I notice at the coast near Vancouver in the past few weeks - an increasing silver blue-gray haze on the leaves of the native "Big Leaf Maples" (the really big native ones many of have in our gardens and pastures (or out at UBC forest for example) Acer macrophyllum - Wikipedia (Acer macrophyllum) why is that? It happens each year to some of the Acer m. trees it seems - but there has been no rain and lots of sun - so why does mildew develop (I associate mildew with moisture - it would seem i'm not correctly assuming cause-effect) is it mildew - is there a technical term? Just curious. i notice other deciduous like alders or salmon berries or even blackberries don't have mildew - so why are maples the chosen host? I don't intend to fix or remedy this - as it doesn't seem to kill the tree - just looks strange in late summer before the October rains and leaf drop. thank you!
The problem you are observing on bigleaf maples seem to be one of many they are being hit with in recent years. I have been researching diseases of Acer macrophyllum lately because my large tree suddenly dropped almost all its leaves in a 2-week period before the end of July. At first I assumed drought conditions were the cause although there are lots of healthy-looking trees in the area that receive less water than mine. Another clue that it is under stress is the huge bunches of seeds still clinging to the almost-bare branches. What I have learned is that there are quite a number of potential causes for the decline of big-leaf maples for the past decade or so. In fact, the University of Washington, WA Dept. of Natural Resources (DNR) and US Forest Service (USFS) forest health specialists conducted a study on the increased levels of bigleaf maple decline and dieback observed in western Washington. They ruled out Verticillium Wilt among other root rots but have not found a specific cause. There are many websites that talk about the situation. For a good summary, see: What’s Going on with Bigleaf Maple? As far as the silver blue-gray haze on the leaves you see on the leaves of some trees is concerned, here are a couple of possibilities . . . A species of powdery mildew in the genus Sawadaea has been found in lower mainland British Columbia (on big leaf maple). Maple (Acer spp.)-Powdery Mildew (I learned that some powdery mildew fungi attack only certain hosts.) Chondrostereum purpureum (Silver Leaf Disease - refers to the silver or leaden luster of leaves that occurs on some hosts. CTD — Sap Rots: Silver Leaf Disease
thank you Margot - very interesting - the other point I observe on the east side of Salish Sea here is that ... the powdery silver haze seems to be on smaller maples (ie either less mature - or - ones that are repeatedly trimmed back either for improving ocean views or under utility lines. there are many HUGE maples of same species that seem un-touched by haze. plus - the haze disease doesn't seem to leap from one tree to the other right next to it. Strange. I notice that my beloved Acer circinatum (vine maples; native species) that I plant in my garden intentionally are un-touched (touch wood!) by this silver haze. And I don't really water them (water restrictions, laziness, etc) (ps - my other current question - which might be observed in Nanoose too - blooming dogwoods right now? - see other topic thread same forum - any insight?) thank you
I moved the reply about the dogwood to the dogwood post, at BC dogwood re-blooming in August? Cornus. And I'm moving this thread to Maples so that some maples people will see it.
Powdery mildew in maples usually has to do with water regulation. Either too wet or too dry can bring it on, but very large well established trees are less prone, obviously because their larger root systems make them less prone to water stress. Some species, like A. macrophyllum, campestre, negundo are particularly sensitive. Here in Normandie where it's usually very wet, dry summer periods can cause outbreaks. This year we are having a spectacular drought, and paradoxically maples in pots have been over watered. I have a young macrophyllum that has some mildew on it, whereas a much larger one in the ground has none. Although unsightly powdery mildew isn't dangerous for the tree. It can be treated with a copper foliar spray, or other fungicide, but as usual once you see it, it's too late to help.
