I have seen some on a few purple Norway maple trees here - other than 'Crimson Sentry', which mildews a lot. Since Norway maple is a pest species in North America my view is that any specimens becoming mildewy to a bothersome degree should just be cut down. Vicious infestations of the tree like those that are commonplace in parts of northeastern USA do occur on some sites out here also. The quantities of it in one wooded Seattle park, before volunteer removals were undertaken had to be seen to be believed.
I have seen a small spontaneous grove of the partly purple form on Vashon Island, again (like the worst Seattle infestation I have seen) in a ravine where presumably the summer drought is tempered enough to allow significant seedling survival. A good place to look such things up is the book Wild Plants of Greater Seattle - Second Edition by Arthur Lee Jacobson, who present a comprehensive listing of plants both native and not known to have grown wild there.
Jeanne, Go to your local good nursery and ask them for a mildew spray for trees Do not apply it in the direct sun - better in the early morning before the sun is too warm. My greenhouse seedlings get mildew which if I do not address can be bad but usually the plants can handle it. Watch for tinly new leaf growth and throw away any mildewed leaves that drop. Good luck.
Thank you Katsura! We had been told to try a baking soda mixture, but to no avail. Any local landscapers want to spray with a sulphur based spray, which will probably work, but our youngest daughter has a severe allergy to sulphur. Hopefully the local nursery can help us find a mildew spray that doesn't contain sulphur. Thank you so much for your information. Jeanne
