wet or humid weather. If your rose is already infected with black spot, or in a weakened condition from any other fungus on roses, it is more susceptible to being invaded by this canker.
What does it look like?
You will notice yellow, reddish, or brown colored areas on the canes that are sunken in.
These sunken areas may have a dark purplish area beside the sunken area. They may also be cracked.
The affected leaves could be spotted, wilting, or yellow. The stems of the plant will die back.
What do you do?
You should cut out and destroy the bad canes at least 5 inches below the area that is infected.
Note: Be sure to disinfect your pruning tools after each cut by dipping in a solution of bleach water, so you won't spread the rose fungus to your other rose plants.(Bleach water is a good disinfectant any time you prune different bushes)
After pruning, you could spray the canes with a Sulfur Organic Spray. Repeat spraying every 10 to 14 days for as long as the weather is wet or humid.
How can you avoid it?
You can keep your rose plants healthy by watering, feeding, and pruning them regularly. Healthy plants are less likely to be affected by a troublesome rose disease.
To help protect the canes from this disease; don't let the canes rub against each other, or against other objects such as the trellis or the support structure.
Always remember: Sometimes, even with your best efforts, your roses will be affected by one or more rose diseases. Do your best to treat them organically, and learn from each episode. (Like children, you can't always protect them from everything!).........
If Stem canker is a concern, read about more Rose Disease and Pests
More Diseases of Rose Plants;