Officials watching for disease that kills oak trees say it hasn't yet been found in Minnesota

Monitoring for plants infected with fungus-like organism that causes sudden oak death will continue

Phytophthora ramorum symptoms
Phytophthora ramorum symptoms on a rhododendron include large, brown blotches on the leaves.
Photo courtesy Joseph O'Brien, USDA Forest Service, Bugwood.org via Minnesota Department of Agriculture

Minnesota agriculture officials monitoring for the possible spread of a fungus-like organism that has killed millions of oak trees on the West Coast say it has not yet been found in the state.

But they remain on the lookout for a disease that could harm the state's iconic oak trees.

The Minnesota Department of Agriculture put out a call in August to anyone who purchased rhododendrons this year, to be on the lookout for sick plants.

Rhododendrons infected with the organism — called Phytophthora ramorum — were found this year in Iowa, Indiana and Illinois. The shrubs can be a carrier for the disease that causes sudden oak death.

Minnesota did not get a shipment of the infected rhododendrons, but officials asked that anyone in the state who purchased or planted rhododendrons this year to watch for leaves with large, brown blotches, as well as young green stems and shoots that turn brown and shrivel.

Michelle Grabowski, a plant pathologist with the state Department of Agriculture's Plant Protection Division, said they received more than 40 calls from the public. Those cases were investigated, which led to several site visits.

"We were able to collect samples from those plants, bring them back here to our lab ... where we ran the tests," she said. "And I am very happy to report that Phytophthora ramorum was not found on any of the plants that we sampled here in Minnesota."

Grabowski said protocols are in place to test shipments from nurseries on the West Coast that should catch infected plants in the future -- and testing and monitoring is conducted in Minnesota and other states as plants arrive.

State officials said Phytophthora ramorum "is responsible for killing an estimated 30 to 45 million oak trees in coastal forests" of Oregon and California. It destroys the tissue that transports food and water in the trees.

While the oak species dying on the West Coast are different from those native to Minnesota, Grabowski said tests have shown that Minnesota's red and white oaks are susceptible to the disease.

Grabowski noted that Phytophthora ramorum would need to arrive in Minnesota before any oak trees could be affected by sudden oak death -- so any oak trees in the state showing signs of stress now likely are affected by other common insects and diseases, such as oak wilt.

Find more information about insects and diseases affecting Minnesota oaks and other trees here.

Sick rhododendrons?

The symptoms of Phytophthora ramorum on a rhododendron include leaves with large, brown blotches, as well as young green stems and shoots that turn brown and shrivel.

Anyone who spots those symptoms on a recently purchased rhododendron — one that was purchased in 2019 — is asked to call the state's "Arrest the Pest" line at (888) 545-6684, or email arrest.the.pest@state.mn.us. Find tips on making a report here.

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