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ARS Home » Pacific West Area » Davis, California » Crops Pathology and Genetics Research » Research » Publications at this Location » Publication #320677
Title: Phytophthora ramorum causes cryptic bole cankers in Canyon line Oak

Author
item SWIECKI, TEDMUND - Phytosphere Research
item ARAM, KAMYAR - University Of California
item Kasuga, Takao
item Bui, Mai
item RIZZO, DAVID - University Of California
item BERNHARDT, ELIZABETH - Phytosphere Research

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2015
Publication Date: 2/18/2016
Citation: Swiecki, T., Aram, K., Kasuga, T., Bui, M.Q., Rizzo, D., Bernhardt, E. 2016. Phytophthora ramorum causes cryptic bole cankers in Canyon line Oak. Plant Health Progress. 17:20-26.

Interpretive Summary: Unusual mortality of large canyon live oaks was observed in natural stands in San Mateo, and we suspected it was caused by the Sudden Oak Death pathogen, Phytophthora ramorum. We found that symptomatic trees were spatially associated with California bay, the primary source of P. ramorum spores in this forest type; however, the pathogen could not be isolated from affected trees. Artificial inoculation of logs, and later, trees, confirmed that P. ramorum caused cankers on canyon live oak, but cankers showed either no or minuscule external bleeding. Tree declines associated with similarly cryptic Phytophthora cankers could remain undiagnosed or misdiagnosed for many years, thwarting detection and management efforts.

Technical Abstract: Unusual mortality of large canyon live oaks was observed in natural stands in San Mateo, California 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 this forest type. Trunk canker symptoms on affected trees were similar to late-stage symptoms caused by Phytophthora 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.