146 Sitz et al.: Drippy Blight, a Disease of Red Oaks in Colorado, US. Arboriculture & Urban Forestry 2018. 44(3):146–153 Drippy Blight, a Disease of Red Oaks in Colorado, U.S., Produced from the Combined Effect of the Scale Insect Allokermes galliformis and the Bacterium Lonsdalea quercina subsp. quercina Rachael A. Sitz, Marcelo M. Zerillo, Jacob Snelling, Jorge Ibarra Caballero, Kathleen Alexander, Kendra Nash, Ned A. Tisserat, Whitney S. Cranshaw, and Jane E. Stewart Abstract. Drippy blight is an emergent disease of red oaks, caused by the interaction between a kermes scale insect (Allokermes gal- liformis) and a bacterium (Lonsdalea quercina subsp. quercina). Multi-locus sequence analysis was used to confirm the bacte- rial pathogen’s identity and its relationship to other phylogenetically related Enterobacteriaceae species. Further, Koch’s postulates were performed on sapling red oaks. Prior to the discovery of drippy blight disease in Colorado, in the United States, the bacterium was reported on oak trees in California but was limited to acorn infections. The scale insect, A. galliformis, was previously known to occur on pin oak in the eastern United States but was not previously associated with either this bacterium or the production of significant branch dieback associated with drippy blight. In addition to a description of this new disease, this research docu- ments a host range expansion of L. quercina subsp. quercina to northern red oak (Quercus rubra), Shumard oak (Q. shumardii), and pin oak (Q. palustris) and extends the reported host range of A. galliformis to include northern red and Shumard oaks. Key Words. Allokermes galliformis; Bacteria; Colorado; Drippy Blight; Lonsdalea quercina, Red Oak; Scale Insect. Northern red oak (Quercus rubra), pin oak (Q. palustris), and Shumard oak (Q. shumardii) comprise a small but important component of the urban tree landscape along the eastern urban corridor adjacent to the Rocky Moun- tains in Colorado, U.S. Since the early 2000s, some municipalities within this region have noted sites where all ages of these oak species and their hybrids have experienced signifi- cant dieback of unknown origin. The disease incidence has accelerated in some locations. For example, in Boulder, which has the largest concentration of northern red oaks on public property in the state of Colorado, trees showing extensive twig dieback throughout the crown more than doubled over three years, from 11% (50/450) in 2012 to 24% (110/450) in 2015. As the trees have shown progressive decline, there ©2018 International Society of Arboriculture has been a resultant increase in tree removals (K. Alexander, personal communication, 2016). Affected trees initially exhibit leaf scorch- ing and leaf drop, followed by dieback of small- diameter twigs throughout the canopy (Figure 1a; Figure 1b). As branch cankers form or as twigs die, tree parts become brittle and snap off the tree, especially during windstorms. At the point of breakage, new shoot growth often results in small witch’s brooms, or twig dieback from successive years may result in major limb dieback. Another feature associated with the dis- ease, particularly in northern red oak, is copious gummosis that drips from cankered, damaged twigs, and onto sidewalks and parked cars, creat- ing a nuisance (Figure 1c; Figure 1d). This com- bination of symptoms has led to this condition being described as “drippy blight” of red oak.
May 2018
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