Pacific Southwest

Title: Relationships between Phytophthora ramorum canker (sudden oak death) and failure potential in coast live oak

Author: Swiecki, Tedmund J.; Bernhardt, Elizabeth; Drake, Christiana; Costello, Laurence R.

Date: 2006

Source: In: Frankel, Susan J.; Shea, Patrick J.; and Haverty, Michael I., tech. coords. Proceedings of the sudden oak death second science symposium: the state of our knowledge. Gen. Tech. Rep. PSW-GTR-196. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture: 427-453

Station ID: GTR-PSW-196

Description: In autumn 2002, we conducted a retrospective study on coast live oak (Quercus agrifolia) failures in Marin County, California, woodlands affected by Phytophthora ramorum canker (sudden oak death). The objectives of this case-control study were to quantify levels of bole, large branch, and root failure in these woodlands and determine how various tree and stand factors are related to failure potential. Non-failed trees were used as a control population to identify factors that might contribute to tree failure. Rates of bole and large branch (more than 20 cm diameter) failures were significantly higher between about July 2001 and December 2002 than in the period from 1992 through July 2001. Bole failures were the most common type of failure. Based on the estimated date of failure, for the years 1992 through 1996, boles failures occurred in 0.5 percent of the trees each year. The incidence of bole failures increased to 5 percent per year between mid-2001 and the end of 2002. Among recent failures (2001-2002), 39 percent of the bole failures and 30 percent of the scaffold (primary branches arising from the main stem) failures occurred in live stems. Most root and root crown failures also occurred in live trees. Among trees with recent failures (2001-2002), 83 percent showed symptoms of P. ramorum canker. Branch, scaffold, and bole failures were strongly associated with advanced symptoms of P. ramorum canker, which include evidence of wood degradation by Hypoxylon thouarsianum and/or various wood boring beetles.
Early P. ramorum canker symptoms, consisting of only bleeding cankers without secondary invasion, were not associated with an increased likelihood of failure. Wood decay was the most consistent and important factor influencing failure potential. Decay was present and rated as a contributing factor in almost all failures. Fruiting bodies of H. thouarsianum and other wood decay fungi, decay columns, and canker rot symptoms were significantly more common among failures than among non-failed controls. Several variables indicative of wood decay were highly significant predictors of failure in both recursive partition and multivariate logistic regression models. Beetle boring was also significantly more common among failures than among non-failed trees. Other factors associated with increased failure potential include overtopping of the tree by other trees, local alteration of the stand canopy due to dead or failed trees, multiple trunks, multiple branches arising from the same point, and asymmetric canopy shape. Failures in both live and dead trees were largely influenced by the same factors.

Keywords: tree failure, wood decay fungi, Hypoxylon thouarsianum, ambrosia beetles

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Citation

Swiecki, Tedmund J.; Bernhardt, Elizabeth; Drake, Christiana; Costello, Laurence R.  2006.  Relationships between Phytophthora ramorum canker (sudden oak death) and failure potential in coast live oak.   In: Frankel, Susan J.; Shea, Patrick J.; and Haverty, Michael I., tech. coords. Proceedings of the sudden oak death second science symposium: the state of our knowledge. Gen. Tech. Rep. PSW-GTR-196. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture: 427-453.