Pathology Several species of fungi belonging to the genus Phytophthora cause root rot of herbaceous and woody ornamentals. Phytophthora cinnamomi attacks woody ornamentals including arborvitae, azalea, Chamaecyparis , dogwood, forsythia, Fraser fir, hemlock, Japanese holly, juniper, Pieris , Rhododendron , Taxus and white pine. Therefore, poorly drained soil or wet sites favor the disease.
Zoospores infect feeder roots just behind the root cap.
Soil pH plays little role in this disease. Phytophthora overwinters in the soil. The fungus can be splash-dispersed during heavy rains or overhead irrigation. Also, the fungus can be carried in run-off from plant to plant in the field or from an infected plant to the drain holes of containers of nearby healthy plants. Purchase Phytophthora -free plants.