The Potential for a California Oak Wilt Epidemic
David N. Appel
Abstract: The relative disease susceptibilities of 14 Quercus species from Texas and California were tested in three inoculation studies with different isolates of the oak wilt pathogen, Ceratocystis fagacearum. Ten of the oaks were evergreen or semi-evergreen live oaks while the remaining four were deciduous species. The species represented 3 different Quercus sub-genera. All species were susceptible to the pathogen, but there was significant variability in disease response. In the first study, there was variability among different selections of live oak from Texas, and there were distinct differences in disease response among the California oaks in the other two experiments. In two of the studies, the most susceptible species was the exotic cork oak, Q. suber. Q. kelloggii, the only deciduous red oak inoculated, responded similarly to Q. suber, an evergreen white oak. The two evergreen red oaks, Q. agrifolia and Q. wislizenii, were the least susceptible in one of the studies while the two evergreen "intermediate" oaks, Q. chrysolepisand Q. tomentella, were the least susceptible in another. Significant differences in pathogenicity were observed between two Texas isolates of C. fagacearum. The implications of these results forthe California oak woodlands is discussed.