Your live oak trees (Quercus virginiana) can create a romantic setting in your landscape, with their spreading crowns and hanging Spanish moss coverage. However, the otherwise pleasing green foliage and general appearance of your trees can quickly become a bit of an eyesore when powdery mildew takes over. Familiarize yourself with this disease and possible treatments to prevent damage and to extend the health of your live oaks.
1. Preventive Care
Provide the proper care for your live oak trees, because healthy, vigorous plants have a heightened capacity for avoiding and recovering from diseases such as powdery mildew. Grow live oaks in areas of the yard that offer full sun to partial shade. These trees thrive in moist, well-drained soil; while they prefer acid conditions, they will tolerate alkalinity as well as most soil types, some wetness, drought and salt. Live oaks grow best in U.S. Department of Agriculture plant hardiness zones 7b through 10b.
Powdery mildew is a fungal disease caused by a variety of species. On oak trees, those species include, but are not limited to, Erysiphe species and Sphaerotheca lanestris. This infection is most prevalent during warm, not hot, humid weather, particularly in a temperature range of 60 to 80 degrees Fahrenheit, explains the University of Georgia College of Agricultural and Environmental Sciences. However, while humid weather promotes disease growth, unlike many fungal diseases, dry leaf surfaces are also a necessary component for development. Disease overwinters on plant debris and spreads sporadically on wind and water to new host plants.
By the time symptoms become visible on your live oak tree, the disease has typically been active for several weeks. Look for areas of white, powder-like spots on foliage surfaces. As the disease progresses, the spots typically become larger and may inhibit photosynthesis, thereby diminishing the tree's health. In addition, leaves may become brown or yellow and fall early from the plant, and often take on a distorted appearance. Early defoliation leaves the live oak tree susceptible to sun damage. An overdevelopment of shoots on the ends of branches, referred to as witches' broom, may also occur, resulting in further cosmetic damage.
Because this disease primarily results in cosmetic damage and will not lead to plant death, fungicides are typically unnecessary on live oaks. In addition, by the time you notice symptoms, the effectiveness of potential fungicide application is greatly decreased. Healthy trees will typically develop new foliage and regain their health by the next season. You should, however, remove and destroy areas affected by witches' broom to improve the appearance of your tree. Prune heavily infected sections of the tree; always gather and destroy debris and removed plant material to avoid spreading disease or causing reinfection. Protect yourself from contact with fungi and from injury when handling sharp tools by wearing gardening gloves.
- University of Florida IFAS Extension: Quercus Virginiana -- Southern Live Oak
- University of California Cooperative Extension: Powdery Mildew Prevalent on Foothill Oaks This Year
- University of Georgia College of Agricultural & Environmental Sciences: Key to Diseases of Oaks in the Landscape
- University of California Cooperative Extension: Powdery Oak Mildew -- It's Everywhere!
- University of California Statewide Integrated Pest Management Program: Powdery Mildew
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