My understanding and my (one) experience with powdery mildew is that when the temperature soars above 90F during the day for a few days in a row powdery mildew is just about finished off. Large temperature swings like those you mention will bring humidity pretty low for most of the day, again dealing powdery mildew a pretty strong setback. So in inland CA daytime temps are too warm for powdery mildew and nights are too cool for blackspot. Finally the relative humidity for most hours of the day through most of the growing season is just too low for any of the fungal infections to thrive. It's just about as good as it gets for growing roses.
It sounds like Austin realized that he could sell roses not fit for England or any part of the US east of the Rockies into a market that already embraced quite a lot of such roses.
Again, it seems to me that roses bred and sold where climatic conditions are similar to those of coastal California might offer more resistance to powdery mildew than roses bred in and for the inland California market. In addition to roses offered by David Austin I'd check out roses from Palatine. Heirloom Roses has a whole line of roses bred by Harkness that might be promising. I'm tempted to suggest roses offered by ARE, but they tend to be targeted at resisting black spot more than mildew. Mme Alfred Carriere, for example, seems to be my most vulnerable rose right now and it's from ARE: I'm hoping it performs better once established.
Zuzu, My limited experience with powdery mildew suggests that the roses most vulnerable are the ones growing the fastest. The mildew sets in principally on new foliage. Very little established foliage has been touched by it. So I wonder, if one waters a little more lightly and fertilizes a little less - especially with established roses - does this provide a measure of protection to roses against mildew? It seems to me that there might be a spot of evidence to support this. In his book on climbing roses Quest Ritson claims that Mme Alfred Carriere was panned as a terrible rose in England in the years following its introduction (though he doesn't say why), but forty years later it was by far England's favorite white climbing rose. Established roses would spend less time making new foliage, perhaps, and so would have less difficulty with the disease.
