Plum trees dying one by one, help!

I'm afraid I'm extreemly ignorant of gardening but am trying to learn. A couple of years ago we bought a house in Italy with several fruit trees. Short version of the story is when we took on the house we noticed a couple of the trees were long dead so we cut them down. Then this winter we noticed that two of our plum trees didn't survive (it wasn't a harsh winter). And now we are noticing that the last three trees are going the same way. The leaves came and the fruit but now the leaves are all dying off and the fruit is dissapearing. It all looks pretty fatal unfortunatly. Before this we always got a great crop of plums from the trees. I presume there must be some disease spreading from tree to tree, but it's only affecting the plum trees. It will be such a shame to lose them all. Does anyone have any ideas?

 

Could possibly be Silver Leaf....it does spread very quickly...did you notice the leaves having a silvery hue to them?...