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Laboratory Report

Articles: Laboratory Report

Foliar and Canker Hosts
We have learned a lot about sudden oak death (SOD) and its causative agent, the water mold Phytophthora ramorum (P. ram), since our last report (see Pacific Horticulture, October ’04). Many of us are now aware of the central role played by California bay laurel (Umbellularia californica), a foliar host that provides an ideal environment for P. ram to reproduce. Foliar hosts produce the spores that are responsible for spreading SOD; some, such as Rhododendron, Camellia and Pieris, may be killed by P. ram. Without suffering significant damage, bay laurel acts as a primary driver of the disease, for several reasons: it produces copious quantities of spores, it frequently overtops oaks and spores are literally “rained down” on oak canopies, and it is ubiquitous in many woodlands, where it grows in close association with susceptible oaks.

Canker hosts include several tree species in the red oak group (Quercus spp.), and tan oak (Lithocarpus densiflorus), which is unique in acting as both a foliar and canker host. Canker host species generally do not produce spores that spread the disease, but suffer varying levels of mortality. While several species of red oa...


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