You're looking at a bad case of powdery mildew. Immunox is the go to fungicide for control of it. Follow the label directions. I would not remove the infected leaves because even the lightly infected ones may still be carrying on photosynthesis. The diseased leaves will drop off as the fungicide goes to work. You should end up all right, with weakened roses, but with good care they should bounce back. Keep a diligent eye out for powdery mildew. It does not start out very powdery, but with twisty leaves and slowed down tip growth. Nip it in the bud in the future if it should return.

Moses

Thank you, Moses!

What would be a good, after Immunox, rose care that you would recommend?

Where are you, inna? Climate is a consideration. Some roses mildew if they get too dry.

Agree with Moses on Immunox.

Sheila is right. Usually in zone 6, Pittsburgh, S/W Pennsylvania, hot, humid summers, our main fungal adversary is the dreaded black spot...year after year after year it is the same intense pressure that by The 4th. Of July, the most black spot susceptible roses, i.e., the old time hybrid teas like Chrysler Imperial, Crimson Glory, Tropicanna, etc. are almost completely defoliated unless sprayed relentlessly.

Here, in the 'Burgh,' we encounter powdery mildew mostly in spring and fall only if the days are very warm and the nights are cold, and there is little or no rainfall, that is, it is DRY. As Sheila stated, dry conditions effect some roses really badly. There was one rosarian here that fought a powdery mildew infestation by spritzing his roses' foliage with plain tap water frequently every day. He claimed it worked and I had no reason to doubt him.

Moses

I'm in California. It hasn't been hot or dry yet. We've been getting more rain this spring. Could that be the cause of the mildew?

I wish a Californian rose enthusiast would chime in on how they combat powdery mildew. That way you get site specific care instructions. Climates vary considerably in California so if you are more specific as to coastal or inland, and which city you are in, that will help you get the information you need. So lets wait and see who steps up to the plate!

Moses

Don’t forget to rotate your anti-fungals.

Some bedtime reading:

https://hgic.clemson.edu/factsheet/powdery-mildew/

I see rust, too.

I would not have thought more rain was the cause of PM.

I’m in Seattle and PM is a regular visitor mid spring when the days are dry and warm but the nights are cool and damp. I think I must have read the same comment Moses mentions from an old rosarian because I learned about spraying down in the mornings on sunny days with the hose a while back and it has made all the difference for me. I still get some PM on my more suseptible roses but not so much they can’t shake it off when the weather gets better for the roses in late spring. This also has the advantage of keeping spider mites at bay and giving your beneficial bugs a drink!

California is a big state, I’m from coastal SoCal originally and I know PM can be an issue on the coast with cool nights and warm days.

I have had powdery mildew, but this post is not very responsive. Most of my problems in NE Iowa will be concerning Botrytis, and blackspot. So, in the spring, I usually spray Daconil for all fungal diseases and it is good with Botrytis. Daconil should not be sprayed at temps above 78F, and voila, botrytis is a cold damp weather disease.

I spray with any and all other fungicides when it gets warmer as those other ones prevent blackspot, and hopefully powdery mildew, although occasionally, but not often, it has visited.

After winter, I cleaning up every dead leaf I can find under my rose bushes. I have put down a layer of cypress mulch or red cedar mulch under them. Most diseases have mostly ended with this protocol and some spraying, but there is less spraying when you keep things clean under your roses, and mulched, as described.

Thank you All for your suggestions, advice and bedtime read :)! Super helpful!

I live in Southern California, inland, about 13 miles from the ocean.