Since 2004, diagnosticians have been concerned about a disease killing oaks and tanoaks in California and Oregon. Although the disease was reported in 1995 on tanoak in California, it was not until 2004 that the pathogen was found on many other hosts, some of which had been shipped to 22 states in the United States via nursery shipments. A nursery inspection program, quarantines, and a national nursery survey were initiated in 2004. A federal order now regulates the interstate movement of plants from all commercial nurseries in California, Oregon, and Washington.
The main concern is that Ramorum blight (also known as sudden oak death, ramorum leaf blight, and ramorum dieback) might spread to midwestern and eastern U.S. forests, killing oak stands. To date, Ramorum blight has not been found in Illinois. It was identified in an Indiana nursery in 2006 and was quickly eradicated. The potential for spread is still present, and spring is the best time to observe symptoms. The University of Illinois Plant Clinic can do the initial testing to help identify the pathogen. Become familiar with symptoms so you can help keep this disease in check.
The cause is a fungal-like pathogen, Phytophthora ramorum. This Phytophthora species is found above-ground on the foliage or trunk but not on roots. This disease can cause symptoms including leaf blights, stem tip dieback, and oozing cankers (on trees). It may be hard to distinguish from other problems with similar symptoms. Pictures and symptom descriptions can be found on the sudden oak death fact sheet produced by the North Central Region Pest Management Center. That fact sheet provides other helpful links as well. A more comprehensive site to find information on foliar blight symptoms caused by this disease and look-alike problems is located at the APHIS web site (Adobe PDF). Look for browning of some leaf tips, often with a darker border or a water-soaked edge to the lesions. Often the lesions have diffuse borders. Uniform tip dieback of all leaves or leaves on the south or west side of plants usually indicates environmental stress rather than this disease. Infected trees develop oozing cankers. The wood under the oozing bark is also discolored, with a black border.
There are now 115 hosts of P. ramorum. The pathogen forms spores on blighted tissues of leaves and stems of infected shrubs. These plants may be shipped to nurseries, purchased by landscapers, and planted near established trees, which could become infected. Ultimately, the main concern is that oak forests will be threatened. The disease has killed thousands of oaks in California. The APHIS list of hosts can be viewed at this web site. Scroll down to the section where online resources are listed. This list is updated frequently. Many of these hosts will grow in Illinois. Some plants of concern in Illinois include rhododendron, lilac, honeysuckle, viburnum, witch hazel, camellia, pieris, Douglas-fir, European beech, magnolia, white fir, horse chestnut, and others.
Many states, including Illinois, have conducted nursery surveys to monitor plants received from the west coast. The University of Illinois Plant Clinic tested samples collected by the Illinois Department of Agriculture and Illinois APHIS inspectors in 2004, 2005, and 2006. ELISA (enzyme-linked immunosorbant assay) tests for the Phytophthora genus were performed on those samples.
National funds for that survey were not continued in 2007. Phytophthora ramorum has not been found in Illinois.
Keep an eye on rhododendron, viburnum, pieris, kalmia, witch hazel, honeysuckle, camellia, or lilacs planted in the last 3 years and obtained from the west coast. If these plants develop extensive cankers, blights, or dieback, review the information above. If you own a nursery and receive suspect material, keep it quarantined until you are certain it is not infected. Feel free to call the University of Illinois Plant Clinic with questions. The telephone number beginning May 1 is (217)333-0519. As new information is available, we will post it in this newsletter.