Paul Shirville, Photo By: Irene Hannestad
I saw several lesions that had dried up and looked hollow. Above the lesion there was strong new growth. How do you get canker and what can you do about it? - Heide
This is definitely a symptom of Stem Canker, a fungal disease that can affect any part of the plant, but most commonly attacks stems and canes (hence, the name). The fungus enters through wounds caused by crossing canes, improper pruning, even deadheading. It is spread by water splashing on the plant from irrigation systems or rain.
Cankers come in three flavors. The pathogens scientific names are Coniothyrium wernsdorffiae, Cryptosporella umbrina, and Coniothrium fuckelii. C. wernsdorffiae is the most severe looking of the three. C. umbrina is the least noticeable. And your symptoms describe C. fuckelii.
In general, the cankers start as small yellowish or reddish spots on the bark, gradually increasing in size. Eventually, the canker turns brown, increases in size, and may eventually girdle the cane. The tissue within the canker begins to dry out and shrink, giving it a somewhat shrivelled appearance. If the canker only covers part of the cane, growth above the canker will continue. If it girdles the cane, however, growth will die.
One more thing for the northern rosarians who may be reading this article. After removing winter protection in spring, cankers can appear as black saddles on the cane. As they are exposed to air and sunlight, however, they quickly change in appearance to those symptoms described above.
Controlling Stem Canker is a combination of proper pruning procedures and appropriate fungicides.
As stated above, keep canes from crossing each other. Even if they appear not to be touching,
wind and growth can change that condition.
Pruning out dead or dying wood during your regular visits to the garden is also essential.
And when planting new plants, use care not to damage canes. If you should damage a cane seriously, consider
removing the cane entirely.
Likewise, when pruning or deadheading, cut back to the node. Failing to do so, results in dieback
to the node, and the resulting dead wood is an invitation to canker spores.
As for fungicides, those that control black spot on ornamentals will also control canker.
By Mark Whitelaw, Kindly Provided by Laura Whitelaw