What is happening to my Victoria plum tree leaves?

[Ally0007]

Hello!
I planted a Victoria plum sapling three years ago. It is now 12 feet tall. Each year, the leaves become progressively perforated throughout the summer, with brown spots and some leaf yellowing too (see photo). I would be extremely grateful if someone could identify the problem and recommend a treatment. The tree gets adequate water. The soil is clayey and alkaline, but there are adequate nutrients, as I dug manure and bonemeal into the soil. I have another plum tree, 10 feet from the affected one. That one is not affected. Here is a photo of a couple of leaves. The greener leaf is typical, representing 99% of the leaves. The yellowed one is representative of about 5% of the leaves. There are no obvious bugs or caterpillars present. A couple of ants and spiders; that's all. Affected leaves are beginning to fall off the tree. I did make the mistake of pruning during winter. All branches are equally affected. There are no gooey fissures in the bark. Thank you kindly for any help!

[devonuk]

I think the holes and the brown discoloration are shot hole aka coryneum blight, if you do a search for those terms. Not sure about the yellowing but it's probably liveable with if only 5% affected.

For shot hole I have tried removing affected leaves but it comes back; I don't think it is life-threatening. The advice was treat with Bordeaux mixture which is now banned. It is a mixture of two readily available ingredients, but I have no idea whether identifying those ingredients and making it up yourself for home use is caught by the ban or not.

[Brack]

Bordeaux mix is copper sulphate and slaked lime, both pretty easily obtainable. Not sure on proportions for the mix.