The subject of this post is a bit late, but I wanted to get the info out to everyone so you can be prepared. Wisconsin state plant health officials are advising consumers who bought rhododendrons or azaleas this spring and summer to be on the lookout for signs of a fungus that could spread to oaks and kill them. It is called Phytophthora ramorum or P. ramorum or Sudden Oak Death, and it was found in one Northern Wisconsin nursery.

This fungus causes Sudden Oak Death, which has never been found on the landscape in Wisconsin. There is no human health risk. Brian Kuhn, director of WIDATCP’s* Plant Industry Bureau said “These plants entered Wisconsin legally, with the proper documentation, and all the businesses involved have cooperated with us.” Susceptible plants at the supplier and the nursery have been destroyed and owners are disinfecting soil and equipment.

Nevertheless, DATCP has imposed an external quarantine on items that could carry P. ramorum. A list of plants covered by the Quarantine can be found at Quarantine.

Serious Issue

Consumers should look for leaf and shoot dieback as potential symptoms. Suspect plants should be sent the University of Wisconsin – Madison Plant Disease Diagnostic Clinic. There is no cost. For information about how to submit a sample, visit Plant Disease Diagnostics Clinic.

Please note sudden oak death is not the same disease as Oak Wilt or Oak Anthracnose, which exists in much of Wisconsin. Information about Oak Wilt is available on the Department of Natural Resources website. Information about Oak Anthracnose is available on the University of Wisconsin-Extension website.

Although the disease may not necessarily kill rhododendrons and azaleas, it could be transmitted to as many as 100 different plant species, including hardwoods, softwoods and shrubs. Oaks are at the greatest risk, because P. ramorum is incurable and has killed 30-45 million oaks in California and Oregon. Spores from the disease can travel in plants; soil, gravel and potting mixes; and wind-blown rain and other water sources. Contaminated pots, shovels and other equipment could also transmit the disease unless sterilized with bleach.

Maryann Whitman, who was Editor of the Wild Ones Journal March April 2005, wrote a good piece entitled It’s All One Piece: The Unraveling of an Ecosystem: Sudden Oak Death Affects More Than the Oaks. It touches on the problems encountered with trying to control diseases such as Phytophthora ramorum.

See also Pathogen That Causes Tree-Killing Disease Found in Illinois for more info about the nursery sources for the plants with this fungus.

*WIDATCP stands for the Wisconsin Department of Agriculture, Trade and Consumer Protection

The above information was taken in part from WIDATCP press release entitled Bought Rhododendrons or Azaleas This Season? Check for Disease, Plant Health Officials Caution.