Hi there i have noticed that my neighbours either side have brown lilac trees that do not seem to have produced many flowers i did notice this in the spring so it may be something to do with our early heatwave in April and lack of water
We also have lleyandii trees which are brown, but we use a Bayer Greenfly spray that was recommended to us and it seems to have stopped the cypress aphids which cause this
Hope this helps

Hi Mrssmall I hope you get an answer to what caused this as mine did exactly the same, it started off well in the spring then the leaves shrilled and it has stayed like that all summer in spite of feeding with blood and fish, mulching and watering. Like Primrose suggests I too put it down to the hot dry spring.