Posted: 06/05/2017 at 10:56
One possibility is that it has "Silver leaf", a fungal disease which usually enters by pruning at the wrong time. Is there any possibility that the tree has been pruned between Autumn and Spring in the past? Plums should only be pruned during the Summer to prevent infection.
The only other thing I can think of is that a "weed 'n' feed" product has been used on the lawn which won't have done the tree any good. Lawns with trees in should still be fed but not with products containing weedkillers.