Your plum has brown rot. No cure for it and very contagious. Last year I was destroying an average of 30 a week so I decided to cut the tree back hard and spray with a fungicide. Hardly a fruit this year but I thought I may break the sequence. Cleaned up the soil and treated that as well. This year I have still got brown rot. My neighbour has the same tree (Victoria) and maybe that is the cause of the infection. You will need to inspect your fruit on a daily basis and cut off the infected fruit and burn it.