Black knot fungus is caused by Apiosporina morbosa, which is endemic in this area and affects plants in the genus Prunus. This includes cherries, plums, flowering almond, apricot, and Mayday trees. All of the boulevard trees (Schubert Chokecherry) in my neighbourhood have black knot. As the galls produced from the infection release additional fungal spores, it was an uphill battle to keep my tree free of black knot.
Needless to say, I have replaced my Mayday tree with a flowering crabapple which is not susceptible to black knot fungus.