I would question any table if I didn't know how they came up with their evaluations (well, it if contradicted my own experience, that is). Maybe it is a question of state, but in the sites I manage Methely is much more susceptible than any of the many plums growing- this includes over 20 different varieties, including Damson. In my nursery I have Methely growing with other varieties and over the years it is so hard to manage for black knot that I'm not going offer it anymore. You can see all the extra wounds in this variety from cutting, cutting , cutting. In home orchards there is usually some sun blockage and if this occurs from the east the equation will be much different than on any university testing site. I have 3 Damsons on a site with good exposure and it doesn't suffer more than other plums there. However, on that site Methely doesn't suffer a whole lot either although a bit more than the Damsons. But then, I've resisted Damson's tendency to be a dense upright growing tree- a form that makes drying difficult. You cannot know about how easy or hard black knot is to manage unless you have managed it at multiple sites. There is huge variability on the problem that often can't even be explained by sun exposure or nearby native cherries.

Both 2 and 3 will not be helpful. Even the most affective synth fungicides have limited efficacy against this disease but chlorathalinal is said to be helpful although it's hard to tell in my experience. It certainly doesn't make a big difference. You needn't treat with alcohol between wounds if you cut to clean wood. Next year cut out knots as they occur. Keep plum growing with good vigor.

My parents have a plum tree in their backyard. It would bloom and attempt to fruit, but never got much fruit due to drop and what looked like disease. My dad finally cut it down to about a foot above the crotch. We placed a big flat rock on top of the branch stubs so it fit really nice, almost like a table. The tree ended up growing around the rock holding it securely, and the new growth looks nice compared to the gnarled, black knotted part that was there before. I'd say if you're at the point where you're going to cut it down, try cutting all of the top growth off.

when did you get the house .. from the last pic.. the pruning cut.. makes me think its a brand new planting ... straight from the seller .. meaning.. a shipping cut ... if so ... we dont know if it was properly planted .. properly watered ... or anything ... it is rather surprising.. how a stressed tree .. is attacked by all kinds of problems... but unless you solve the stress .. then solving all the secondary problems is useless ... lets have a pic of the whole ... and then see what we can intuit ... it might be simpler.. and cheaper.. to be done with it ... i never understand.. spending 20.. 30.. 50 dollars on remedies.. for a tree not worth 20 bucks ... sometimes.. its just easier to start over.. if in fact you want a plum... which is subject to all kinds of problems.. like black know and gummosis ... on older trees .. i gave up on plum decades ago ... just a PIA tree ... also.. there is a very active fruit forum ... no reason not to try both forums ... ken