Sudden oak death/P. ramorum blight is a disease that can affect a wide range of ornamental hosts. It is not limited to oak as the name would seem to imply. The causal agent, Phytophthora ramorum (Fy-TOFF-thor-uh ruh-MOR-um), was first identified in 1993 in Germany and the Netherlands on ornamental rhododendrons. Since then, this disease has been observed in Belgium, Denmark, France, Norway, Poland, Republic of Ireland, Slovenia, Spain, Sweden and the UK. The geographic origin of the pathogen is unknown at this time, however because of its limited known geographical distribution in relation to host distribution, it is believed to be recently introduced to the United States.
P. ramorum was first isolated in the United States in 1995, in Mill Valley, California, where it was found to kill oaks and tanoaks. It is now known to cause damage to a wide range of trees and ornamental plants. The disease has been found on various native hosts in 14 coastal counties in California, in Curry County Oregon and in Nassau County, NY. The pathogen has been confirmed in nursery stock in California, Oregon, Washington and British Columbia.
This disease appeared to be a serious, but localized problem until March of 2004 when it was discovered that plants had been shipped from some P. ramorum-infected nurseries in California to other states. USDA-APHIS traced those shipments and the recipient nurseries and suspect plants were sampled and tested for P. ramorum. As a result of these inspections, the pathogen was found in 22 states across the nation; infected plants were subsequently destroyed. As of April 2017, P. ramorum has not been found in Illinois nor any other Midwestern state.
As of August 2013, 46 plant species have been proven hosts and another 71 plants have been associated with the disease. Some of these species can be grown in Illinois and include Japanese pieris, Douglas fir, rhododendron, witchhazel, viburnum, beech, and lilac. See the following link for the official P. ramorum U.S. host list is provided by USDA-APHIS. Keep in mind that the list changes often, so note the revision date on any such listing.
The introduction of this pathogen to Illinois would be devastating for the affected nurseries, forests and landscapes. Thus, we must all be aware of this disease and the need for accurate and rapid diagnosis and response.