If all of a sudden the leaves on fruit trees turn brown and die, it could be an infection with Monilia fungi. Phytophthora and Verticillium pathogens also frequently attack the trees. In all cases, entire branches die quickly without prior signs, leaves turn brown and dry up, and fruit hangs brown from the branches.
Cut off dead branches
The only thing that helps here is the immediate and thorough cutting out of infested branches. Cut right into the healthy wood. In your plum tree, many branches are affected. Nevertheless, only a quick cut will help. Coat the wound areas with wound protection paste (for example with Malusan from Neudorff). This prevents further pathogens from entering the wood. Disinfect the cutting tool (knife or saw) before you start cutting.
If many twigs and larger branches are affected, cut down more branches after the plum harvest so that the tree looks halfway good again. Spraying with pesticides will do little good if the tree is already very large. Moreover, treatment must be done at a very specific time (right after budbreak); by now it is too late for that.
Fruits in the residual waste
In the fall you still have to collect all fruits and twigs from the ground and dispose of them in the residual waste. This way you can prevent re-infection.
We keep our fingers crossed that you can still save your tree!