Sudden Oak Death
Phytophthora ramorum
Overview
Sudden oak death is caused by Phytophthora ramorum, an oomycete that kills tanoak and some oak species and causes ramorum blight on many ornamental hosts such as rhododendron, camellia, and viburnum. It spreads via nursery stock, wind-blown rain, and movement of infected plant material. In forests, canopy dieback can be rapid on highly susceptible hosts. Quarantines regulate movement of host plants in many regions. Home gardeners should treat suspicious oak decline and ornamental leaf blight seriously where the pathogen is known.
Symptoms
On oaks and tanoak, bleeding cankers on the trunk ooze dark sap, followed by crown wilting, leaf browning, and tree death. On ornamental hosts, leaf spots, shoot tip dieback, and twig blight appear, sometimes without killing the whole plant. Cankers may girdle stems. Symptoms can resemble other Phytophthora diseases and abiotic decline, so laboratory diagnosis is important in regulated areas. Multiple trees or shrubs may show blight after cool, wet periods.
Environmental Conditions
Humid
Prevention
Buy nursery plants from reputable sources and inspect for leaf blight before purchase. Avoid transporting rhododendron, camellia, and other hosts from infected regions. Keep irrigation off oak trunks and ornamental foliage when possible. Diversify landscapes so entire oak stands are not uniform in susceptibility. Clean soil and plant debris from tools and shoes after hiking or working in known infested forests before visiting clean properties.
Management
Follow local extension and regulatory guidance; do not move symptomatic host material out of quarantined zones. Remove and dispose of badly infected ornamentals as directed. For high-value oaks, professional phosphonate treatments are sometimes used preventively in high-risk areas; they are not a guarantee. Reduce splash onto trunks and avoid wounding. Improve airflow in dense ornamental plantings. Report suspected sudden oak death in new areas to authorities.