Before the leaves return to your trees, now may be a good time to look for Black Knot Disease.
According to the Government of Alberta, this has become a significant and widespread problem across our province.
"Black Knot, caused by the fungus Apiosporina morbosa, is a very common disease of plants in the genus Prunus."
"This disease reduces the aesthetic value of affected specimens, as infections spread rapidly; high levels may result in the eventual death of the plant."
Here is a list of plant species affected by Black Knot:
The most distinguishing symptom of Black Knot is the characteristic black, tar-like swellings that develop on branches of the infected plant.
What can be done to control Black Knot?
- Removal of sources of inoculum (prevents population build up)
- Prune out all knot-bearing branches during late fall, winter or very early spring when plants are dormant, and knots are easy to see
- Remove infected branches to at least 15-20 cm (6-8 inches) below knot. NOTE: It is preferable to prune an infected branch further back to an appropriate location, such as a healthy collar, rather than leave a stub
- As a precaution, cutting blades should be cleaned and disinfected after pruning, if possible, especially if cuts have been made through obviously infected material
- For knots on scaffold branches or trunks that can't be removed, cut away diseased tissue down to good wood and at least 1 cm (1/2 inch) beyond the edge of the knot
- Failure to remove branches beyond the internal growth will result in re-growth of the fungus
- Diseased wood must be destroyed immediately (burned, buried or removed from site). Diseased knots can produce and release spores for up to 4 months after removal. Proper composting can help to accelerate the breakdown of infected materials
- Ensure plants are healthy and free from stress (not a guarantee from disease)
- Regular monitoring
- Ensure adequate canopy ventilation through proper pruning
- Chemical control (preventative not curative)
- Few choices available
- Not usually recommended unless for valuable plantings, such as collections, orchards, arboreta or for severe infestations
**with information provided by The Government of Alberta
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