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Individual Highlight

Drippy blight: A new disease complex of red oak

Photo of Bacterial exudates covering second instar kermes scales feeding at the junction of new and one-year-old growth.Bacterial exudates covering second instar kermes scales feeding at the junction of new and one-year-old growth.Snapshot : Disease complexes, the result of insects and plant pathogens interacting to compromise their plant hosts, are becoming increasingly common worldwide. These disease complexes represent an imminent threat to plant biodiversity, because the combined activity of insects and pathogens can cause more damage with a potential for increased spread than diseases caused by a single organism. Forest Service scientists are seeking to determine the extent of damage and document the disease in an effort to better understand the disease and protect red oaks.

Principal Investigators(s) :
Sitz, Rachael A.  
Research Location : Colorado urban corridor
Research Station : Rocky Mountain Research Station (RMRS)
Year : 2017
Highlight ID : 1353


This research describes the emergent disease complex, known as drippy blight, which affects red oak species in Colorado. To determine the causal agent of drippy blight, Forest Service scientists conducted studies on red oak saplings including pin, northern red, and Shumard oaks to determine if the bacterium isolated from cankers and exudates was causing the disease. They also performed tree surveys to determine the extent of damage throughout the community, document the identifying characteristics of the disease, and note interactions with plant feeding insects. Drippy blight disease is caused by a plant pathogenic bacterium in the family Enterobacteriaceae, Lonsdalea quercina subsp. quercina, in association with the kermes scale insect, Allokermes galliformis. The scientists confirmed the bacterium as the causal agent, because the red oak species inoculated with the bacterium all showed symptoms of drippy blight, and the bacterium was recovered from these inoculated oaks. The bacterium consistently exudes from kermes scale feeding sites on the newer branch growth throughout the upper canopy, lending to the name “drippy blight.” The disease symptomology includes twig abscission, leaf drop, and branch dieback; after successive years of infection whole-limb dieback occurs.

Forest Service Partners

External Partners

  • City of Boulder, Colo.
  • Colorado State University
  • Netherlands Institute of Ecology
  • Oregon State University