By Melissa Koesler
Garvin County County Extension Director
Ag/4-H Youth Development Educator
Earlier this year, an Eastern Oklahoma nursery received rhododendrons from a west coast source. The plants were unloaded on the Oklahoma property in late February.
Over the next two months, the imported rhododendrons were loaded onto trucks along with plants that were grown at the Oklahoma site. These trucks were sent to several states including Oklahoma, Indiana and Kansas.
Unfortunately, some of the imported rhododendrons had a plant disease known by the common names Ramorum blight or Sudden oak death (SOD).
When plants arrived at garden centers, it became clear there was a problem. It is likely, that humid conditions during shipping encouraged development of the disease and expression of symptoms.
Nursery inspectors in Indiana collected samples to be tested for plant pathogens.
The pathogen present was determined to be Phytophthora ramorum, the cause of SOD or Ramorum blight.
Other states that received the rhododendrons had similar findings, all traced back to a common, west coast source.
At this time, the only confirmed cases of P. ramorum in Oklahoma are from one site, a Home Depot in Oklahoma County. There were four ‘Cunningham’s blush’ Rhododendron plants that have tested positive for P. ramorum.
It is possible that a few other Home Depot or Walmart stores in Oklahoma also received contaminated rhododendron plants.
The Oklahoma Department of Agriculture, Food and Forestry (ODAFF) has visited garden centers that received contaminated plants and the remaining plants at the stores have been destroyed.
It is possible that some rhododendron plants were purchased before the contamination was discovered.
ODAFF is also sampling at the original nursery to determine if the pathogen spread during the limited time the plants were present at that Oklahoma nursery.
All samples are being tested by the Plant Disease and Insect Diagnostic Laboratory at Oklahoma State University and possible P. ramorum findings will be confirmed by the USDA.
The pathogen P. ramorum is a fungus-like organism that causes spots, blotches and foliar dieback of landscape plants such as rhododendron, azalea, viburnum, lilac, camellia, laurel and Japanese Andromeda.
In California, it has been found to cause decline and bleeding cankers of oak trees. It should be noted that other disease problems may cause similar symptoms, so home gardeners should not immediately assume that their plants have SOD.
The rainy spring weather has led many plants to develop disease symptoms and most of these diseases occur commonly in Oklahoma.
If home gardeners are concerned about rhododendrons recently purchased from Home Depot or Walmart stores, they should dig up the plant (including roots and potting mix) and place the plant(s) in double plastic bags.
If plants are tightly packed, it may also be helpful to remove plants within 2 feet of the rhododendron since spread is generally through splashing water.
The plant(s) should be disposed of in the landfill, not the compost pile. After removing the plant, clean equipment (shovels, tools, shoes) with a commercial disinfectant.
It may be possible to get a refund from the store where the rhododendron was purchased.
It is likely that home gardeners may be concerned that their plants are suffering from Ramorum blight or SOD.
At this time, the only landscapes where there may be concern is those that installed new rhododendron plants purchased at Home Depot or Walmart from February to June 2019.
Even if contaminated plants were installed, it is unknown if the pathogen can survive our harsh environment and it is not likely to have spread much in the short time. As a reminder, with the substantial rainfall received in many areas of Oklahoma this spring, plant diseases are a normal and natural occurrence.
(Information provided by Jennifer Olson, Associate Extension Specialist, Department of Entomology and Plant Pathology, Oklahoma State University.)