University of Maryland Extension

Powdery Mildew - Trees and Shrubs

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powdery mildew infection
Powdery mildew on euonymus

Key Points

  • Powdery mildew is the common name for the disease and symptoms caused by a closely related group of fungi.
  • These fungi grow on the upper and lower leaf surfaces, young stems, shoot tips, flower buds, and/or blossoms of plants.
  • As they grow, they produce microscopic chains of spores that give infected areas their characteristic white powdery appearance.

oak leaves infected with powdery mildewsycamore leaves with powdery mildew
 Powdery mildew on oak leaf                   Sycamore leaves infected with powdery mildew



powdery mildew rosepowdery mildew on lilac
Powdery mildew distorting rose leaves      Powdery mildew commonly infects lilac

Problems Caused by Powdery Mildew and Conditions that Favor the Disease

  • The fungi parasitize the tissues of the plant causing a decline in its vigor. They also block light needed for photosynthesis.
  • Infection is rarely lethal, but does cause leaf yellowing and browning, leaf distortion, premature leaf drop and blemished or aborted flowers, and slower-than-normal growth.
  • Young plants grown in heavy shade are the most seriously affected by this disease.
  • The optimum conditions for powdery mildew development are warm days followed by cool, humid nights.
  • Dry daytime weather allows spores to spread to other plants on air currents.
  • On a cool evening they absorb enough moisture from the air to germinate and cause infection.
  • The entire powdery mildew life cycle can take place in less than a week under ideal conditions, and many overlapping infection cycles can occur within a single growing season. These fungi overwinter in the bud scales for initiation of infection next season.

dogwood leaves with powdery mildew
Powdery mildew is a problem seen on dogwood

beginning powdery mildew infection on dogwoodpowdery mildew on dogwood
Early symptoms of powdery mildew on        Dogwood with powdery mildew. Notice the
dogwood
                                                   red tint to the leaves                                                               

crape myrtle infected with powdery mildew
Powdery mildew infection on crape myrtle

Management 

  • Control begins with the selection of plants resistant to powdery mildew.
  • Place susceptible plants where there is adequate sunlight and good air circulation to reduce humidity levels.
  • Allow proper plant spacing for the same reasons.
  • Pruning (thinning out plants) for better air circulation also may help.
  • Registered fungicides may be needed if disease is severe. Check the label registration on horticultural oil products for powdery mildew control listings.
  • When selecting new dogwood varieties, choose powdery mildew resistant cultivars of kousa, flowering dogwood or hybrids such as 'Cherokee Brave' (flowering), 'National' (kousa), 'Milky Way Select' (kousa), 'Stellar Pink' (hybrid), 'Stardust' (hybrid), 'Constellation' (hybrid), 'Aurora' (hybrid), and others.
  • There are also many powdery mildew resistant crape myrtles available for planting in the nursery trade. The U.S. National Arboretum introductions are mildew resistant. (https://www.usna.usda.gov/science/plant-introductions-and-releases/plant-intros-genus1/), look under Lagerstroemia.

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