Oak-killing Pathogen Found in Indiana Nurseries

June 5, 2019

Earlier this month, the Indiana Department of Natural Resources intercepted plants containing a pathogen called sudden oak death in grocery stores. As its name suggests, SOD is a threat to Indiana’s oak trees if it’s not contained.

According to a statement released by the DNR, the disease has been found in more than 70 Walmart stores and 18 Rural King stores across the state. SOD was delivered to those stores in shipments of rhododendrons from a nursery in Oklahoma. Shipments containing infected materials were sent to nine other states.

The disease is prevalent along the West Coast, where it has spread so expansively that Janna Beckerman, a botany and plant pathology professor at Purdue University, says it cannot be removed.

“It’s absolutely impossible to get out at this point,” she said. “It’s spread all throughout the state of California, Washington, Oregon, British Columbia as well.”

This is a photo of an oak leaf infected with sudden oak death.
Foliar dieback symptoms in viburnum infected by Sudden Oak Death. Photo courtesy of Janna Beckerman.

This is the first time in about 10 years that the disease has been seen in Indiana, and it has not been established in the Midwest, to date. However, scientists are worried about the disease spreading. According to Beckerman, there are two ways this could happen.

First, the nursery business is a global industry, which is why the Indiana DNR inspects plant materials shipping into the state to be sure they aren’t infected with pathogens like SOD.

“It has spread across the world mostly through the transportation of infected plant material,” she said.

The second way SOD spreads is through the release of spores by an infected plant in its new environment. Beckerman said host plants can form a wide variety of spores but, given the amount of rain Indiana has had recently, waterborne spores have scientists most concerned.

The disease does not kill its host plants but uses them to produce more spores to infect more plants in the environment. This makes its spread incredibly difficult to stop without eradicating infected plants.

“This thing is really well evolved to spread and persist once it gets in somewhere,” said Beckerman.

This makes SOD dangerous for more than just Indiana oak trees. The understory of forests and popular landscaping plants are also in danger. According to the Indiana DNR, the disease travels in more than 100 species of host plants.

Beckerman says the broad plant host range is one reason the Indiana DNR Department of Entomology & Plant Pathology considers SOD its “top priority.”

This is a photo of an oak tree infected by sudden oak death.
Oak tree infected with Sudden Oak Death. Photo courtesy of Janna Beckerman.

Other phytophthora, SOD’s pathogen family, have been introduced in parts of the world and are known to be destructive. For example, in Australia, phytophthora cinnamomi is responsible for the extinction of 80% of plant life.

“This has the potential to be ecologically devastating,” Beckerman said.

Lindsey Purcell, Purdue University Urban Forestry Extension Specialist, warns of yet another source of damage by SOD. The pathogen has the potential to kill trees in urban forests and green spaces.

“Left untreated and unchecked, it could further deplete the urban canopy, which is so desperately needed for our quality of health and life,” Purcell said.

Though the economic impacts of SOD infecting other plant species is not fully understood, Purcell says the disease can have profound effects in cities and towns everywhere.

Not only is the loss of oak trees a burden on cities economically, but infected trees die and become a danger when they are in places with people present, since the trees are liable to fall. Purcell says keeping green spaces safe for people is a major consideration when trees are present, and pests create an even greater issue for cities.

“We have already lost many trees to emerald ash borer, gypsy moth and Asian long-horned beetle as well as other pests,” said Purcell. “This is just one more serious pest which can kill one of our most important tree species.”

According to Beckerman, SOD is most dangerous in urban forests, so people living near these forests should be observant for changes in landscape shrubs, including rhododendrons, azaleas, euonymouses, lilacs and viburnums, and groundcovers like periwinkle.

She said if someone suspects any of their rhododendrons or other known host plants have the disease, they should send a sample to the Purdue Plant and Pest Diagnostic Laboratory. The lab also has an application called Purdue Tree Doctor, where people can see pictures of the disease to know what they are looking for.

The Indiana DNR says anyone who has purchased rhododendrons in the last four weeks from Walmart or Rural King should destroy them or call 1-866-NO-EXOTIC (663-9684) or the local county extension office at 1-888-EXT-INFO (1-888-398-4636) for instructions.

Oak-killing Pathogen Found in Indiana Nurseries