We have a 30 ft (at least) double trunked holly tree in our garden which used to look beautiful. Last year it lost a lot of yellowed leaves during the summer and one side of the tree partly defoliated. We thought this was due to drought but this year it is clear that the tree has bad holly tar spot. As it is too big to spray or prune, we have only picked up and disposed of fallen leaves. However, the leaves have not grown back on the bare branches; in fact, they have increased and have turned bright yellow with black patches up them. Is this a further disease affecting the tree and is there anything we can do to bring it back to its former glory?