maple mold

Discussion in 'Maples' started by Daniel Mosquin, Jun 4, 2003.

  1. Daniel Mosquin

    Daniel Mosquin Paragon of Plants UBC Botanical Garden Forums Administrator Forums Moderator 10 Years

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    Vancouver, British Columbia, Canada
    The following was received via email:

    I would like to know how to get rid of and control white mould on a very large red maple tree. It has been happening every summer for several years now. Someone said to spray it with liquid seaweed which was done but it came back the next year. The branches were thinned to allow more air circulation. Any other suggestions and advice would be greatly appreciated.

    Thank you .
  2. Douglas Justice

    Douglas Justice Well-Known Member UBC Botanical Garden Forums Administrator Forums Moderator VCBF Cherry Scout Maple Society 10 Years

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    Vancouver, Canada
    In the Vancouver area, and perhaps elsewhere, powdery mildew is seldom a significant problem on any maple except the red leaf forms of Acer platanoides (Norway maple). Locally, the most susceptible is the cultivar called 'Crimson Sentry'; virtually every street planting in the area is affected.

    In late spring as the disease becomes established in leaf tissue, plants look as though they've been dusted with powdered sugar, which is somewhat showy, or at least interesting. Later, however, affected leaves become dull and rather tattered looking.

    Given that this disease is so common in this plant here, the assumption is that the fungus that causes it is well-established and the conditions for its proliferation are ideal. This suggests that complete control is probably impossible, or at least impractical.

    Increasing aeration by thinning the tree's canopy is a good start (as long as the tree's health is not further compromised by poor pruning). Liquid seaweed may ultimately benefit the tree (seaweed is high in potassium, which is a required plant nutrient and also known to increase disease resistance), but applying it to the foliage may actually have increased infection by powdery mildew or other fungi. Some seaweed emulsions contain nitrogen (also a required plant nutrient), the application of which has been shown to increase infection by fungi. On the other hand, it may have had no measurable effect at all (other than on your time and wallet).

    Elemental sulfur is well-known as a fungicide, and wettable sulfur has been shown to have a negative effect on the fungus that causes this disease. However, applications of sulfur-containing products at temperatures exceeding 10C often cause leaf burn. Dormant applications of lime-sulfur (i.e., in the winter), together with a thorough clean-up of leaves and other potentially infected debris from the immediate area may provide some control.

    These trees appear to tolerate a fairly high level of infection by powdery mildew, but Crimson Sentry should probably not be recommended for mass plantings or street plantings (unless a ghostly pallour on purple foliage is actually wanted).

    It would be useful to hear from other forum participants who are familiar with this phenomenon.
  3. I have planted six crimson sentry maples on my property two years ago and have been seeing this problem. I have tried sulfur but the mildew keeps coming back every late spring. Have you had any success in reducing or eleminating the powdery white mildew. Seems to be common in the northwest. I live in Camas, Washington. Please feel free to e-mail me at


  4. Dale B.

    Dale B. Active Member Maple Society 10 Years

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    Atlanta, GA

    Neem oil spray has been recommended to me as a control for powdery mildew. I have used it once or twice and it seems to keep it away for a while. The package recommends regular treatments as prevention and treatment. What do you think about a normal routine of Neem oil spray every month during the growing season?

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