According to Jim Hole's newsletter, Reflect, Relax, Reconnect; the City of St. Albert has found a method which seems to be effective in preventing black knot disease.
Black knot disease has been responsible for killing many trees in the Prunus family, namely Schubert Chokecherry, Mayday, Cherry and Plum. The disease appears as a black mass on the branches of any of the aforementioned trees and, if left unchecked, will spread through the tree. If it girdles a branch, or worse yet the trunk, it will kill the branch/tree, respectively. 
As mentioned in my post, Fungal Alert - Black Knot on Local Schubert Chokecherries, if you find this disease on your tree, you must trim the branch back at least one foot below the infected area. Then destroy the affected trimmings as the disease is air born and spreads easily. Disinfect your pruning equipment thoroughly afterwards.
The city tree care team has been out trimming the Prunus trees and applying a dormant spray. Why, you wonder? According to Kevin Veenstra (City of St. Albert and a certified arborist), a spray of a combination of lime sulphur and horticultural oil helps protect the trees from the disease, stating that the trees they've sprayed remain disease free.
What wonderful news! I wish I'd known or thought of this years ago when my Schubert Chokecherry had the disease. Since we have moved, I no longer have that particular tree to care for, but you, my readers, can certainly benefit from this development!!
Please, give it a try. Spray when there is no wind and the temperature is above zero but before the leaf buds begin to swell. Let me know if you feel this makes a difference for your trees, please.
*Warning, the lime sulphur does indeed smell like sulphur but the odour dissipates in time and exposure to fresh air. ;}
**photos taken by myself and previously posted at Fungal Alert - Black Knot on Local Schubert Chokecherries.
