Plum and cherry trees suffer from a number of leaf blights and fungal diseases and cool wet weather makes this problem worse. Even frequent spraying with a fungicide has not shown good results if the tree is susceptible and weather conditions cause a fungal outbreak.  The most practical approach may be to just wait it out but try  to rake up the fallen leaves so that they don't' reinfect the tree.  Good air circulation and sunny days will help as well. So sorry about your plum and I wish I had a more optimistic prognosis. You may want to take a sample of the foliage to your nearest Master Gardener workshop and see if local experts can identify if it is  indeed a fungus among us. Keep growing, Marianne Binetti