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Serving: IL

Deadly plant disease now in Illinois

Tree Talk: Sudden oak death is confirmed in 10 counties. Here’s what to look for and which plants are susceptible.

The plant disease sudden oak death has found its way to Illinois via retail stores in a number of counties throughout the state. Phytophthora ramorum, the causal fungus of SOD, was confirmed in ornamental plants at Walmart locations in Cook, Jackson, Jefferson, Lee, Macon, Monroe, St. Clair, Stephenson, and Will counties, and one Hy-Vee location in McDonough County. The pathogen is a fungus that has killed numerous oaks and other native plants in Oregon and Washington.

SOD is a water mold fungus that can attack plants directly through the bark, killing plant tissue and resulting in a bleeding canker. As the cankered area grows, the plant eventually wilts and dies. In rhododendrons, a common landscape plant, the pathogen causes a leaf blight and twig dieback. Blight symptoms include leaf spots and wilting, and stems and/or trunks with a brown or black discoloration.

Despite being under an international quarantine, the fungal spores can easily move with irrigation water, rainwater and flowing water. Once the spores land on the bark or leaf of a plant, they can penetrate the leaf cuticle or bark. Like with many invasive pests and pathogens, humans are the most efficient agent for long-range spread of the disease. Infected plants and movement of spores on shoes, bike and automobile tires, tools, implements, and in infected soil can spread the disease over large areas.

To date, SOD has been positively confirmed on the following plants and varieties in Illinois:

Rhododendron. Cat Cunningham Blush, Firestorm, Holden, Minnetonka, Nova Zembla, Percy Wiseman, Purpureum Elegans, Roseum Elegans, Wojnars Purple

Lilac. Common Purple, Persian Lime

To help limit the spread of SOD, know the symptoms associated with the disease, and realize there are many other causes (such as poorly drained or high pH soils), and other diseases and insects that can cause similar symptoms. There is no cure for SOD.

If you think your plant might be infected, contact the Illinois Department of Agriculture or the local University of Illinois Extension office for further guidance, especially if you have purchased rhododendrons from Walmart, Hy-Vee or Rural King since April 2019, and if those plants are looking symptomatic or in poor condition. The most likely source of infection would be plants purchased this spring; older, established plants are not likely to be infected. It is not recommended to destroy or dispose of these plants without state or federal guidance.

Miller is a horticulture professor at Joliet Junior College in Joliet, Ill., and a senior research scientist in entomology at The Morton Arboretum in Lisle, Ill. Email your tree questions to him at fmiller@jjc.edu.

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