August 19, 2019

A fungus-like organism known to kill oak trees on the West Coast has made its way to the Midwest, and officials are warning Minnesotans to be on the lookout for symptoms of the disease.

The invasive plant pathogen Phytophthora ramorum causes sudden oak death. In California and Oregon, Phytophthora ramorum is responsible for killing an estimated 30 to 45 million oak trees in coastal forests. The microorganism also infects over 100 other plants, including rhododendrons. Now, several midwestern states have discovered rhododendron shrubs infected with Phytophthora ramorum at retail nursery sites.

Although no infected plants have been found in Minnesota, the Minnesota Department of Agriculture (MDA) is asking anyone who purchased a rhododendron in 2019 to carefully inspect the plant for symptoms of infection by Phytophthora ramorum and to report any concerns.

In rhododendrons, infected leaves have large brown blotches. Young green stems and shoots turn brown and shrivel. Leaves attached to infected stems wilt and may have a dark brown line extending down the center of the leaf from the base. Phytophthora ramorum often does not kill rhododendrons.

“While rhododendrons may not die from this plant disease, our main concern is that the plants act as carriers of Phytophthora ramorum and could spread spores to Minnesota’s oaks,” said MDA Plant Pathologist Michelle Grabowski. “Phytophthora ramorum has never been identified in Minnesota but it could have significant impact on the state’s forests and landscapes. Tests have shown that native trees like Northern red oak and white oak can be infected with Phytophthora ramorum.”

In oak trees, Phytophthora ramorum infects the main trunk of mature trees. This infection causes a “bleeding” canker in which wood and bark turn reddish brown, and red liquid seeps out from cracks in the bark. The pathogen destroys the tissue that transports food and water throughout the tree. When most of the trunk is infected, leaves quickly turn brown and the tree dies.

If symptoms of Phytophthora ramorum are found on new rhododendrons, photos or a description of the symptoms should be reported to the Minnesota Department of Agriculture’s Arrest The Pest line at arrest.the.pest@state.mn.us or 1-888-545-6684.

Click here to download a photo of Phytophthora ramorum on a rhododendron. Caption: Phytophthora ramorum symptoms on a rhododendron include large, brown blotches on the leaves. Photo courtesy Joseph OBrien, USDA Forest Service, Bugwood.org. 

Click here to download a photo of a tree with sudden oak death symptoms. Caption: In oak trees, Phytophthora ramorum causes a “bleeding” canker in which wood and bark turn reddish brown, and red liquid seeps out from cracks in the bark. Photo courtesy Bruce Moltzan, USDA Forest Service, Bugwood.org

Media Contact
Allen Sommerfeld, MDA Communications
651-201-6185
Allen.Sommerfeld@state.mn.us