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New plant disease: Sudden Oak Death Disease

A reddish brown "bleeding" canker that signals the appearance of
Sudden Oak Death Disease. Red liquid seeps out from cracks in the bark.
Photo: Bruce Motzan, USDA Forest Service,
If you have a rhododendron in your yard or neighborhood, take a closer look at the leaves. The Minnesota Department of Agriculture (MDA) is warning Minnesotans to be on the look out for a fungus-like organism that's known to kill oak trees that's making its way here.  

That's right--the rhododendron plant is a carrier of this invasive plant pathogen called Phytophthora ramorum. The spores spread to oak trees and cause sudden death.

So far it has NOT been found in Minnesota, but the potential is there, say MDA officials.

Why should we worry?

MDA officials report that the pathogen is responsible for killing an estimated 30 to 45 million oak trees in the coastal forests of California and Oregon. It also infects over 100 other plants, including rhododendrons. 

The microorganism has been detected at retail nurseries in Midwestern states.

What happens to oak trees? 

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.

Look for symptoms on your rhododendron

Be on the lookout for this new plant disease which is carried
by rhododendrons and ultimately kills oak trees.
The MDA is asking anyone who bought a rhododendron plant in 2019 to carefully inspect the plant for any symptoms of infection and to report any concerns. 

What to look for: 
  • Infected leaves of rhododendrons have large, brown blotches.
  • Young green stems and shoots turn brown and shrivel.
  • Leaves attached to the infected stems wilt and may have a dark brown line extending down the center of the leaf from the base.
Just FYI--this invasive pathogen often does not kill rhododendrons. 

Impact on Minnesota's trees

“Phytophthora ramorum has never been identified in Minnesota but it could have significant impact on the state’s forests and landscapes," said MDA Plant Pathologist Michelle Grabowski. "Tests have shown that native trees like Northern red oak and white oak can be infected with Phytophthora ramorum.”

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 or 1-888-545-6684.

This information was excerpted from a Minnesota Department of Agriculture press release from August 19, 2019. 

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