Understanding and Managing Sudden Oak Death in California

1.3.2 Variation in Host Susceptibility

SOD canker hosts differ in their relative susceptibility to P. ramorum. Tanoak appears to be more susceptible than the oak species (see 1.5. Stand-Level SOD Impacts. Among oaks, coast live oak appears to be more susceptible than California black oak. Effects of tree size and condition—

Tanoaks are susceptible to trunk cankers, foliar infections, and twig cankers over their enter lifespan. In contrast, oaks susceptible to SOD rarely develop twig infections (see 1.2.2 SOD in Oak / California bay Forests) and cankers generally do not develop on trunks less than 10 cm in diameter. Large diameter oaks show a higher incidence of SOD cankers than do small diameter oaks.

As oak stem diameter increases, so does bark thickness. The bark of tanoak and most susceptible oaks has a thick layer of living phloem tissue. Phloem is the primary tissue that P. ramorum invades when it forms trunk cankers (see 1.4 SOD Development on Trunk Canker Hosts). In coast live oak, bark thickness is a better predictor of SOD risk than is trunk diameter (Swiecki and Bernhardt 2005). Coast live oaks with thicker bark tend to be infected more readily and show faster rates of disease progress.

Large tree canopies may also capture higher amounts of inoculum than smaller trees (see California bay–oak distance and disease risk ). This may also contribute to the greater incidence of SOD seen in large trees.

Unlike diseases that preferentially attack stressed trees, P. ramorum canker is more likely to affect relatively vigorous, faster growing coast live oaks (Swiecki and Bernhardt 2005). Oaks that are growing slowly because of suppression, water stress, or severe wood decay are less likely to be infected than more dominant, healthier, trees. Faster growing coast live oaks develop wide bark fissures with unweathered brown bark as a result of rapid bark expansion. New infections are often associated with these fissures (fig. 1-3), which may be preferred sites for infection by P. ramorum. Slow growing coast live oaks that show weathered gray color in almost all bark fissures are less likely to be infected by P. ramorum (Swiecki and Bernhardt 2004, 2005).