Location: Foreign Disease-Weed Science
Title: Effects of inoculum density and wounding on stem infection of three Eastern U.S. forest species by Phytophthora ramorum Authors
Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 24, 2014
Publication Date: March 28, 2014
Citation: Tooley, P.W., Browning, M.E., Leighty, R.M. 2014. Effects of inoculum density and wounding on stem infection of three Eastern U.S. forest species by Phytophthora ramorum. Journal of Phytopathology. 162:683-689 (2014) - doi: 10.1111/jph.12251. Interpretive Summary: Phytophthora ramorum is a destructive forest pathogen that is a threat to the oak forests of the Eastern U.S. To investigate the epidemic potential of P. ramorum, we applied different concentrations of pathogen spores to stems of three species of forest trees prevalent in the Eastern U.S. The tree species were northern red oak, chestnut oak and red maple. Our goal was to determine how many spores are required for infection of these trees, and also whether wounding is necessary to get infection. We found that, with wounding, very low concentrations of spores were effective in producing disease on the tree stems. With increasing spore concentration, the two oak species showed higher rates of disease development compared with red maple. These results indicate that wounding is necessary for infection of tree stems by P. ramorum, under our conditions, thus calling attention to the potential role played by insect activity and other natural forms of wounding on epidemic development.
Technical Abstract: Seedlings of three Eastern US forest species (red maple, northern red oak, and chestnut oak) were inoculated by applying Phytophthora ramorum sporangia to stems at different inoculum densities with and without wounding. Disease occurred in all treatments involving wounds, and no disease was observed in unwounded treatments. Younger seedlings (3 - 4 year old) did not differ significantly from older seedlings (5-6 year) in disease incidence, but older trees sustained smaller lesions compared with younger trees. For both old and young trees, disease on wounded stems was observed down to the lowest sporangia concentration utilized (500 sporangia/ml for old trees and100 sporangia/ml for young trees). The results show that in the presence of wounding, even very low sporangia concentrations can result in disease, and further suggest that wounding caused by insects and other factors may play an important role in P. ramorum epidemiology in forest environments.