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Drippy blight: A new disease complex of red oak

Date: August 31, 2017

Red oaks are an important part urban forests in Colorado, but have experienced dieback recently as a result of an unknown disease complex. Understanding more about this complex is an important first step in protecting red oaks.


Background

A northern red oak showing symptoms of drippy blight disease.
A northern red oak showing symptoms of drippy blight disease.

Bacterial exudates covering second instar kermes scales feeding at the junction of new and one-year-old growth.
Figure 2. Bacterial exudates covering second instar kermes scales feeding at the junction of new and one-year-old growth.

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, studies were conducted 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. Furthermore, tree surveys were performed in order to determine the extent of damage throughout the community, document the identifying characteristics of the disease, and note interactions with plant feeding insects.

Findings

  • 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.

  • We 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 is observed.

Upcoming Publication

Sitz, R.A., Zerillo, M., Snelling, J., Ibarra Caballero, J., Alexander, K., Nash, K., Tisserat, N.A., W.S. Cranshaw, and Stewart, J. 2017. Drippy blight, a disease of red oaks in Colorado produced from the combined effect of the scale insect Allokermes galliformis and the bacterium Lonsdalea quercina subsp. quercina. Accepted manuscript. Journal of Arboriculture and Urban Forestry.

 
 
 


Principal Investigators: 
Principal Investigators - External: 
Whitney Cranshaw (CO-PI) - Colorado State University
External Partners: 
Marcelo Zerillo, Netherlands Institute of Ecology
Jacob Snelling, Oregon State University
Jorge Ibarra Caballero, Ned Tisserat, and Jane Stewart, Colorado State University
Kathleen Alexander, City of Boulder Forestry
Kendra Nash, City of Fort Collins Forestry
Research Location: 
Colorado urban corridor