Seil - that is VERY interesting about rust! I thought it only plagued those of us in California. In my garden, only some of the HTs get it - nobody else, which I find fascinating. Peace, Golden Showers, Duet, and Sutter's Gold all get it occasionally. None of the teas, chinas, tea noisettes, hybrid musks, hybrid multifloras, hybrid giganticas, polyanthas or other OGRs get it. None. This is one of the reasons that I early on decided not to ever purchase HTs. Rust is really ickky! You practically have to strip all of the foliage off a rose bush, and then any susceptible ones nearby get it because of the spores flying around in the air. Sorry to hear it has popped up where you are. Jackie

I get it, too on my pumpkins and gourds. I am still researching this problem myself. So far, I try to cut off any dead leaves on the vines and any tendrils that have turned brown. I try not to water too late in the evenings, either, which is getting harder to do w/ the evening getting so much shorter all the time! Only thing I have found that sort of works is to get a powerful spray bottle, and mix up a solution of powdered milk and water, and a couple of good drops of dish deterg. I dilute the powdered milk mix pretty well -- 9 to 1 is usually recommended. Spray liberally about mid afternoon when sun is out. Spray after it rains. I know it won't help w/ the leaves that have it the worst, but I believe it does help keep the newer growth and healthier leaves healthy! I think there are a few other effective ways but not sure myself! I am reading some things about copper right now. I don't know about that.

Planting under your lilac might make some of the preventative chores more difficult, and the extra watering and fertilizing might make your susceptible lilacs even more so. Just something to think about... Also, powdery mildew is not a universal fungal disorder. There are many different strains of powdery mildew, and each are rather specific as to the groups of plant families that they can infect. For example, your lilac can't share its case of PM with your roses, or with your cucumbers! I don't believe that there is a strain of PM that infects hosta.

I have used milk diluted 1 part -2 parts water as an organic method for mildew control on my squash and cucumbers. Sometimes it works but usually it just slows it down. That is why some of these vegetables do not grow through the summer/ But in weather like this it is hard to fight once it has gotten started. I do not water in evening but the rain has been coming down anytime it wishes. I doubt the mildew will kill the rockrose but it might not look to good until the weather changes.