Black knot is a serious problem in Calgary. Other than selective pruning and hope, there isn't much to be done. Spoke with someone from City of Calgary urban forestry and he is estimating that Calgary will lose 100,000+ trees to black knot. I pruned it out for 3 years on my two big mayday trees but ended up losing them both this year. Called in three arborists for evaluations and all three said they were too far gone. It just sucks.
If you are pruning, the general advice is remove the pruned wood from the property ASAP (landfill or burn) and wash all your saws etc with bleach. My thought is that if it is in the neighborhood, it will keep reinfecting vulnerable species. I bet we are a dozen trees down on my block alone and there are still a half dozen I have noticed with some infection still.
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