Gardening and Horticulture - fungus on lilac bush

My lilac has never bloomed except for one flower two years ago. It is about 5 ft tall, in full sun and good soil. We have had a drought the past three summers and the leaves often turn brown on the ends and wither at the end of the summer and I assumed it was lack of water.

But this year we have had tons of rain and my local gardening store employees said it is a fungus. I was so surprised. So I have been spraying it but the fungus was well established by the time I started spraying. It isn't helping and most of the leaves have fallen off. Some new growth has come out but it keep raining and the spray isn't on for long. Is there anything else I can do? What about the Bayer systemic liquid for roses? Would that work next year if the lilac doesn't die? It has fertilizer in it too which I know the lilac doesn't really need.