Many Illinois readers have been concerned about the health of mature oaks on their property. The most serious diseases of oaks in Illinois include oak wilt and bacterial leaf scorch, both of which were discussed in previous issues. Oaks are also susceptible to decline following compaction of the root system, construction injury, Armillaria root rot, and many other factors. Many of you have probably heard of another threat to oak--sudden oak death. We have not seen it in Illinois but address it here because of the U of I Plant Clinic receives frequent questions.
This awesome-sounding disease is caused by Phytophthora ramorum. Although we find many species of Phytophthoras in Illinois on rhododendron, many greenhouse plants, and even soybeans, the sudden oak death species does not occur in Illinois. In fact, it has not been found outside California and Oregon.
Sudden oak death (SOD) is a fungal disease that causes a rapid decline and death of oaks. The name is a bit misleading because it also occurs on other tree and shrub species, 17 in all. Symptoms vary from leaf spots to twig blight to trunk cankers. Roots show no symptoms. Oaks affected are tan oak, coast live oak, and California black oak, none of which grows in Illinois. Research has shown that red and pin oaks are susceptible when artificially inoculated. In addition, some other plants grown in Illinois can serve as hosts, including rhododendron, azalea, and Douglas-fir. Obviously, there is great concern that the disease might move on these plants to other areas of the country.
SOD causes rapid decline and death of oaks, usually resulting in death in 1 to 3 years. On oak, the disease causes a bleeding canker on the stems or trunk. You might also see bleeding cankers on Illinois oaks; but the SOD cankers ooze a black or reddish fluid, and the wood under them has black zone lines evident when bark is removed. Once crown dieback begins, the leaves turn brown in a few weeks.
Because of the concern of movement of this disease within and out of California and Oregon, USDA began regulating the shipment of all host plants in February 2002. The Animal and Plant Health Inspection Service (APHIS) performed surveys for some southeastern states (Georgia, South Carolina, North Carolina, Tennessee, and Virginia) where the disease is most likely to take hold. These surveys were slated for spring 2003. If the disease moves outside Washington and Oregon, this information will be shared. Oklahoma plant pathologists have initiated nursery surveys and have not yet found the disease.
There is a tremendous amount of information available on this disease on the Internet. A good source to start with is USDA’s “Sudden Oak Death Pest Alert,” NA-PR-02-02, at http://www.na.fs.fed.us/spfo/pubs/pest_al/sodeast/sodeast.htm.