150 not differ from cankers on northern red or pin oaks (P = 0.53 and P = 0.35, respectively). Oaks inoculated on stems three weeks post budbreak developed bacterial exudates within ten to fifteen days of inoculation, and small cankers formed on stems within 21 days of inoculation (Figure 3b). Although the cankers on northern red oak trees averaged 1.25 cm and were over twice as long as cankers on Shumard and pin oak trees, there were no statistical differences in the canker lengths (P = 0.12 and P = 0.47, respectively). Cankers found on Shumard and pin oaks were also similar in size (0.53 cm and 0.57 cm, respectively, P = 0.76). Sitz et al.: Drippy Blight, a Disease of Red Oaks in Colorado, US. oak (Q. wislizenii), producing a condition some- times described as “drippy nut of acorns” because of the copious bacterial ooze manifesting at acorn wound sites (Hildebrand and Schroth 1967). Although L. quercina causes infections of acorns in California (Hildebrand and Schroth 1967), a canker disease of Populus in China and Hungary (Tóth et al. 2013; Li et al. 2014), and contributes to oak decline in Britain and Spain (Biosca et al. 2003; Brady et al. 2012; Denman et al. 2012), drippy blight of red oak differs from these previous doc- umentations. Branch dieback, canker formation, twig abscission at the junction to the current sea- son’s growth, leaf drop, epicormic branching, and witch’s brooms are symptoms associated with this newly described condition. Furthermore, during the spring and summer, bacterial oozing occurs throughout the canopy. Exudates may be so copi- ous that dripping ooze results in large sticky areas on sidewalks and other surfaces under the canopy throughout the middle of the summer. Twig can- kers, indicated by maroon discoloration and clear margins (Figure 1d), appear in the late summer near kermes-scale-feeding sites and wounds. If the bac- terium is present in the fall, it dries and hardens. Historically, tree damage accompanying kermes Figure 3. Inoculation experiment: a) Shumard oak, 14 days post leaf whorl inoculation with Lonsdalea quercina subsp. quercina (note the bacterial ooze at inoculation sites, arrows); b) canker formation in northern red oak on one- year-old stems. In the course of field collections of three north- ern red oak trees in 2010, A. galliformis was found on 70%–81% of the twigs. Cankers typi- cal of those produced by L. quercina subsp. quer- cina were present at one or more feeding sites of scales on 51%–57% of the kermes-scale-infested twigs. Further, cankers were only present at scale insect feeding sites, which strongly supports an interaction between A. galliformis and L. quercina subsp. quercina. This combination of kermes scale feeding and subsequent bacterial canker formation resulted in 30%–42% twig dieback. The occurrence of twig cankers associated with the production of large amounts of bacterial ooze from infection with L. quercina has not previ- ously been reported. In California, L. quercina subsp. quercina infections are limited to acorns of coast live oak (Q. agrifolia) and interior live ©2018 International Society of Arboriculture scale infestations was attributed exclusively to the insect feeding (Turner and Buss 2005; Turner et al. 2005; Kosztarab 1996; Pellizzari et al. 2012). In con- trast, these results document a situation in which the combined activity of a kermes scale insect and a bacterium produce the tree decline symptoms described as drippy blight of red oak (Snelling et al. 2011; Caballero et al. 2014). The exact manner of how these two organisms interact to produce drippy blight disease remains unclear. In drippy nut of acorns, the bacterium establishes at wound sites caused by seed-feeding weevils, filbertworms, and cynipid gall wasps (Swiecki and Bernhardt 2006). Similarly, in drippy blight of red oak, the wound- ing associated with kermes-scale-feeding injuries may provide entry or exit courts for the pathogen. Alternatively, the interaction may be more indirect, where kermes scales act as a stressor to facilitate growth and spread of the pathogen within the host. Additional outstanding questions on drippy blight disease remain. For example, efforts to man- age this disease complex have been disappointing and are complicated by the presence of two causal
May 2018
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