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.

Here is some goof info. http://www.ipm.iastate.edu/ipm/hortnews/node/1472

You have brown rot on the cherry and the beginnings of black knot on the plum. Also looks like some shothole fungus on the plum leaves. About the only thing you can do at this point in time is to prune out the areas of the tree affected by the black knot. The other problems are fungal in nature and likely developed due to overly wet spring weather and mild temperatures. These are problems we see virtually every spring here in the PNW :-)) You can spray to help control but too late now to be very effective. I would recommend removing all the dead/.browned spurs on the cherries as the brown rot can travel into the fruit as well as the leaves and twigs