Horse-chestnut (Aesculus spp.)-Powdery Mildew

Cause The fungus Erysiphe flexuosa has been reported in Washington and Idaho and observed in Oregon. Powdery mildews are highly specialized and form a close association with the host such that this fungus only infects species of Aesculus. The disease is favored by shade, warm conditions (60°F to 80°F) and cool nights. Overcrowding of plants is also favorable for disease development. These fungi may survive in sexual fruiting bodies (chasmothecia) associated with plant debris and/or as mycelia within infected buds.

Symptoms Infection of leaves may produce sparse or dense white mycelial colonies. Leaves may have discrete spots or colonies that cover the entire leaf surface. Heavily infected leaves may show yellowing along the midrib and margins. Discoloration may also be seen on the upper leaf surface in relationship to colonies underneath.

Cultural control

  • Rake and destroy fallen leaves at the end of the year.
  • Space plants for good air circulation.
  • Avoid excessive fertilization.

Chemical control 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. Alternate or tank-mix products from different groups that have different modes of action. Limit the use of any one group during crop production.

  • Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
  • Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Group 3 fungicide. 24-hr reentry.

Reference Glawe, D.A. and Dugan, F.M. 2006. First report of Erysiphe (Uncinuliella) flexuosa in western North America. Pacific Northwest Fungi 1:1-11.