I think that looks more like damage caused by a few sap-sucking insects such as aphids and nothing yo worry about.
I can’t see any sign of powdery mildew anywhere on your photos.
However, Powdery mildew usually affects plants which are already stressed, often by drought. This spring has been unusually dry and even after recent rains our soil is still dry as dust a few inches below the surface … and that’s where the roots of our shrubs are.
I would give that lilac two buckets full of water poured slowly over the root area do that it really soaks in. I would repeat that at least twice a week, more in warm spells … until the leaves turn colour in the autumn. Start again eg the beginning of March. I think that will solve the problem without resorting to fungicides.
Worryingly, doctors and scientists are seeing cases of fungicide-resistant health conditions which are causing serious medical problems in humans, due to the over-use of fungicides … just as the over-use of antibiotics has caused the proliferation of antibiotic-resistant infections in hospitals throughout the world.
Gardeners and farmers need to act responsibly now and save the use of fungicides for occasions when there really is no alternative 👍
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.