Drippy blight is a disease of red oaks including northern red oak (Quercus rubra), pin oak (Quercus palustris), and Shumard oak (Quercus shumardii) in Boulder. It was first detected here in 2010 and has led to the decline and removal of large, significant red oaks throughout the community. Drippy blight is a striking disease where the unique association between a kermes scale insect and a bacterium contribute to severe tree decline. The pin oak kermes scale (Allokermes galliformis) produces wounds while feeding, damaging tissues and allowing entry (and exit) of the bacterial pathogen (Lonsdalea quercina var quercina). The bacteria causes cankers that contribute to twig dieback and produces viscous ooze that drips from trees. You will see oozing, dripping, twig dieback, flagging, and abscission of twigs that often look like “squirrel clippings” on the ground in late June and July.
Treatment of the scale insect with dormant oil sprays or trunk injected azadirachtin have had some success with managing the disease. Colorado State University extension recommends pruning oaks with drippy blight only during the dormant season (October 15 – March 15) and spraying pruning tools with a disinfectant (e.g., Lysol® All Purpose Cleaner) between cuts to prevent the spread of the bacteria within the canopy. It is not permitted to plant any red oaks species in Boulder due to this disease.
