Strange happenings in the garden

Last year in summer my Sumac suddenly lost all its' leaves for no reason that I could determine. It lost one branch, but is budding OK this year.
This year it's my lilac that has done a similar thing- most of its' leaves are browning and only one flower bud is coming to anything. Next to the little tree are two very healthy older lilacs both with flowers, so I don't think it can be a virus. The tree is a variegated lilac with beautiful pale lilac double flowers which smell gorgeous. It is a grafted tree and I am wondering if now it has grown a bit the graft is failing- is there anything I could do to help if this is the case, or has anyone any other ideas, please?

 

It sounds to me like water shortage, if the more established lilacs are quite close to the younger one thay could be leaching the moisture and nutrients from the soil. Give the little a couple of good soakings and see what happens. David.

 

Thanks dai, I have been watering it copiously and some leaves have recovered. You may well be right, I'll cut back the other 2 after they've flowered.