I believe the picture of the leaf underside displays a leaf infected with a fungus. I don't know what fungus it is, but I believe there is a fungus infecting that leaf.

Those diseased areas on the cane all appear on one side of it. Could it be sunburn?

Masha

It is possible that the cane I described as mid-stage could be sunburn, but not the other. The purple blotches appear all over the cane, and it happened to nearly all of the canes, about twenty canes. This is a very vigorous rose.

Your plant is lovely. We are having a wet winter, and we do want to be on the lookout for various kinds of uck. It's not a clear case of DM from those shots. It's a lot easier to Dx when you see it on new spring growth: it drops when the plant is jiggled. I don't think I'd sweat those canes. I found LO to have sensitive, soft canes. It's long gone from my garden, but my plant was never that nice. I don't think I'd worry yet unless it looks different from anything you've ever seen before.

I think, from looking at Baldo's website, you have diagnosed correctly. Your rose looks healthy enough to me, I think I'd put up with it.

Here is a link that might be useful: Baldo

I have had similar splotches on leaves and canes of my Climbing Devoniensis that grows on the north side of my house.
Last year I was very concerned that it was DM and since I was propagating plants for cemetery sale I felt compelled to have a definitive answer. I took a sample to the state plant pathologists (Baldo gave me their names) - turns out it was POWDERY mildew, not Downey.
The pathologists said downey mildew was pretty uncommon.
As Cass said, we've had a lot of rain, so more fungal dz is present. Maybe the copper sprays will keep it in check and it will not affect the entire plant.

I have cut this rose back rather brutally, to just 8 or 9 canes, each 15 to 30 inches high. Given that this means I cut out about 90% of the plant mass, you can see I was preparing for removal. However, I'll give it another chance. In a few hours I'll give the whole front garden a good spray with copper fungicide and see how things develop.

Thanks for your thoughts and encouragement, everyone.

I think it's probably DM -- it's a match for what we were told was DM, and L.O. suffered badly with it here.
It's more of a problem in a climate like ours, where it never stays hot enough for 24 hours to suppress it.

Avocados have been a huge crop here, but Avocado Root Rot (a closely related organism) is a big problem.
They've found that drenches won't eliminate it, and that their best control is to mulch around affected trees, out to the drip line, with fresh horse manure.
The resultant bacterial colony seems to be more effective than any chemical approach, over the long haul.

Of course, many have given up, and sold the land for development.

Jeri