Table of apple cultivar susceptibility to the black knot fungus, Apiosporina morbosa.

^zVR = very resistant. No control needed. (Very few cultivars in this category for any disease.)
R = resistant. Control only needed under high disease pressure.
S = susceptible. Control usually needed where disease is prevalent.
HS = highly susceptible. Control always needed where disease is prevalent. These cultivars should receive first priority when control is called for.