Powdery Mildew

    Powdery Mildew

    Many fungal species

    powdery mildew

    Powdery mildew on lilac (Whitney Cranshaw, Colorado State University, Bugwood.org)

    powdery mildew

    Powdery mildew on dogwood (Penn State Department of plant pathology and Environmental Microbiology Archives, Bugwood.org)

    powdery mildew

    Powdery mildew (Whitney Cranshaw, Colorado State University, Bugwood.org)

     

    Hosts, Symptoms & Signs

    • many hosts; powdery mildew species are usually host- or group-specific
    • white powdery growth on green plant tissue
    • commonly occurs on the tops of leaves, but may be on the undersides of leaves, young stems, buds, flowers and young fruit

    Disease Cycle

    • overwinters on plant debris as survival fruiting structures (cleistothecia) or mycelia
    • in spring, spores from overwintering cleistothecia are spread to new tissue via rain, irrigation, wind or insects
    • young, succulent growth is most susceptible
    • only moderate humidity is needed to germinate spores
    • powdery mildews are common in warm, dry climates

    IPM Recommendations

    • Manage trees to improve or maintain overall health.
    • Powdery mildew in ornamentals is an aesthetic problem; tolerate powdery mildew.
    • Use powdery mildew-resistant plants.
    • Avoid late summer nitrogen applications.
    • Apply fungicides when the first small white spots are discovered. Once the entire leaf is colonized, fungicide applications will be less effective.
    • Apply a fungicide (chlorothalonil; potassium bicarbonate; sulfur) to affected foliage. Note: Sulfur can only be applied at temperatures below 90 degrees F; temperatures must stay below 90 degrees F until the fungicide has dried.
    • Test ornamental fungicides for phytotoxicity (plant damage) on a few leaves before applying fully.