Thank you for that tidbit. I had never read that. Hmmm.

Take it with a grain of salt vaporvac. Until I understand how it really helps I'm almost more inclined to think it might be other circumstances or coincidental. Even though it has seemed to help in my own experience.

Although... I am now going to mulch the areas that I left open.

Maybe the mulch helps to reduce the microclimate humidity within the actual bush? Since it helps to hold moisture in the soil?

I'm honestly science minded at heart, so I am quite curious in the "how" and now that the question is stuck in my mind I probabaly won't let it go until I figure it out!

Maybe the mulch acts as a barrier to hinder the mildew spores circulating from the soil to the foliage?

These are all just my random guesses.

Real knowledge would be appreciated. :)

I had original written that , but then I read it's caused by humidity, so I erased it as I figured mulch would probably increase humidity levels. However, if the mulch is dry it would absorb moisture. I think the mildew spore are probably everywhere as the one plant I had that had it was in a very small pot. I sprayed it with very hot water and it went away. I had read about dunking plants, but that seemed too hard. Milk is also supposed to work, but I agree, if mulch works to prevent it, that would be MUCH better.

I think I jinxed myself, as when walking the garden I notice one of my Dark Desires has some. Say what? I guess it's been the perfect temp and humidity lately, but why just that one? I have all the mulch pulled back there as I needed to plant a NOID rose in a temp spot for the winter and decided I might as well add some compost while it's removed. Hmmmm, hmmmm.

Here, powdery mildew is a disease of hot, dry weather. So the idea is the mulch helps with water retention.

Makes sense! Thank you for the concise answer! :)

My curiosity is now satisfied.

And my raised beds are now mulched like the rest of my yard. :)

Good post! I think of p.mildew as being more of a problem when temperatures start going up and rainfall diminishes,like mad_gallica. Often even the mature oaks in my garden get it in summer. It really bothers me that some of my favourites- chinas and noisettes- seem especially prone, so this (improving mulching around these roses in particular) is an encouraging idea indeed. Thank you, all!

There appears to be at least two causes of mildew to my mind. One is the genetic proclivity to mildew due to cool, damp air that doesn't circulate well. The second is a forced response to water stress. Even roses and other plants which may not succumb to it when the "conditions are right", can be forced to mildew when too water stressed. I have witnessed parallel susceptibilities to rust, one where the plant rusts when the conditions would have you expecting to see it and one as a response to extreme water stress. At least R. Arkansana and its hybrids are susceptible to the forced type. There may be others, but I have played with Arkansana and demonstrated it in that species. Cal Poly is a very healthy yellow mini in the climates in which I have grown it, but when potted and dried out severely, it will rust like an old iron skillet. This has led me to believe that Nature uses fungal issues to "instruct" plants when to do what. Where Arkansana is indigenous, it is very Arctic hardy and disease resistant...until late in summer, when the rains are over and the ground water is drying up. Then, it begins rusting, causing the foliage to fall and the plant to go into winter dormancy so it is sufficiently cold hardy to survive the extremes in those climates. Perhaps the forced mildew is a natural trigger to force summer dormancy in response to lack of summer water?

It definitely makes sense. Especially in cases like Bart and Mad Gallica describe. In my yard, I seem to only have issues with powdery mildew when it becomes more humid (here - that means rainy), but when it's still warm during the day and cooler at night. That usually equates to early fall. I don't seem to have issues with it during the dry heat of summer.

The powdery mildew I just found on my roses that weren't mulched was the worse I can remember. And it really wasn't that bad. It was probably the worst on Iceberg of all roses! Bizarre! That one never gets mildew in my yard. I usually keep my roses very well spaced and mulched, with drip irrigation. I think that has really helped. I don't spray (usually). I have only really purposefully gardened in Northern California though, and our conditions are so different from most. I'm inland with a lot of dry heat ... and over the last few years with such severe drought conditions and weird climate changes.

Although, since I found these forums I was thinking about squeezing everything together in order to fit more roses in! On second thought, I'm going to pull back and stick to my usual spacing after all. More roses will not be better if they're covered in mildew. More disease maintenance and pest control takes all the fun out of it!

Thanks for explaining Roseseek. Nature using mildew and rust to help the plant defoliate in order to survive colder or hotter/dryer climates makes total sense.

Your explanation for how mulch helps ward away mildew makes sense too... and fits with your explanation of what causes mildew. It's wonderful and invaluable to have such knowledgeable professionals like you on these forums. Thank you so much for making yourself available and sharing your wisdom.

Thank you, Cori Ann, you're welcome! Don't be surprised at mildew on Iceberg. In SoCal, mildew, particularly on the flower stems, sepals, ovaries and new growth tips is its major "failing". Otherwise it is extremely disease resistant. If pruned too harshly, it can be forced to black spot in those climates, which also is very logical. Plants have immune systems, also. Reducing stored nutrients by removing too much wood and limiting the plant's ability to feed itself by removing or preventing it from producing the necessary foliage mass, can cause malnutrition. Starve any organism and you can force it to "get sick".