Invasive tree pathogen that causes red oaks to ‘bleed’ found for first time in Minnesota

A devastating pest that has killed tens of millions of oak trees in California was discovered for the first time in Minnesota, researchers announced this week.

The “sudden oak death” pathogen, which behaves like a fungus and is always fatal to red oaks, was recently discovered on a rhododendron plant at a Hennepin County nursery. The plant has been destroyed, and the pathogen does not seem to have spread to any others in the nursery or in the nearby soil, researchers said.

“It appears to be an isolated incident so far,” said Nick Rajtar, a postdoctoral associate at the University of Minnesota. “So it’s not time to sound the alarm bells quite yet, but this is a scary pathogen that nobody wants to see get established here.”

The rhododendron sample was taken in July as part of a testing program led by researchers at the U’s Invasive Terrestrial Plants and Pests Center. The U has been using a grant from the state’s Environmental Trust Fund to hunt for invasive forest pests and diseases that could potentially be introduced to Minnesota through nurseries.

The microscopic pathogen, Phytophthora ramorum, isn’t a fungus but is closely related to one. It originated in Asia and spreads spores that can lie dormant in the soil, and even swim through water.

“So you can imagine how that could be problematic at a nursery when they’re watering plants that are very close together,” Rajtar said.

The pathogen was found in California in 2000 and has been steadily spreading east.

It takes a couple of years for the pathogen to kill an infected tree, but individual oaks often don’t show any signs of illness or of being under attack until just a few weeks before they die. That gives the appearance that the pathogen kills them almost immediately.

The pathogen can live on rhododendron and other popular ornamental garden shrubs without killing those shrubs. It can quickly spread from those shrubs to oaks, where it is always deadly.

Outside of California and Oregon, the pathogen has established a foothold in Appalachia and is spreading in British Columbia. So far it has not been known to spread in the Midwest. Iowa found an infected plant at a nursery in 2019, apparently an isolated case, Rajtar said. Infected plants were also found in Wisconsin nurseries, he said.

Researchers hope that the bitter winters of Minnesota are a defense against the pathogen, but they don’t know yet how well it could survive here.

Oak trees may be the single most important plant for fostering wildlife in the state’s ecosystem. Their acorns provide ready food for everything from squirrels and rodents to black bears. Their leaves harbor and feed massive numbers of caterpillars, which, in turn, support entire populations of songbirds.