In my area black knot is extremely prevalent, it’s bad enough that damsons, plums, and green gauges are basically impossible to grow (I’ve tried, they end up being more black knot than healthy tree annually, despite pruning and spraying religiously). Despite this, I have a Montmorency cherry with no signs of black knot ever, so clearly it, and perhaps other sour cherries, have a high resistance to black knot.
This gets me wondering: what is the level of resistance to black knot in peaches, almonds, apricots, sweet cherries, etc? Does anyone in an area with high levels of black knot have any experience with other prunus species? I’m especially curious about almonds, as I’m interested in getting an almond tree soon.
Toka plum and other american type plums have shown no knots for me. Shiro plum seems ok as well. Rosy Gage is the worst for me, but all other euro plums have knots as well, even President had one once. I use a blow torch on the knots now, seems to work. I have never seen them on any other stone fruits in my orchard (peaches, cherries or apricots)
Of my named varieties Red Heart plum has been black knot free for 20 years. Unfortunately Red Heart rarely produces and I’m top working it with other varieties. I have/had an old heritage prune type plum tree that was 80+ years old that never had an instance of black knot. Unfortunately, it looks as if this tree has finally given up the ghost this summer, as the minimal green growth it had this year has withered and died. Luckily I had a graft of this tree finally take on a root sucker of another heritage plum tree. The other heritage plum tree I believe to be a yellow egg plum. The yellow egg plum to the best of my recollection only had 1 instance of black knot in 60+ years.
On the other end of the scale is Methley, which I took out a couple of years ago because of its frequent infections. A new tree that is supposed to be Satsuma is my most problematic tree these days. Shiro gets a strike here and there, but nothing too drastic. Mirabelle had one knot this year.