At this time in the season, I wouldn't advise you to do anything. The trees will be entering dormancy in just a few weeks and dropping their leaves anyway. Just clean them up completely and dispose of them.

When dealing with woody plants like your trees, it is important to remember that powdery mildew is more of a cosmetic issue than a real threat to the health of the tree. But it is best to avoid anyway, if nothing more than to maintain an attractive appearance.

PM often tends to be a late season issue in that is most common in late summer and early fall. This is when environmental conditions are prime for its development. Soils are at their driest and warmest and the air still and humid, creating an ideal breeding ground for the pathogen. And plants located in more shade and in closer proximity to neighbors can be more affected. Make sure your trees have adequate light and air circulation.

You may be contributing to the problem with your watering program. Sprinklers that throw water above ground (as opposed to something like drip irrigation or soaker hoses) increase humidity. And watering in this manner 3 times a week for 20 minutes may be unnecessarily often but with the effect of insufficient water reaching down properly into the soil profile. It would be much better to water less frequently - perhaps only once a week - but for a longer period of time. And in a manner that delivers water directly to the soil at the root zone. Low and slow :-))

I can't tell you exactly how often to water - that depends on your soil conditions and weather - but even once a week may be more frequently than necessary for trees established in the landscape. Ideally you want the soil to be just moist to the touch throughout the root zone, Not too wet and not too dry. You may have to do some experimentation and testing to get the timing correct. And proper mulching can help to maintain soil moisture between waterings.

Thank you, gardengal48. Next year I shall try the soaker method and hope that my trees look will once again be beautiful.