I was hit pretty hard by downy mildew this year, very early in fact, and was so surprised because I rarely ever get it. The last time I got it was late in the year during an unusually warm and moist fall about 15 years ago. But it affected the rhododendron leaves, not the roses, which were already going dormant and losing their leaves to the cold. I used Aliette to combat it then.
My garden was full of it this year by the time May rolled around, before I realized what was happening. It's easy to mistake for blackspot at first, until you look carefully. At first I thought I must have missed spraying a few bushes with my usual fungicides, so I resprayed, but it continued to spread rapidly. So I looked more carefully at the leaves and realized the spots weren't black and feathery, but rather purplish or dark brown and splotchy. And as noted above, the spots often start at the top of the bush and move their way down, instead of the reverse as with blackspot. Downy mildew is known for spreading very fast, and can lead to rapid defoliation, even on leaves that aren't yet discolored. I noticed all of this happening in my garden a few weeks ago and took quick action.
Importantly, many fungicides that are used to control blackspot and powdery mildew do not control downy mildew, which is more closely related to algae. This explains why my normal spray program didn't prevent it. I remembered I had used Aliette many years ago so I read up on it and it's still considered an excellent fungicide for downy mildew. Unfortunately, I couldn't find any small bottles of it like I had years ago; instead, it comes in a big 5 pound bag of soluable powder. It's expensive, but I shopped around and found the best price (there is a nearly 100% price difference between sellers). However, I also read what I already knew about other fungicides, which is that it's important to rotate between products to avoid resistance. So I also purchased a bottle of Heritage, which is recommended for downey mildew and other fungi.
I have about 125 roses, and they need to be sprayed every few days for a few weeks to get rid of the downy mildew, so I'll make good use of these products. But it's still an expensive treatment.
Took me some time to realized it. Never had it before. Brownish spots are started form the top, canes are also discolored in some roses. Investigated other sides of the leaves, did not find any spores, just reddish color.
, starting from Mancozeb early spring, than OxiDate 2.0 and Actinovate SP...I do not have black spots so far, but missed badly downy mildew. 
. I have veggie garden next to roses.
