Abstract
Mortality of large canyon live oaks suddenly appeared in natural stands in San Mateo, CA, starting in 2007. A survey of affected stands showed that symptomatic trees were spatially associated with California bay, the primary source of Phytophthora ramorum spores in California coastal oak forests. Trunk canker symptoms on affected trees were similar to late-stage symptoms caused by P. ramorum on other oak hosts, but the pathogen could not be isolated from affected trees. Artificial inoculation of logs, and later, trees, confirmed that P. ramorum caused phloem cankers on canyon live oak, but cankers showed either no or minuscule external bleeding. Knowledge of early bark symptom appearance facilitated successful isolations from naturally infected trees. Tree declines associated with similarly cryptic Phytophthora cankers could remain undiagnosed or misdiagnosed for many years, thwarting detection and management efforts. Accepted for publication 12 February 2016. Published 18 February 2016.
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