We were looking at mildewed leaves on a maple at a friend's house tonight, and people wondered why in such dry conditions there would be so much mildew. The maple with the mildew seems to be in an open sunny location with good air circulation. Is it just that if the spores overwintered, they'll appear the next season no matter what the conditions are? I think I remember learning on a nature walk that the mildew doesn't hurt the trees, and the problem disappears with the leaves, so there is no need to really do anything about it. This site: http://plant-disease.ippc.orst.edu/d...m?RecordID=713 indicates that not much can be done once the mildew appears anyway. Is that true?
How dry are you talking? Dry roots for long periods are said to encourage powdery mildew which seems a little counter-intuitive for a fungus that likes shade, damp air, poor air circulation etc. For example, I have seen it on partially neglected container trees that were not really getting enough water. Seems to be one of those pathogens that needs a stressed host to really thrive. A short article from the Daily Telegraph about powdery mildew (on Azaleas) that addresses this: Disease: how to combat powdery mildew Powdery mildew can attack plants during dry weather. Here's how to cope Hope that helps. Sorry I can't be much help on the possible treatments, other than avoiding stressing the plant.
I can confirm what Maf says. I have the worst mildew situation that I can remember and the weather has been unusually dry and hot this summer. In fact last time my garden had measurable rain was July 23rd while the normal rainfall for the period would have been around 50 mm (2"). It has mostly affected the new summer growth which has shrivelled and fallen. Gomero
You may want to read this article below. Master Gardeners - Santa Clara County - Powdery Mildew Keep in mind I am in an area that seldom ever gets Powdery Mildew unless plants are grown in very moist conditions in deep shade. One thing that some people may want to keep track of is which Maples and certainly which palmatum types are more susceptible to Powdery Mildew than others are. Generally, in areas that are experiencing unseasonal or unusual dry Summer weather, it is recommended to wash off the leaves, either from overhead sprinklers or from jets or streams of water such as power washer attachments to a garden hose or just from a water hose itself. Spring forms of Powdery Mildew can better be treated with an oil spray preventative of some kind. I've even had success with Oregon grown plants in Oregon using a tincture of Wintergreen oil as a hand applied topical wipe on leaves. The problem of course is that this can be rather labor intensive but for the few leaves we first see that can be dealt with, the wipe does indeed lesson the spread to other leaves and to other nearby plants. Jim
Thanks, everyone. What I've learned from the Master Gardeners article link in mr. shep's posting is that letting things be can be the right action (though that article didn't talk about maples specifically), and that if you consider spraying, you need to choose the right spray depending if you want to ward off the disease (preventative) or kill it once it’s established (eradicant). The article emphasized cultural control, which for smaller plants, includes spraying them with water to wash the mildew off. Well, today's the first good rain in a long time, so that might help.
The west coast sees mildew in summer because the combination of dry roots and damp leaves (from summer fogs or dew) that occurs here is perfect for it. Attempts to suppress mildew with washing should be undertaken in the morning, on a dry day, so plants do not remain damp overnight.
