Help with ID of disease, and treatment - Ask Extension
Mildew and brown spot on deciduous trees, mostly Japanese maples. Early leaf drop this year on one of the green-leafed varieties. It’s been going on...
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Help with ID of disease, and treatment #813912
Asked October 11, 2022, 11:56 AM EDT
Mildew and brown spot on deciduous trees, mostly Japanese maples. Early leaf drop this year on one of the green-leafed varieties. It’s been going on for a couple of years, since the summers got ridiculously hot. No huge changes in sunlight.
Tried topical fungicide and neem oil during summer with little effect.
Baltimore County Maryland
Expert Response
Powdery mildew on Japanese maple foliage (and on a range of other tree, shrubs, and perennial species) is quite common by late summer; the fungus thrives in warm weather, cooler nights, and humid conditions. While fungicide treatments can sometimes suppress fungal outbreaks, they need to be applied early (before symptoms appear) and regularly (repeatedly over the course of the growing season as conditions persist that allow for infection). Even then, they do not always work perfectly, but fortunately powdery mildew usually doesn't harm the long-term health of the host plant very much.
Trees are difficult to spray since good coverage of the entire canopy is needed for a treatment to be effective, which is one reason we discourage fungicide use in this situation. You can rake-up and dispose of the fallen leaves from infected plants, though while this may reduce infection, it's not a foolproof way of preventing disease in future years. Treatment this late in the season is not necessary though, nor would it be very effective. Neem oil is not a great fungicide, even though it can be labeled for such use; it's mainly an insecticide.
From what we can see in the photos, the maple leaves aren't showing obvious powdery mildew symptoms (though sometimes the powdery attribute isn't very prominent), but they do have what looks like symptoms of scorch and possibly minor Anthracnose or other fungal leaf spot infections from earlier in the year. These are all typical for Japanese maple and not much can be done about it except watering when needed to lessen the tree's stress, which will keep the leaves from developing as many "burnt" spots. (Feel the soil moisture around six inches deep in the tree's root zone; when it's becoming relatively dry to the touch at that depth, water thoroughly.) In high summer during a dry spell, this may mean about one soaking per week would be needed to keep the tree well-hydrated as a rough guideline.
Nothing needs to be done for now. Aside from watering when needed and raking-up fallen leaves going forward, no actions would have much impact on plant health and appearance at this point. Some premature leaf drop near the end of the growing season is to be expected when trees are stressed (cherries, honeylocusts, black locusts, and river birches do this all the time to varying degrees) but as long as they keep leafing-out normally in spring and retaining those leaves into most of summer, they should be okay.
Miri
Trees are difficult to spray since good coverage of the entire canopy is needed for a treatment to be effective, which is one reason we discourage fungicide use in this situation. You can rake-up and dispose of the fallen leaves from infected plants, though while this may reduce infection, it's not a foolproof way of preventing disease in future years. Treatment this late in the season is not necessary though, nor would it be very effective. Neem oil is not a great fungicide, even though it can be labeled for such use; it's mainly an insecticide.
From what we can see in the photos, the maple leaves aren't showing obvious powdery mildew symptoms (though sometimes the powdery attribute isn't very prominent), but they do have what looks like symptoms of scorch and possibly minor Anthracnose or other fungal leaf spot infections from earlier in the year. These are all typical for Japanese maple and not much can be done about it except watering when needed to lessen the tree's stress, which will keep the leaves from developing as many "burnt" spots. (Feel the soil moisture around six inches deep in the tree's root zone; when it's becoming relatively dry to the touch at that depth, water thoroughly.) In high summer during a dry spell, this may mean about one soaking per week would be needed to keep the tree well-hydrated as a rough guideline.
Nothing needs to be done for now. Aside from watering when needed and raking-up fallen leaves going forward, no actions would have much impact on plant health and appearance at this point. Some premature leaf drop near the end of the growing season is to be expected when trees are stressed (cherries, honeylocusts, black locusts, and river birches do this all the time to varying degrees) but as long as they keep leafing-out normally in spring and retaining those leaves into most of summer, they should be okay.
Miri
