So, no strong opinions. It could mean that it is neither recommended nor discouraged?

You probably won't kill it by doing it now but often pruning early in the growing season exposes the tree unnecessarily to the disease pathogens that are present then. And the sap is already running so you can get some bleeding.

Ideally, fruit trees are pruned while still dormant....late winter or very early spring. Long before signs of any budding. The second best time is in summer....late June to mid July.

Unless you live in the British Isles. In which case it is specifically recommended to prune plums, unlike other fruit trees, in full growth to avoid silver leaf disease. But plums don't need much pruning anyway. Is there a reason you feel yours needs it?

Thanks gardengal. I know that the best time for pruning is very early in spring and agree with you. However pruning fruit trees in the middle of summer when they are ripening their fruit sounds controversial to me. Could you, please, elaborate on this a little bit?

" But plums don't need much pruning anyway. Is there a reason you feel yours needs it? "

Over twenty five years since it was planted the tree never stopped developing new growth. Going straight up suckers, little branches growing in all directions criss-crossing with bigger branches and each other, bigger branches growing bigger still. I usually removed growing up suckers. A particularly heavy snow one winter broke one of the biggest branches, and unusually heavy load of fruit a few years ago broke another. What remains is a thicket of criss-crossing branches on one side of the tree only. I opened the tree already several weeks ago by cutting everything what was going down, up, and in-between, but would like to cut a few more little branches. Hence my original question.