Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Maple -- Powdery Mildew
 
Cause: Phyllactinia guttata, a fungus that attacks many hardwood species such as hazel, birch, and alder. Conidia are readily wind disseminated. Chasmothecia develop within the external hyphae and form appendages late in their development. The uneven wall thickness and bulbous nature of the appendages allows the entire cleistocarp to be dislodged from the host. When dry, the appendages flex like a spider's legs and allow the cleistocarp to be dislodged and carried by wind. These fungi may overwinter as chasmoothecia or through buds infected the previous season.
Symptoms: A white mildew grows, often with small black fruiting bodies (chasmothecia), on the undersides of mature leaves.

Note the white patches on these leaves.

White colonies of this fungus can cover the leaf surface.

Norway maple in nursery with powdery mildew.
 
Chemical control: Most are not labeled for use on trees to be used for syrup production. Fungicides will do best when used before symptoms develop. Few materials have good eradicant activity. Use at 7- to 14-day intervals; using shorter intervals when environmental conditions favor disease development.
  1. Banner MAXX at 5 to 8 fl oz/100 gal water may be effective. 24-hr reentry.
  2. Bayleton 50 T&O at 5.5 oz/275 to 550 gal water. Landscape only, not for use on plants for sale. 12-hr reentry.
  3. Cygnus 50 WG at 1.6 to 3.2 oz/ 100 gal water plus a non-organosilicone spreader-sticker. Best used before symptoms develop. Do not use near Norway maples. 12-hr reentry.
  4. Eagle 20 EW at 6 to 12 fl oz/100 gal water. Do not use treated trees to produce syrup. 24-hr reentry.
  5. Heritage at 1 to 4 oz/100 gal water plus a non-silicone based wetter sticker. 4-hr reentry.
  6. Insignia at 4 to 8 oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventatively only. Listed for Amur, Japanese, Norway, Sugar, Soft and Negundo maple. 12-hr reentry.
  7. Safer Garden Fungicide (Ready To Use 0.4% sulfur) thoroughly sprayed over the entire plant. Do not use when the temperature is over 85°F or within a few weeks of an oil spray.
  8. Spectracide Immunox at 1 fl oz/gal water.
  9. Strike 50 WDG at 1 to 2 oz/100 gal water. 12-hr reentry.
Biological control:
  1. Cease or Rhapsody at 2 to 8 qt/100 gal water is registered for Japanese Maple. Active ingredient is a protein from Bacillus subtilis strain QST 713. Efficacy in the PNW is unknown. 4-hr reentry.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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