Moving to full sun helps with my roses with mildew. Mildew is often caused by cold nights and warm day-time temp, plus DRY condition. The roses topped with salty soil (with turkey manure) were the 1st one that broke out in mildew.

Mary Magdalene never have mildew in its 12 years as own-root, but back in 2012 I put too much ACID fertilizer high in salt with a 10 in nitrogen, and it broke down in mildew. I had to water it profusely with my ALKALINE tap water at pH 9, and mildew went away.

I never have mildew in my garden in late fall, until in 2014, I installed 7 rain-barrels (50 gallons each). Rain is acidic at pH 4.5.

Baking soda is very alkaline at pH over 10, but it's also salty. Mildew is often from potassium NOT available. Potassium regulates water osmosis (it pumps up water to plant tissue). Potassium is most available at neutral to alkaline pH.

From University extension research: Potassium is NOT available in very dry soil or soaking wet soil, but most available at moist soil.

Agree to the above: I get mildew from soaking wet poor drainage clay, as well as mildew in hot & dry with zero rain for 1 month.

Please observe without intervention (aside from ensuring adequate hydration) and report back in 10 days. I’d like to learn if Snow Pavement continues to tolerate the little bit of powdery mildew.

Thank you both for these replies!

@roseseek I will move her to more sun and check the soil humidity.
I can't do much about air humidity, being about 150 meters from the sea, the air is humid year-round even though it doesn't rain for almost two thirds of the year. Currently we hover between 70-80% humidity, with temperatures around 25°c. I got rugosas because they are supposed to be seaside roses, I'm surprised they'd suffer from air humidity! Oh well...
Snow Pavement is a really pretty rugosa with such divine scent, I want her to thrive!

Regarding the spray, my experiment with thyme extract came after a recommendation from the rose introducer Ducher. They said never spray rugosas, except maybe with nettle extract (against aphids). I figured if thry can handle nettle maybe they can handle thyme (against spidermites), and it worked. Unfortunately neither of these extracts helps with getting rid of mildew.

@strawchicago z5 your comment is giving me an idea:
I remember reading somewhere that potassium bicarbonate as a better alternative to baking soda, supposedly it is more effective and alkaline. Also the fact that it is potassium not sodium removes the risk of salt damage. So perhaps a potassium bicarbonate solution might provide both a boost of potassium and alkalinity while also suppressing the mildew.
I will try to find some tomorrow, and will move the rose in the meantime. Perhaps I will start by doing a test spray on just one or two leaves to avoid a defoliation disaster.
I now remember that I had fertilized all the non-rugosa roses with Masterblend 4-18-38 and its CalMag additive just a few days ago (at half strength). I usually never fertilize my rugosas with anything synthetic, but I had half a watering can of Masterblend left over so I dumped the remainder into Snow Pavement's pot... Maybe she didn't like that?

@rifis (zone 6b-7a NJ) I will wait before I spray, but I am going to move the pot tomorrow and check the soil humidity.

Snow Pavement is new here this year and I had her in a part shade spot to protect her from our blazing zone 11 summer sun. But by this time of year that corner may be getting too shady for her.

I will definitely update this thread with any news.

Something you might consider... Potassium bicarbonate. It's used in wine making as an acid reducer. You can buy it on line for very little and you can even drink it as you might sodium bicarbonate for indigestion. @AquaEyes 7a NJ suggested it to me to use on coveted seedlings to prevent mildew. I bought a pound ($8 on Amazon) and never got around to using it as I was able to alleviate the issue by clearing the growth around the seedling table. But, that MIGHT would. I would NOT add soap or oil as the "wetting agent" as so many recipes suggest as that can easily cause the Rugosa genes to go into over drive.

@roseseek wow! Yes, that's just what I was writing in my reply to Straw!

Ok potassium bicarbonate it is. I will try to find some locally, I think it's sold in pharmacies. I didn't know it was used for wine making too, that's fascinating!

I will do a spot test on a leaf or two when I get some and will report any good or bad effects here.

seasiderooftop Rugosa doesn't like salty chemical fertilizer. Master Blend NPK 4-18-38 has potassium chloride with salt-index of 116.2, even higher than nitrogen fertilizer (salt idex from 90 to 80).

That's how I killed decade-old Sharifa Asma (has Rugosa genetics) .. I put a wad of SALTY chicken manure around its crown.

Potassium bicarbonate is more effective than baking soda. Here's an excerpt from the web: "GreenCure® is a potassium bicarbonate-based fungicide that has been proven to cure and prevent powdery mildew, blackspot, downy mildew, blights, molds and other plant diseases."

