McConnell, Megan ElizabethThe association of <italic>Phytophthora cinnamomi</italic> with declining white oaks was studied at 102 sites in mid-Atlantic oak forests. Seven <italic>Phytophthora</italic> species were isolated from 44 sites. <italic>P. cinnamomi</italic> was restricted to USDA plant hardiness zones six and seven, and <italic>P. cinnamomi</italic> propagules in soil were significantly higher in zone seven than in zone six. When white oak fine root lengths of infected and <italic>Phytophthora</italic>-free trees were compared, infected trees had significantly lower fine root amounts. However, infected trees in zone seven had more fine roots. Little difference in pathogenicity between 32 <italic>P. cinnamomi</italic> isolates was found during white oak seedling stem inoculations. Fine root lengths of inoculated white and red oak seedlings decreased most during the spring. Propagule density in soilless potting media decreased with increasing temperature, except at 8°C and 16°C. These studies demonstrated that the impact, survival and spread of <italic>P. cinnamomi</italic> are strongly linked to environmental conditions and host species.enThesisPlant pathologyForestryPlant sciencesforest pathologymid-Atlantic regionoak declinePhytophthora cinnamomiwhite oakwhite oak decline