the fruit themselves arnt poisonous but the stones contain cyanide but i would be more worried about stones causing a blockage, another concern would be if too many plums are eaten they may ferment in the stomach and could cause the stomach to bloat.
I also have a plum issue from next door's tree, plus pears from the other side. Dylan is keener on the pears! If you are clearing them regularly then I wouldn't have thought there would be too much problem - maybe the 1970's poos will grow a bit like Dylan's do in pear season! If you watch him go to them and give a firm "NO" or "LEAVE" followed by a treat, you should be able to train him to leave them (Hopefully!!).
You should see my garden,i have 4 plum trees and an apple too,its like could i have some grass with my plums! the place is peppered,am on a major pick up session today,and going to make some jam from the ones still left on the trees
Pixie is not interested in them at the mo which is a relief.
omg ive just found out Damsons are also poisonous and so are conkers!!! so that means we will have to fence off a section of the garden !!!
Mind you just had designer thought and i could pack it full of fruit trees and have my own little orchard ...... nice
Delta likes plums, but yeah its a good idea to teach im to leave, also teach him to bring back whatever he has in his mouth. Delta ran about with a bag of the prosesed cheese slises that she found on a walk when she was a puppy, was so worried she would eat the bag, didnt get it off her till she had eaten most of the cheese.