To Ken_adrian: Thanks for the advice, but I am not one to give up on a plant because I had a problem with it for a little while during 1 season. I have dealt with issues with dahlias as well from time to time, like slugs, spider mites and tree hoppers, but I stiIl grow them. I think some things are worth a little extra effort, and its really not that much more effort anyway to be honest. I grew the same variety of zinnias about 2 years prior to last year's powdery mildew, and at that time I had more zinnias in a more shaded area that tended to hold moisture/humidity moreso than this new area does, yet I didnt have any problem with powdery mildew at that time. I dont mind having to take 10-15 minutes 2-3 times a season mixing up some spray solution and coating the plants with a safe, relatively non-toxic spray that helps prevent fungus if it means having 5-6 foot tall flowering plants that continuously flower from June til early November. I will also try to keep them more ventilated. Maybe they were too close last year. But we also had a generally wet first half of summer.

I do not use fungicides so my advice is more in the organic way of growing. Powdery mildew does do better in dry conditions as when it rains it washes the spores off the plant. It happens usually when night time temperatures are low and day time temperatures are warm. If this happens every spring then you can use a horticultural oil (like winter oil but more diluted) but this should be done carefully and in cool temperatures because when applied in warm weather it can burn foliage. This works like a coating that the mildew spores cannot penetrate and is best done in early April before mildew is present. Right now it might be best to cut back rose and wait for better weather. Overhead watering can help, but only if it is done early in the day. The weather will change and the mildew problem will go away, but meanwhile it may really slow down the growth of the plant and it may not be attractive for the remainder of this season if it has had a severe case of mildew.

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.

Anything other than water on the JM foliage can cause leaf drop. These are extremely sensitive to anything sprayed on their foliage.....so I don't :-) I doubt the vine maples will have as strong a reaction but at this point in the season, there is nothing you can do. The leaves will begin dropping shortly and you can't "clean up" PM damaged foliage.....you can only attempt to prevent it from happening to begin with and it is too late for that. Fall coloring on any tree is a factor of many conditions, mostly due to climate and watering. Colder evening temperatures and warm, sunny days as we move into fall should help. And if you start backing off of the watering a bit, that should help. A bit of drought stress (not too much - that can cause early leaf drop) combined with cooler temps (especially at night) helps to condense the sugars in the leaf tissue fluids and stimulates the colored pigmentation (anthocyanins, carotenoides, etc) at the expense of the chlorophyll.