Your Snow Pavement should recover from the mildew as soon as it's planted into the ground. The fungus infected leaves will stay damaged, that damage is irreversible, but new growth will be clean.

SP is one of my favorite hybrid rugosas, love the color, scent and cupped blooms. Grew it some years ago along with many other hybrid rugosas.

SP will send out runner growth from the original plant like so many hybrid rugosas do, making a colony in time. Just remove the runners as necessary to prevent invasion into neighboring plants, nor give SP enough room to make a colony. You can keep the colonial spread in bounds as you wish. SP is not aggressive or rampant in this regard, spreading slowly, but standardly practiced rose distancing from its neighbors, as say with adjacent floribundas, will present future issues as SP 'spreads its wings.' Give it more room than you would a 'normal,' rose bush.

Spray nothing onto the foliage. Believe me on this. Even foliar feeding will cause severe leaf damage on the foliage of a hybrid rugosa. Only water on the foliage is the rock hard rule with hybrid rugosas. Stick to it, please!

Moses

“ … damaged …”

What damage?

Listed as a Hybrid Rugosa and looking the part this one can be expected to be less clean than pure Rosa rugosa. As fairly often seems to be the case with named rugosa roses having genetics of non-rugosa roses included in their makeup.

'Snow Pavement' Rose (helpmefind.com)

Powdery mildew damages rose foliage. Where is the question?

Moses

@strawchicago z5 thank you for that info! I was not aware of the saltiness of Masterblend, or of the salt index of various fertilizers. I've been looking into that and it is very useful information. Living so close to the sea, my plants definitely already have to deal with their fair share of salt as aerosol from the ambient air and winds, so I really should avoid giving them too much. I'll take it into account for future fertilizing.

@Moses, Pittsburgh, W. PA., zone 5/6, USA I agree, Snow Pavement is absolutely lovely! Mine has no chance of suckering as she is grafted (on laxa rootstock). As for getting her into the ground... I wish! I garden on my rooftop so all my plants will be living in containers for the foreseeable future. SP is new here this year and currently in a 5 gallon pot, I will be potting her up to a larger container next year.

@Embothrium I wish I knew what SP's parents were. She was bred almost 40 years ago and the hybridizer (Baum) has passed away. I wonder if the lineage info might survive in his notes somewhere and if there's any hope it could be made public. I would think that would already have happened if such notes were still around though, so I guess it'll remain a mystery.

I have moved Snow Pavement to the sunniest part of the roof. So far I am not seeing any further mildew spread. However, our rainy season is starting and we are expecting rain next week, a little on Monday and a lot on Friday, so I will see how that affects it.

I have purchased the potassium bicarbonate but haven't used it yet.

I did some more research and found that horsetail (equisetum) extract is reputed to be effective against mildew. Since Snow Pavement responded well to previous spraying with plant extract (thyme), I will also puchase this one to test.

@seasiderooftop the rains just might help. If it's water stress, the rains will alleviate it. And, a film of water on the leaf surface is an extremely effective "fungicide". Anything which prevents the fungus spore from making contact with the leaf surface can prevent germination. They require the correct temps and humidity levels to germinate on a dry leaf surface. Good luck on getting all the rains you need!

Not to hijink this thread but I just sprayed my Snow Pavement with deer repellant this morning. Is that going to cause problems?

Not for next year, but foliage exposed now to the presumed putrescent egg concoction will register its displeasure in no uncertain terms.

What brand (if you used a commercial product)?

Deer Out

Update: 

I ended up not spraying, and just cutting off the affected parts.

Also I now am 100% convinced that you were right @strawchicago. The mildew was caused by the Masterblend. Almost all my roses got some degree of mildew from it in the week that followed application. I have since stopped fertilizing and am just letting the season's abundant rains wash all that salty mess out. Never using that stuff again on roses. Thank you for helping me figure this out Straw!

New update because this might be useful to rugosa lovers.

The mildew continues to appear on many of my roses. The extremely humid weather this season is probably not helping.

Snow Pavement got mildew again. This time, at the first sign of mildew I did spray her with Horsetail (Equisetum) extract.

A few days later, the mildew progression seems to have stopped and she kept all her leaves.

This confirms to me that plant extracts can be sprayed on this rugosa. Obviously I would never spray her with any synthetic fungicides but the organic plant extracts seem to be ok, she handled both the horsetail and the thyme extract without any problem.

I don't know if other rugosas would be the same, as my other rugosas didn't get mildew, but this one was fine with it and it seems to have worked.