A close up of the leaves and the effect is even more startling. On this lilac it seems to affect the whole leaf. On plants like garden phlox (phlox paniculata) which are also especially prone to this fungus, it will cause the leaf to yellow and drop as well giving the flowers a “lollipop” effect.
With a fungus like this, it’s very tempting to want to treat and treat immediately. Not only is the fungus disfiguring, but in the case of the phlox, it’s unsightly as well.
You can treat–but as I remarked yesterday, prevention is much better than treatments since fungicides only control and do not cure once the fungus is present (remember for next year–it’s too late for this year.)
But powdery mildew doesn’t invade the tissue of the leaf so it’s not really doing the plant any harm (despite the fact that it may be doing harm to your eyes to look at it!) So not treating at all isn’t going to affect the plant from one year to the next.
Now in the case of the phlox, where you have leaf drop, I’d advise you to clean up the leaves and remove them from the garden and do not compost them so that you don’t have any spores that might over-winter there to re-infect. But chances are you have the cultural conditions present that this fungus likes and you will have the fungus again next year.
In the case of phlox there are some things you can do for existing plants to try to control the fungus. You can remove every other stem to improve circulation within the plant, or cut down the outer stems by 1/2 to achieve the same effect. It’s worth a try.
With a lilac it’s a bit harder. Mine’s in a bit too much shade. And unless I want to take down some trees–which I don’t–or move this established plant–which I also don’t–I have few options.
This bulletin from Colorado State discusses the disease and its controls in a bit more detail than I have. It also give you some chemical controls–which you know I don’t endorse.