Looks like Powdery Mildew, a kind of fungus. I've never had Neem oil or any organic spray work well for Powdery Mildew unfortunately. You might try "Bayer Advanced Garden Disease Control for Roses, Flowers and Shrubs".

The people on the Rose forum recommend it. It's systemic, in that you water it in and it protects pretty much all season. It is not organic but you apply it by watering it in so it goes up through the roots. So no spraying. Even organic spays can be irritating to the lungs. And you're not going to eat it.

Is it in the full sun? Like for all day? Lilacs love full sun and will mildew in shade or part shade. Sometimes full sun plants will mildew in a shady area. If so, than just moving it into more sun will help too.

That being said, they really don't do well in California in general, especially in the south. Lilacs love and need winter chill. Like several months of ice and snow. If they don't get it, they tend to not bloom. They also love humidity, which we also don't have much.

There are low chill varieties, but their blooms are generally smaller. And truthfully for me, a plant that only blooms for about 4 weeks a year and is just an average green bush the rest of the growing season, and just sticks in winter isn't worth my garden space. But that's just me.

If I'm going to have a plant be bare in the winter here, without the beauty and cover of snow, I want it to have some interest in fall, like pretty leaves, or winter itself, like pretty, colorful or interesting branches. Lilac has none of that. I really think it's just the scent. Not enough draw for me.

But that just my rant. It all may be worth it to you. If it is, you should get the Bayer product.

Serenade works well on powdery mildew too. Is your bush getting enough air flow and sun?

Am I supposed to spray daily or just one soaking application?

Unless someone here has used the same product they can't answer. What do the directions recommend regarding 'Safer Brand' garden fungicide?

Any product you use will tell you how often to spray. Whatever you use you will need to read the label carefully.

In general when you spray any product it's best to spray until you see it drip off the leaves. Too light of a spray may be less effective.

You can Google "Safer Brand" fungicide and see a picture of the label to read before you buy.