Sudden Oak Death
Emerging Threat Note: This pathogen is not established in Tennessee, but we actively monitor for it and encourage residents to report potential sightings.
First discovered in 1995, sudden oak death has become a serious problem in northern California and Oregon. The pathogen, Phytophthora ramorum, is currently killing tanoaks, coast live oaks, and California black oaks in these western states and threatens the vast oak forests of the eastern United States. The name “sudden oak death” is somewhat of a misnomer, as trees often die over a period of several years. The disease causes severe girdling cankers on the main stem and large branches. The pathogen is known to infect hundreds of plant species, most of which show no obvious symptoms. Therefore, there is a serious risk that the pathogen will be introduced to new areas on infected landscape plants shipped from nurseries in the Pacific Northwest. While this disease is not established in Tennessee, annual surveys for the pathogen are conducted by the Tennessee Department of Agriculture to prevent its accidental introduction.
On oaks and tanoak, cankers develop in the inner bark and outer sapwood. Cankers rapidly expand to girdle infected trees, cutting off the tree’s supply of water and nutrients. Black or reddish ooze is often observed from stem cankers. Bleeding cankers will stain the surface of the bark; ooze may be difficult to see if it has dried or has been washed off by rain, but the dark staining will remain. Black “fungal” lines may be visible in the sapwood beneath cankers, especially in dead trees. Leaves will turn from green to pale yellow to brown within a few weeks of girdling; succulent shoots may also wilt. Crown dieback begins in the upper branches shortly thereafter. Infected trees may survive for one to several years. Declining or dead trees may be attacked by secondary insects or pathogens such as ambrosia beetles, bark beetles, twolined chestnut borer, Hypoxylon canker, and Armillaria root rot.
Symptoms on other woody and herbaceous plant species vary, but infection occurs mostly on leaves and shoots. Small or large leaf lesions may be present; premature leaf drop, shoot dieback, wilting, or death may occur.
Because this disease may be present on plants without showing obvious symptoms, contaminated nursery plants are the primary concern. Shipments from quarantined areas are monitored to prevent accidental spread of the pathogen. However, because oaks are a dominant species in many eastern forests, particularly in Tennessee, sudden oak death remains a serious threat to nursery and forest industries in the state. The implications of the disease being introduced are unknown, so proactive measures to prevent its introduction in Tennessee are a priority.
There are no known cures for sudden oak death. Large scale eradication of susceptible hosts in areas where the disease occurs has only been partially successful in slowing the spread. It is critical to prevent the introduction of the disease on infected nursery plants.
What Can You Do?
Purchase plants from local sources when possible; do not purchase or plant diseased or unhealthy plants. Report any suspected cases of sudden oak death to the Tennessee Department of Agriculture.