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Canker
A canker is another fungal disease that causes the canes to turn black or brown. All you have to do is prune the canes just below the canker in the early spring.
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.
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.
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.
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.
