A potential crisis with regard to oaks is at hand in Indiana. And, if you recently bought rhododendrons from certain stores for your landscape, you may be able to help prevent this crisis.
Recent news alerts have been sent out by Purdue Extension, Indiana Department of Natural Resources, and other groups regarding the disease, Sudden Oak Death (SOD), Phytophthora ramorum, (pronounced Fī-toff-thor-ă ră-mor-ǔm) in Indiana. Janna Beckerman, Professor of botany and plant pathology at Purdue, said that sudden oak death, as the name suggests, is a disease that is capable of rapidly killing certain species of oaks. It was first identified in California in 1995.
Beckerman said this disease kills oaks, but also several other commonly used landscape plants. Despite its name, sudden oak death primarily spreads through foliar hosts that are sold throughout the United States. Foliar hosts include rhododendrons, azalea, viburnum, lilac,and periwinkle (Vinca minor). She said that currently, over 120 hosts in addition to oaks have been identified, and more continue to be added to this list. “What is most unusual about sudden oak death is the severity of disease symptoms coupled with the broad host range of the pathogen,” she said. “This leads to difficulty in diagnosing and managing this disease.”
“If the pathogen becomes established in Indiana, sudden oak death could change the structure of American forests and landscapes, just as emerald ash borer is currently doing, and how Dutch elm disease did 50 years ago, or the chestnut blight did 100 years ago,” Beckerman said.
On May 22, Indiana DNR confirmed that it had intercepted plants containing P. ramorum for the first time in about 10 years. They confirmed May 29 that more than 70 Walmart stores and 18 Rural King stores in the state received rhododendron plants infected with SOD.
Indiana DNR also reported that they have destroyed approximately 1,500 infested rhododendron so far, and pulled another 1,500 from stores, with an order to stop selling rhododendron until further notice.
Beckerman said that oaks in the red oak group (leaves having lobes with bristle-tips) are susceptible to this pathogen, which includes red, black, pin and other oaks in the group. “To understand this disease, it is important to recognize that there are two categories of hosts: bark canker hosts and foliar hosts,” said Beckerman. “Diagnostic symptoms of infected red oaks include oozing sap and red-brown cankers that often lead to death.” Other Phytophthora diseases, root rots or boring insects may produce symptoms easily confused with SOD. For this reason, Beckerman recommends that all Phythophthora infections be screened by the Purdue Plant and Pest Diagnostic Laboratory, https://ag.purdue.edu/btny/ppdl.
Beckerman said that other landscape plants are infected via the leaves and small branches. Symptoms can easily be mistaken for other diseases or environmental disorders. “Although symptoms from these infections are not severe, and are rarely fatal, the infections produce enormous numbers of spores that can infect neighboring, susceptible oaks – and other species,” she said. “For this reason, we are asking people to examine any rhododendrons (or other co-mingled hosts like azalea, viburnum and lilac) purchased this spring from Walmart and Rural King, while the disease may still be controlled and the pathogen contained.”
What can we do?
Dr. Beckerman recommended the following:
At this point, you should be aware that this is a new disease problem, with the potential of spreading throughout the Midwest. No one can predict what the consequences could be if it gets introduced and becomes established in Indiana. Sudden oak death is most dangerous at the urban-forest interface, where forests meet with development, and people living at this interface should be observant for any changes in their landscape shrubs (rhododendron, azalea, euonymous, lilac, viburnum) and groundcovers, like periwinkle.
If you believe you have an infected plant purchased from Walmart or Rural King in the last few months, the plant should be double bagged and destroyed by throwing in the garbage. Sanitize any of garden tools and boots that that may have come in contact with the suspect infected plant. Please contact the store where purchased to discuss a refund. The IDNR cannot help with issues about refunds.
If the material came from a store that is not listed above OR was bought any time before February 2019 then it is not part of this infested shipment and to our knowledge should be clean of pests and pathogens. Learn more about symptoms you should be looking for at suddenoakdeath.org/maps-media/photos/plant-symptom-photos.
For more general information about SOD see the following link: suddenoakdeath.org/about-sudden-oak-death.
Find recent news releases and updates on SOD from Indiana DNR at in.gov/dnr/entomolo/4532.htm.
If you believe you have an oak tree that is infected, call 866-NO-EXOTIC or call Phil Marshall, DNR Forest Pathologist, at 812-358-9034.
Landscapers or nursery industry people may contact Megan Abraham, IDNR Division of Entomology and Plant Pathology, at 317-232-4189.
Read full article by Purdue specialist Janna Beckerman, published on Purdue Landscape Report website: purduelandscapereport.org.