State plant health officials are advising consumers who bought rhododendrons or azaleas this spring and summer to be on the lookout for signs of a disease that could spread to oaks and kill them.

Plant health staff with the Department of Agriculture, Trade and Consumer Protection have identified Phytophthora ramorum, or P. ramorum, on rhododendrons at a northern Wisconsin nursery. This fungus causes sudden oak death, which has never been found on the landscape in Wisconsin. There is no human health risk.

The find resulted from a July survey of 59 garden centers and nurseries in the state, after it was reported that potentially infected shipments originating from a Washington state supplier had gone to these businesses. Potentially infected plants were shipped to at least 28 states, many of which have already found the leaf blight disease at stores receiving the plants. Wisconsin inspectors collected 43 samples, but only one was found positive for the fungus.

Consumers should look for leaf and shoot dieback as potential symptoms. Suspect plants should be sent the University of Wisconsin — Madison Plant Disease Diagnostic Clinic. There is no cost. For information about how to submit a sample, visit https://pddc.wisc.edu.

“Given the results of our surveys, we would expect that many of the distributed plants are negative for P. ramorum. However, due to the positive detection and because many of the plants had been sold before we received notice, we wanted to enlist citizens in keeping an eye out for symptoms,” says Brian Kuhn, director of DATCP’s Plant Industry Bureau. “These plants entered Wisconsin legally, with the proper documentation, and all the businesses involved have cooperated with us.”

Susceptible plants at the supplier and the nursery are being destroyed and owners are disinfecting soil and equipment.

Although the disease may not necessarily kill rhododendrons and azaleas, it could be transmitted to as many as 100 different plant species, including hardwoods, softwoods and shrubs. Oaks are at the greatest risk, because P. ramorum is incurable and has killed 30-45 million oaks in California and Oregon. Spores from the disease can travel in plants; soil, gravel and potting mixes; and wind-blown rain and other water sources. Contaminated pots, shovels and other equipment could also transmit the disease unless sterilized with bleach.

For more information about sudden oak death, visit datcp.wi.gov and search for “sudden oak death.”

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