Unfortunately, there's really no way to tell which cultivar you have by looks alone. Even with close inspection, many cultivars are so similar to others, it can be impossible to tell the difference without a tag. The leaf affliction looks like to powdery mildew to me, but I could be wrong. Neem oil is a plant oil made from the neem tree- it can be used safely on hydrangeas, and is *purported* to combat powdery mildew on the internet, but I'm a bit skeptical about that. Anybody else have any personal experience with that?

Anyway, my course of action in your circumstances would be a bit of surgery and some quarantining- I would remove the most affected areas by pruning, and since it is so late in the season, just let your hydrangea go dormant and drop its leaves for winter, as it's about to do anyway. Just be sure to collect and dispose of anything it drops because many fungi can live in, and spread through the soil. Good luck!

Dormancy for a Hydrangea macrophylla in San Diego's climate is a bit of a no-go :-) Not really ever cold enuf there to encourage a full dormancy (my sister lives in Dana Point and her hydrangeas still bloom in January!). However I would remove the affected foliage and destroy - pruning to any extent now will just remove the flowering potential and is really not necessary.

FWIW, powdery mildew (which is the ailment affecting this plant) is more of a climate/culture related condition than an actual 'disease' issue. Often, dry soils combined with warm, moist air conditions and lack of circulation work to provide the ideal conditions for the pathogen to flourish. For most plants, especially woodies like hydrangeas, the issue is primarily cosmetic as the PM does no real harm to the plant. And Neem oil is a very effective fungicide and does wonders to control PM but like any fungicide is only prophylatic in nature or preventative rather than curative. You must apply (and often frequently re-apply) before you see signs of the problem.

If it were my plant, I'd seriously consider planting it in the ground rather than leaving in a container. For a number of reasons this seems to deter PM to a large extent. Site in an area that receives shade from the hot afternoon sun (early morning sun OK) and in soil well-amended with organic matter. And keep the soil sufficiently and evenly moist - the ability for container soil to fluctate widely in moisture content can lead to PM issues for sure. And to be on the safe side, I might consider treating with an appropriate fungicide (like Neem oil) beginning early in the growing season before you see any signs of developing problems.

I have used Neem oil on my hydrangeas, not a problem. You want to use the Neem oil for pests. I live in Georgia and the bugs have been bad this year, white flies, aphids, the Neem works well, but you have to spray once a week. BTW, the Dogwoods hate the Neem so stay away when spraying