This is a fungus caused by open lesions or cuts on your plants. It will mostly occur on canes and stems. These wounds can happen in many ways. Bad pruning or trimming can easily leave your roses susceptible. Allowing the canes to rub on each other cross wise can also cause friction wounds. Be sure to give proper spacing for your roses.
Here are the three types:
Coniothrium fuckelii - Common, symptoms of dried up legions
Cryptosporella umbrina - Most common but most noticeable
Coniothyrium wernsdorffiae - most difficult to treat
There are different stages to this fungus. In the beginning, they are only small red/yellow spots on the cane. Over the winter in some colder zones, they may initially show as black, but will turn to red/yellow once the cold protection is removed. Thes red/yellow spots will then spread and potentially circle the branch, changing to a brown color. If it is allowed to surround the whole cane, it can kill of growth entirely. Its best to nail this one early if you can.
To prevent stem canker, keep in mind the following:
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