Sporulation of P. ramorum on trees and shrubs in western Washington forests
One of the major factors that will affect the potential spread of Phytophthora ramorum in western Washington forests is the ability of this exotic pathogen to sporulate on known and potential hosts within these forests. In California, the ability of P. ramorum to produce large quantities of inoculum on California bay laurel is a major factor in the epidemiology of sudden oak death in their forest ecosystems. Tanoak appears to be the most important epidemiological host associated with the spread of P. ramorum in southwestern Oregon forests. In the United Kingdom, sporulation on rhododendron within forests appears to be responsible for the spread of this pathogen to several forest trees. Western Washington is considered to be at high-risk for the spread of P. ramorum. Limited information is available regarding the sporulation of P. ramorum on hosts in western Washington forests. Research is underway to determine the potential for P. ramorum to sporulate on known and potential hosts that commonly occur in western Washington forests. This information will increase our understanding of potentially important epidemiological hosts and provide insights on the role they may play in the spread of P. ramorum in forest ecosystems where other important epidemiological hosts, such as California bay laurel and tanoak do not occur.
Studies with known conifer hosts, such as Douglas-fir, indicate that infection occurs during the early stages of shoot emergence. Thus, sporulation on hosts such as madrone and big leaf maple which potentially have foliage within the mid to upper canopy of the forest probably pose the greatest threat to spreading the disease to established conifers. Although one small preliminary study suggests that P. ramorum does not produce sporangia on infected Douglas-fir and grand fir shoots, additional tests are needed to confirm this and determine the potential for it to sporulate on other conifer foliage within Washington’s forests. Although potentially less important as a source of inoculum for standing trees, sporulation on understory shrubs, like Pacific rhododendron and salal, may be important sources of inoculum that may contaminate logs during harvest.