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Powdery Mildew on my Japanese Maples.

9 years ago

Hello,
Please help me and advise me on what to do. My Japanese Maples look awful. First they were invaded by Japanese Beetles even though my yard was treated with Milky Spore a few years ago, then they got this mildew and the leaves have lost their lustre and their colour. They get watered by the sprinkler for 20 minutes 3x a week. They are about 7' tall and are seed grown from a beautiful tree I had in MI. Six are in ground and 3 are in containers. 3 of the in ground maples have the powdery mildew. New leaves are nice and red, and I don't know why the others' are a yellowish colour. I live in NoVa. and any help would be very appreciated.

Thank you,

Susan.

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Comments (2)

gardengal48 (PNW Z8/9)
9 years ago
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.
soozifroozi
Original Author
9 years ago
Thank you, gardengal48. Next year I shall try the soaker method and hope that my trees look will once again be beautiful.
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The sunken area at the base appears to be to be canker. Had to remove a Ukigumo JM earlier in the season for this very reason. I have seen allot of this over the last few years. Have seen allot of it on crabapples, Japanese Maples, and other maples. Plants sometimes beat the pathogen, sometimes not. If they are going to beat it, then expect to see callus to form around the edges of the sunken area. Even if the callus forms, it is far from certain. Once the tree goes dormant for the winter, the canker will often gain the upper hand again. However if the callus does not form by end of summer, then from what I have seen, it's almost certainly a matter of time before the tree is killed. As for how your plant became infected, I suspect that a number of low cost and/or large scale producers have gotten very careless over the last 5-10 years. There has been allot of apparent disease increase in allot of other plant groups as well over the same period (Aster Yellows seems to be everywhere anymore, to the point it's getting rather difficult to get disease free coneflowers, saw this Saturday at the largest local nursery). I'm sure part of it is due to producers having a difficult time the last several years, therefore cutting corners, poor training, poor enforcement, less responsible labor, etc. But also Big Box Store has little incentive, as they know Q Public is unlikely to know anything about disease and will be back again next year to buy another one. Arktrees
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