Lots of black knot here in SW Wisconsin. I have lost Stanley and Seneca to black knot (Seneca seemed only marginally hardy). I have Pozegaca and Ersinger trucking along fine here as long as I am diligent about removing bk strikes (one or two a year). Biggest difference is the latter two varieties are in the middle of my apple orchard away from wild trees while the former were near the woods. So, anecdotally, it seems that isolation from wild inoculum might matter.
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