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"Sudden Oak Death" Pathogen Detected In Plants Shipped To Retail Nurseries Across Missouri

Missouri Department of Agriculture
A wilted rhododendron leaf exhibits signs of Sudden Oak Death, which has been detected in 2,000 plants shipped to retail nurseries throughout the state.

A microbe responsible for the sweeping death of oak trees along the west coast has been detected in over 2,000 rhododendron plants shipped to various retail nurseries throughout Missouri. 

Phytopthera ramorum, commonly known as “Sudden Oak Death,” was first detected in California in 1995 and is under federal quarantine. It hasn’t yet established itself in the Midwest, but the Missouri Department of Agriculture, with the help of the USDA Animal and Plant Inspection Service, has already removed potential host plants from shelves or had them destroyed. 

The rhododendron plants were grown in Washington and Canada, and shipped by Park Hill Plants in Oklahoma to 113 Walmart and Rural King garden center locations. 

State entomologist Collin Wamsley says in addition to the quarantine, the department worked with the retail nurseries to collect samples of symptomatic plants to send to labs at Cornell University and the University of Florida for preliminary diagnostics. 

“Any suspect positives that those labs had from our samples, those samples were shipped to USDA national identifiers for final confirmation,” says Wamsley, adding that they are still being processed.

Researchers are unsure of the potential devastation Sudden Oak Death could cause in the state’s plant life. Over 100 species of trees and shrubs are on its host list, including Northern Red Oaks and Pin Oaks, which are popular landscaping trees. But, Wamsley says it’s all a question of whether Missouri’s climate would be as conducive for the fungus-like microbe as California’s. 

“There’s all that coastal moisture which is ideal for this type of pathogen,” says Wamsley. 

Of Missouri’s 16 million acres of forest land, Wamsley says they would be most concerned with the safety of the Ozark Forest. 

Some potentially infected plants have already been sold to consumers. Consequently, those who bought a rhododendron or certain lilac variety between March and June of this year should immediately and properly dispose of the plants. Wamsley says gardeners can do so throughdeep burial or by double-bagging the plant with its root ball in heavy duty trash bags for disposal into a sanitary landfill. They should also avoid disposing of the plant at city yard waste collection sites, and garden tools used in the disposal process should be sanitized before they’re used again.

Wamsley says if gardeners planted an infected variety in the ground shortly after purchasing it, the disease likely wouldn’t have travelled too far, and won’t if they remove it now. 

“It’s a soil-borne pathogen, and it’s really going to stay localized,” says Wamsley. “It’s a matter of whether there are other susceptible host plants nearby in their landscape.”

Seven varieties of rhododendrons have tested positive in this latest infestation, including Cat Cunningham Blush, Firestorm, Holden, Nova Zembla, Percy Wiseman, Roseum Elegans, and Wojnars Purple. 

Lilac varieties that have tested positive in other states include Common Purple and Persian Lime.

If you have questions about Sudden Oak Death, contact the department’s Plant Pest Control team at 573-571-5505.